Biogeochemical Processes of Biogenic Elements in China Marginal Seas

ADVANCED TOPICS IN SCIENCE AND TECHNOLOGY IN CHINA ADVANCED TOPICS IN SCIENCE AND TECHNOLOGY IN CHINA Zhejiang Univer...

Author: Jinming Song

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ADVANCED TOPICS IN SCIENCE AND TECHNOLOGY IN CHINA

ADVANCED TOPICS IN SCIENCE AND TECHNOLOGY IN CHINA Zhejiang University is one of the leading universities in China. In Advanced Topics in Science and Technology in China, Zhejiang University Press and Springer jointly publish monographs by Chinese scholars and professors, as well as invited authors and editors from abroad who are outstanding experts and scholars in their fields. This series will be of interest to researchers, lecturers, and graduate students alike. Advanced Topics in Science and Technology in China aims to present the latest and most cutting-edge theories, techniques, and methodologies in various research areas in China. It covers all disciplines in the fields of natural science and technology, including but not limited to, computer science, materials science, life sciences, engineering, environmental sciences, mathematics, and physics.

Jinming Song

Biogeochemical Processes of Biogenic Elements in China Marginal Seas With 288 figures

Author Prof. Jinming Song Institute of Oceanology Chinese Academy of Sciences Qingdao 266071, China E-mail: [email protected]

ISSN 1995-6819 e-ISSN 1995-6827 Advanced Topics in Science and Technology in China ISBN 978-7-308-06592-4 Zhejiang University Press, Hangzhou ISBN 978-3-642-04059-7 e-ISBN 978-3-540-04060-3 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2009933161 © Zhejiang University Press, Hangzhou and Springer-Verlag Berlin Heidelberg 2010 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: Frido Steinen-Broo, EStudio Calamar, Spain Printed on acid-free paper Springer is a part of Springer Science+Business Media (www.springer.com)

Preface

Marine biogeochemistry is the study of the interactions of the biology, chemistry, and geology of the ocean, i.e., the roles of distribution, transformation, removal, enrichment and dispersal of chemical components in the ocean controlled by marine biological processes. The marine biogeochemical process is one of the most important controlling procedures of global change. Marine biogeochemical process research in China has made progress in the past 20 years, although the biogeochemical study of biogenic elements in China marginal seas started relatively late. The global research programs launched from the 1980s have greatly accelerated the development of marine biogeochemistry and made it one of the main concerns in the studies of oceanography. Since then, scientists from different fields throughout the world have been devoted to this study, and have made unprecedented progress, which was clearly shown in two aspects, i.e., an unparalleled cross-link between all the specific research fields in oceanographic research, and the systematic new results achieved up to date. Though the oceanic process is very complex, it has been understood more clearly than before. Nowadays, oceanic problems cannot be resolved using only the knowledge from one single field. We can say that, in the past 10 years, the progress in marine research was largely demonstrated by the development of marine biogeochemistry. Global oceanic evolution research, which consists of research of different regions, is concerned mainly with the ocean’s role in the global climate and changing marine environment. So the regional response to global oceanic change is the groundwork of that research. China marginal seas, including the Bohai Sea, the Yellow Sea (YS), the East China Sea (ECS) and the South China Sea (SCS), have their particular environmental characteristics. Extending from the continental shelf to the continental slope, from tropical to temperate seas, with the input from world-famous rivers, and with many developing and developed cities located in the coastal regions, China’s seas almost contain every kind of typical ecosystem, such as an estuarine ecosystem, a continental shelf ecosystem, an upwelling ecosystem, a coral reef ecosystem and a mangrove ecosystem. They are typical regions for studying marine bio-

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geochemical processes. The biogeochemistry process of China marginal seas has been studied continuously with great effort by Chinese scientists and great achievements have been made in the field. This monograph, which is the first book on the biogeochemistry of biogenic elements in China marginal seas in the world, deals with the variations and change mechanisms of biogenic elements such as carbon, nitrogen, phosphorus, silica, sulfur, oxygen, in China marginal seas. It is the main research achievement of the project on “The process and mechanism of ecosystem variation in China marginal seas” which is a “Fund for Creative Research Groups” of the NSFC (No. 40821004), “The degenerated mechanism of main biological resources under composite-pollutant stress in Bohai Bay” which is a National Key Project for Basic Research in China (No. 2007CB407305), “Chemical processes in the sediment-seawater interface and biogenic elements cycling of China’s seas” which is a National Science Foundation for Outstanding Young Scientists in China project (No. 49925614), and the “100 Talents Project” of the Chinese Academy of Sciences (No. 2003-202). The monograph is a landmark in marine biogeochemistry development in China. It also lays a good foundation for further study in this field. It is believed that many scientists and others, who are concerned with the environment, will be interested in the book. This monograph includes 6 chapters. Chapter 1 describes the basic status of China marginal seas, including the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea, and the research progress in marine biogeochemistry in China. From chapter 2 to chapter 5, the research results of biogeochemical processes on biogenic elements in China marginal seas including the Bohai Sea, the Yellow Sea, the East China Sea and the South China Sea are summarized. Chapter 6 provides the main key biogeochemical processes in China marginal seas, the prospects for biogeochemistry in China marginal seas, and the methods, concepts and focus on marine biogeochemical process research in China. There were many people who contributed to the research effort over the past 30 years and their great work has contributed so much to a growing and dynamic field in China. Now, nobody should be in any doubt that the importance of marine biogeochemical processes has been recognized where it counts. Therefore, I want to acknowledge the contributions of the numerous people who made this monograph possible. I owe a special debt of gratitude to my colleagues and students, from whose insights and understanding I have benefited greatly and borrowed freely. These include Dr. Xuegang Li, Dr. Huamao Yuan, Dr. Ning Li, Dr. Peng Zhang, Dr. Liqin Duan, Dr. Yayan Xu and Dr. Sisi Xu. I also greatly appreciate the contributions of Dr. Xuelu Gao, Dr. Jicui Dai, Dr. Guoxia Zheng, and Dr. Xiaoxia L¨ u, for their diligent library research. I thank Miss Hanfeng Lin, Mr. Ian McIntosh, Mrs. Helen (Yuehong) Zhang, and Mr. Jianzhong You at Zhejiang University Press, whose consistent encouragement, hard work, and careful attention to details contributed much to the

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clarity of both the text and the figures. I also express my thanks to authors for permitting their papers published partly cited in this book. Finally, I am grateful beyond measure to my family and friends, without whose patience, understanding and forbearance this monograph would never have been written. I hope that it will be of interest to all those working in the field.

Jinming Song Qingdao, China August 8, 2009

Contents

1

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Basic Status of China Marginal Seas . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 The Bohai Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.2 The Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 1.1.3 The East China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 1.1.4 The South China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 1.2 Progress in Marine Biogeochemical Process Research in China 67 1.2.1 Progress in the Studies in Marine Biogeochemical Processes before 2000 in China . . . . . . . . . . . . . . . . . . . . . . 69 1.2.2 Progress in Biogeochemical Processes of Marine Carbon Cycles since 2000 in China . . . . . . . . . . . . . . . . . . 80 1.2.3 Biogeochemical Cycle of Biogenic Elements . . . . . . . . . . . 94 1.3 Functions of China Marginal Sea Sediments in Cycles of Biogenic Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 1.3.1 Biogenic Elements in China Marginal Sea Sediments . . . 112 1.3.2 Chemical Environments of China Marginal Sea Sediments and Early Diagenesis of Biogenic Elements . . 114 1.3.3 Contribution of Settling Particles to Biogenic Element Recycling in China Marginal Seas . . . . . . . . . . . . . . . . . . . 116 1.3.4 Contributions of China Marginal Sea Sediments in the Recycling of Biogenic Elements . . . . . . . . . . . . . . . . . . . . . 119 1.3.5 Influences of Biological Productions in China Marginal Sea Sediments on the Recycling of Biogenic Elements . . 123 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

2

Biogeochemical Processes of the Bohai Sea . . . . . . . . . . . . . . . . 139 2.1 Change Processes of Carbon in the Bohai Sea . . . . . . . . . . . . . . 140 2.1.1 Partial Pressure of CO2 in Sea Water . . . . . . . . . . . . . . . . 140 2.1.2 Riverine Sources and Estuarine Fates of Particulate Organic Carbon in Seawaters . . . . . . . . . . . . . . . . . . . . . . . 144

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2.1.3 Inorganic Carbon in Liaodong Bay Sediments of the Bohai Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 2.1.4 Biogeochemical Process of Organic Carbon in Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 2.2 Distributions and Transformations of Nitrogen in the Bohai Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 2.2.1 Nitrogen in Seawaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 2.2.2 Evolution of Nutrients and Primary Production . . . . . . 175 2.2.3 Nitrogen Forms and the Decomposition of Organic Nitrogen in Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 2.2.4 Sediment-Water Exchange of Inorganic Nitrogen . . . . . . 193 2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 2.3.1 Distribution of Phosphorus and Silicate in Seawaters . . . 195 2.3.2 Forms of Phosphorus and Silicon in Surface Sediments . 197 2.3.3 Processes of Nutrients across the Sediment-Water Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206 2.4 Behaviour of Heavy Metals in the Bohai Sea . . . . . . . . . . . . . . . . 214 2.4.1 Distribution of Dissolved Heavy Metals in Seawaters . . . 214 2.4.2 Dissolved Heavy Metal Pollution in Bohai Bay . . . . . . . . 219 2.4.3 Heavy Metals in Bohai Bay Sediments . . . . . . . . . . . . . . . 223 2.5 Persistent Organic Pollutants in the Coastal Areas of the Bohai Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 2.5.1 Distributions of Persistent Organic Pollutants in Sediments and Mollusks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232 2.5.2 Composition and Sources of Persistent Organic Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 2.5.3 Potential Risk of Persistent Organic Pollutants . . . . . . . . 250 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 3

Biogeochemical Processes of the Yellow Sea . . . . . . . . . . . . . . . 263 3.1 Dynamic Processes of the Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . 264 3.1.1 Yellow Sea Currents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 3.1.2 Water Exchange Between the Yellow Sea and the East China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 3.2 Carbon Biogeochemical Processes in the Yellow Sea . . . . . . . . . . 273 3.2.1 Carbon Processes across the Air-Sea Interface . . . . . . . . . 273 3.2.2 Biological Carbon Fixation in the South Yellow Sea Seawater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 3.2.3 Initial Carbon Fixed Production . . . . . . . . . . . . . . . . . . . . 290 3.3 Dimethylsulfide and Its Fluxes across the Sea-Air Interface of the Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295 3.3.1 Characteristics of Dimethylsulfide and Dimethylsulfoniopropionate . . . . . . . . . . . . . . . . . . . . . . . . . 296 3.3.2 Sea-to-Air Flux of Dimethylsulfide . . . . . . . . . . . . . . . . . . . 299

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3.3.3 Source and Sink of Dimethylsulfide in the Microlayer . . 301 3.4 Biogeochemical Characteristics Nitrogen and Phosphorus in the Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 3.4.1 Variations of Nitrogen and Phosphorus in Seawaters . . . 303 3.4.2 Dry and Wet Fluxes of Nutrients . . . . . . . . . . . . . . . . . . . . 308 3.4.3 Nutrients in the South Yellow Sea Sediments . . . . . . . . . . 311 3.4.4 Nitrogen in the North Yellow Sea Sediments . . . . . . . . . . 323 3.4.5 Biogeochemical Processes of Phosphorus . . . . . . . . . . . . . 332 3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea . . 337 3.5.1 Behaviour and Variation of Carbon . . . . . . . . . . . . . . . . . . 337 3.5.2 Historical Variation of Nitrogen . . . . . . . . . . . . . . . . . . . . 347 3.5.3 Historical Variation of Phosphorus . . . . . . . . . . . . . . . . . . 366 3.5.4 Biogenic Silica in the Sediments . . . . . . . . . . . . . . . . . . . . 374 3.5.5 Nutrients (N, P, Si) in the Seawaters . . . . . . . . . . . . . . . . 381 3.6 Biogeochemical Characteristics of Heavy Metals in Yellow Sea Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 384 3.6.1 Distributions of Heavy Metals . . . . . . . . . . . . . . . . . . . . . . 384 3.6.2 Annual Variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 3.6.3 Controlling and Influencing Factors . . . . . . . . . . . . . . . . . 388 3.6.4 Pollution Characteristics and Ecological Risk Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396 3.7.1 Polycyclic Aromatic Hydrocarbons in the Sediments of the Northern Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . . . 396 3.7.2 Polychlorinated Biphenyls in the Sediments of the South Yellow Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399 3.7.3 Contamination History of Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls in the 20th Century . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 409 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 418 4

Biogeochemical Processes of the East China Sea . . . . . . . . . . . 425 4.1 Dynamic Processes in the East China Sea and Its Adjacent Ocean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 4.1.1 Circulation and Sea-Air Interaction in the Southern Yellow Sea and East China Sea . . . . . . . . . . . . . . . . . . . . . 426 4.1.2 The Kuroshio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428 4.1.3 Currents East of the Pyukyu Islands . . . . . . . . . . . . . . . . . 430 4.2 Carbon Cycling in the East China Sea . . . . . . . . . . . . . . . . . . . . . 431 4.2.1 Spatial Distributions of Inorganic Carbon in Seawaters . 431 4.2.2 Organic Carbon (Dissolved Organic Carbon and Particulate Organic Carbon) in seawaters . . . . . . . . . . . . 440 4.2.3 Key Biogeochemical Processes of Carbon in Seawaters . 443 4.2.4 Inorganic Carbon in Sediments . . . . . . . . . . . . . . . . . . . . . . 449

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4.2.5 Biogeochemical Characteristics of Organic Carbon in Sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460 4.3 Nitrogen Variations and Budgets in the East China Sea . . . . . . 467 4.3.1 Seasonal Variations of Nitrogen in Seawaters . . . . . . . . . 467 4.3.2 Nitrogen Distribution and Its Influencing Factors in the Sediment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475 4.3.3 Fluxes of Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 4.4 Phosphorus Biogeochemistry in the East China Sea . . . . . . . . . . 482 4.4.1 Distribution of Phosphorus in the Seawater . . . . . . . . . . . 483 4.4.2 Distribution of Phosphorus in the Sediments . . . . . . . . . . 488 4.4.3 Phosphorus Burial Fluxes . . . . . . . . . . . . . . . . . . . . . . . . . . 490 4.4.4 Phosphorus Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 4.4.5 Cycling of Phosphorus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 494 4.5 Silicate and Biogenic Silica in the East China Sea . . . . . . . . . . . 498 4.5.1 Spatial Distribution of the Dissolved Silicate in Seawaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 4.5.2 Distribution of Biogenic Silica in Sediments . . . . . . . . . . . 504 4.5.3 Silica Balance on the East China Sea Shelf . . . . . . . . . . . 506 4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509 4.6.1 Dissolved Oxygen Distributions in Seawaters . . . . . . . . . . 511 4.6.2 O2 Flux across the Sea-Air Interface . . . . . . . . . . . . . . . . . 517 4.6.3 Factors Influencing Dissolved Oxygen Concentration . . . 520 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 5

Biogeochemical Processes of the South China Sea . . . . . . . . . 529 5.1 Water Dynamical Processes in the South China Sea . . . . . . . . . . 529 5.1.1 Circulation and Eddies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 532 5.1.2 Water Exchange via the Straits . . . . . . . . . . . . . . . . . . . . . 537 5.1.3 Dynamics of the Mixed Layer and Thermocline of the South China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539 5.2 Nutrient Budgets in the Seawaters of the South China Sea . . . . 541 5.2.1 Nitrogen Budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 542 5.2.2 Phosphorus Budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543 5.2.3 Silicate Budgets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544 5.3 Biogeochemical Processes in the Pearl River Estuary . . . . . . . . . 546 5.3.1 Nutrients in Coastal Waters of the Pearl River Estuary 546 5.3.2 Carbon in the Pearl River Estuary . . . . . . . . . . . . . . . . . . . 555 5.4 Biogenic Elements in the Northern South China Sea . . . . . . . . . 570 5.4.1 Carbon in the Northern South China Sea . . . . . . . . . . . . . 570 5.4.2 Distributions of Inorganic Nutrients in the Northern South China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575 5.5 Biogeochemical Processes in the Nansha Islands Waters . . . . . . 575 5.5.1 Coral Reefs and Their Affected Factors . . . . . . . . . . . . . . . 577

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5.5.2 Simulated Drift-Net Theory: The New Viewpoint on the High Productivity Supporting the Nansha Coral Reef Ecosystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579 5.5.3 Nitrogen in Sediments of the Nansha Islands Waters . . . 583 5.5.4 Carbon Cycling in the Nansha Coral Reef Ecosystem . . 590 5.5.5 Vertical Transferring Process of Major and Rare Elements in the Nansha Coral Rreef Lagoons . . . . . . . . . 605 5.5.6 Sulfide (−2 Valence) in Lagoon and Off-Reef Sediment Interstitial Waters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 616 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620 6

Prospects for Marine Biogeochemical Process Research in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 627 6.1 Marine Biogeochemical Process Research in China . . . . . . . . . . . 627 6.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 627 6.1.2 Focus on Marine Biogeochemical Process Research in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 631 6.1.3 Research Methods of China Marginal Seas’ Biogeochemical Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 639 6.2 Main Key Biogeochemical Processes in China Marginal Seas . . 642 6.2.1 River Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643 6.2.2 Coastal Anthropogenic Activities . . . . . . . . . . . . . . . . . . . . 646 6.2.3 Biological Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647 6.2.4 Ecological Disasters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648 6.3 Prospects for Biogeochemistry in China Marginal Seas . . . . . . . 649 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 651

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657

1 Introduction

Abstract: In this chapter the basic status of China marginal seas, including the Bohai Sea, the Yellow Sea, the East China Sea, and the South China Sea, and the research progress into marine biogeochemistry in China are described. China marginal seas are typically an ideal area to study marine biogeochemical processes and the progress is significant, especially in biogenic elements such as carbon, nitrogen, phosphorous, and silica.

1.1 Basic Status of China Marginal Seas China marginal seas, which lie between 15°∼42° N and 105°∼134° E including the East China Marginal Sea (Fig. 1.1, Naimie et al., 2001) and the South China Marginal Sea (Fig. 1.2, Kuo et al., 2000) in the western Pacific Ocean, form one of the largest marginal seas in the world. The East China Marginal Sea includes the Bohai Sea, the Yellow Sea, the East China Sea, while the South China Marginal Sea is the South China Sea, extending from temperate, subtropical to tropical zones. China marginal seas form one of the most productive parts of the world’s oceans and have a total area of 4.73×106 km2 , with a continental coastline of 1.8×104 km. The coastal ocean adjacent to three of the largest rivers in the world, the Huanghe River (Yellow River), the Changjiang River (Yangtze River), and the Zhujiang River (Pearl River), is quite active. Recent studies provided evidence that estuaries in China act as a source of nutrients and we can trace species to the ecosystem of the adjacent shelf region. For instance, the total nitrogen input from the Changjiang River was approximately 7.8×109 kg in 1997, which is a threefold increase over the level of 1968. In the northern area (the Bohai Sea and the Yellow Sea), the seasonal variations of sea surface temperature (SST) are large and vary from 0 to 28 ◦ C. In the East China Sea, the SST averages about 21 ◦ C and ranges from 7 to 28 ◦ C. In the southern area (north of the South China Sea), the

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Fig. 1.2. Map of the South China Marginal Sea. Isobaths are in meters (Kuo et al., 2000) (With permission from Elsevier’s Copyright Clearance Center)

1.1 Basic Status of China Marginal Seas

3

SST is high and its seasonal variation is small, varying from 21 to 29 ◦ C. Figs. 1.3 and 1.4 (Chen, 2008) display the SST and sea surface salinity (SSS) of three regions in winter and summer. In winter, the salinity in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea is 26‰∼31.5‰, 31‰∼31.5‰, 19‰∼34.7‰, and 29‰∼34.5‰, respectively. In summer, the salinity in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea is 28.5‰∼30.5‰, 29‰∼32‰, 25‰∼33.5‰, and 30.5‰∼34‰, respectively. Seawater temperatures are much lower in winter (Fig. 1.4a) whereas salinity values are generally higher (Fig. 1.4b). Cooling and stronger wind-induced mixing, supplemented by coastal upwelling, generally causes the concentrations to markedly exceed those in summer (Figs. 1.4c∼e). In general, a higher temperature (Fig. 1.3a) or salinity (Fig. 1.3b) corresponds to lower nutrient concentrations (Figs. 1.3c∼e). On the other hand, a lower temperature or salinity corresponds to higher nutrient concentrations. The monthly wind pattern is displayed in Fig. 1.5 (Lee and Chao, 2003). In winter (Fig. 1.5a), a northwesterly wind was observed in the Bohai Sea and the Yellow Sea. The wind was then roughly parallel to the coast. It became stronger and changed to a northerly wind in the East China Sea and the Taiwan Strait. It again changed to a northeasterly wind in the northern South China Sea. In spring, the wind became weaker and southerly in the Bohai Sea and the Yellow Sea. The wind then became easterly in the East China Sea toward coastal areas (Fig. 1.5b). In summer, the wind almost reversed in comparison to the winter season: a southeasterly wind in the Bohai Sea and the northern Yellow Sea, and a southwesterly strong wind in the South China Sea (circle in Fig. 1.5c). The wind in late fall gradually became similar to that in winter (Fig. 1.5d), and a strong northeasterly wind was observed on the Taiwan bank and in the northern South China Sea. In general, the wind is stronger during fall-winter (Figs. 1.5a and d) than during spring-summer (Figs. 1.5b and c) in the China seas. And the Taiwan Strait seems to have strong winds during the fall-winter season. 1.1.1 The Bohai Sea The Bohai Sea is one of the main China marginal seas, which has its own particular state and hydrography characteristics. This includes the topography, chemical environment, hydrography, sediment pattern, riverine input, and biological characteristics. 1.1.1.1 Topography The Bohai Sea is a semi-enclosed continental shelf sea of the NW Pacific Ocean in northern China, located between 37°07 N to 41° N and 117°35 E to 121°10 E, with a surface area of 77×109 m2 , an average depth of 18.7 m and a coastline of nearly 3,800 km. The size of the Bohai Sea is about 500 km from north to south and 300 km from east to west. The Bohai Sea is a

4

1 Introduction

Fig. 1.3. Distributions of (a) temperature (◦ C), (b) salinity (‰), (c) nitrate concentration (μmol/L), (d) phosphate concentration (μmol/L), and (e) silicate concentration (μmol/L) at surface water in the East China Marginal Sea in August (Chen, 2008) (With permission from Springer)


5

Fig. 1.4. Distributions of (a) temperature (◦ C), (b) salinity (‰), (c) nitrate concentration (μmol/L), (d) phosphate concentration (μmol/L), and (e) silicate concentration (μmol/L) at surface water in the East China Marginal Sea in February (Chen, 2008) (With permission from Springer)

6

1 Introduction N 40 (a)

(c)

(b) 1 dyne/cm

2

1 dyne/cm

Korea

35 30

(d)

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Korea Shikoku Kyushu

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Korea

Shikoku Kyushu

Shikoku Kyushu

Yangtze River

Yangtze River

Yangtze River

Yangtze River

China

China

China

China

25 120

125

130 E

120

125

130

E

120

125

130

E

120

125

130 E

Fig. 1.5. Monthly climatological wind-stress fields in: (a) October, (b) December, (c) April, and (d) August (Lee and Chao, 2003) (With permission from Elsevier’s Copyright Clearance Center)

shallow coastal area with a depth of less than 30 m in 95% of the area. The water depth of the Bohai Sea is shallow at around 10∼20 m in the coastal area, including Liaodong Bay (Li-B, north of 40° N), Bohai Bay (Bo-B, west of 118.6° E), Laizhou Bay (La-B, south of 37.6° N), central Bohai (Cn-B), and the Bohai Strait (Bo-S). The area with a depth of more than 30 m is mainly located in the northern section of the Bohai Strait. The maximal water depth of 70 m is found in the northern part of the Bohai Strait, the connection with the Yellow Sea. The topography of the Bohai Sea is given in Fig. 1.6 (S¨ undermanna and Feng, 2004). N 41

40

39 Bohai Sea Bohai Strait

38

37 Depth 70

118 60

119 50

121

120 40

30

20

122 10

E 0 (m)

Fig. 1.6. Bathymetry of the Bohai Sea (S¨ undermanna and Feng, 2004) (With permission from Elsevier’s Copyright Clearance Center)


7

The Bohai Sea is surrounded by land to the north, west and south and is connected only to the Yellow Sea through the narrow Bohai Strait, approximately 90 km wide. Rivers empty into the Bohai Sea including the Huanghe River, Haihe River, Liaohe River, and Luanhe River. The Bohai Strait is situated in the region between the Liaodong Peninsula and the Shandong Peninsula. At its narrowest part, the Bohai Strait is about 105 km wide. It is separated into several channels by the Miaodao Islands. The north channel is the main channel which has a maximum depth of 86 m. The seabed of the north channel consists of gravel and exposed bedrocks. The southern channels are shallow, with maximum water depths between 20 and 30 m. A wide terrace, with a gradient of only 1/2,000, extends from the north coast of the Shandong Peninsula to the 25 m isobath. The central part of the Bohai Strait is deeper than 40 m, and is characterized by a relatively flat bottom. 1.1.1.2 Hydrographical and Chemical Environment The water temperature of the Bohai Sea is vertically homogeneous from November to March. A thermocline is gradually formed from April to May in the deep area of the Bohai Sea. The thermo-stratification intensifies and becomes greater than 1 ◦ C/m from June to August. Then it decreases rapidly in September. The average annual precipitation in the Bohai Sea is about 500∼600 mm (Feng et al., 1999). In spring, the temperatures of surface waters and sea bottom waters range from 11.6 to 19.8 ◦ C and 10.5 to 19.7 ◦ C, respectively, with the average values being 16.1 ◦ C and 15.1 ◦ C. In summer, the temperatures of surface waters and sea bottom waters range from 23.4 to 28.6 ◦ C and 16.8 to 27.7 ◦ C, respectively, with the average values being 26.1 ◦ C and 22.9 ◦ C. In autumn, the temperatures of surface waters and sea bottom waters range from 17.2 to 22.6 ◦ C and 18.1 to 22.8 ◦ C, respectively, with the average values being 21.2 ◦ C and 21.3 ◦ C. Overall, the temperature is higher nearshore than offshore, but there are some areas of exception. Conductivity-temperature-depth (CTD) data in the Bohai Sea show three low-temperature centers in summer in the middle and lower layers: the central Bohai Straits, Liaodong Bay Mouth, and Bohai Bay Mouth. In addition, there is an evenly high-temperature center in the middle Bohai Sea. In winter the isotherms extend northwestward in the Bohai Sea with a cold tongue-shaped mass off Qinhuangdao towards the southeast, presenting a saddle-like isotherm pattern in the central Bohai Sea. The salinity of the Bohai Sea has obvious temporal and spatial variations. In spring, the salinity in surface and sea bottom waters ranges from 27.75‰ to 31.9‰ and 29.83‰ to 32.46‰, respectively, with the average values of 31.49‰ and 31.40‰. In summer, the salinity in surface and sea bottom waters ranges from 15.39‰ to 31.45‰ and 27.96‰ to 31.47‰, with the averages of 29.73‰ and 30.74‰, respectively. In autumn, the salinity in surface and sea

8

1 Introduction

bottom waters ranges from 23.08‰ to 31.68‰ and 23.9‰ to 31.65‰, with the averages of 29.75‰ and 29.82‰, respectively. In addition, the salinity is higher in the Bohai Bay and lower in the Bohai Straits (Fig. 1.7) (Bao et al., 2004). N 41

N 41

(a)

(b)

40

40

39

39

38

38

37

37 118

119

120

121

122

E

118

119

120

121

122

E

Fig. 1.7. Salinity and temperature of sea bottom waters in (a) winter and (b) summer in the Bohai Sea. Solid and dotted lines denote salinity (‰) and temperature (◦ C), respectively (Bao et al., 2004) (With permission from Bao XW)

Long-term variations of temperature and salinity in the Bohai Sea are displayed in Fig. 1.8 (Lin et al., 2001). The annual mean SSS and SST of the Bohai Sea both show ascending trends during 1960∼1997. The linear trends were 0.074‰ per year for SSS and 0.011 ◦ C per year for SST respectively. The long-term variations of these annual means both showed climate-jump years or inflection years. The dissolved oxygen (DO) and pH are important chemical parameters of seawaters. The DO also varies with the change in seasons. In spring, the DO and pH range from 7.2 to 11.2 mg/L and 7.97 to 8.33, respectively, with averages of 9.59 mg/L and 8.13. In summer, the DO and pH range from 5.4 to 8.1 mg/L and 7.99 to 8.25, respectively, with averages of 6.6 mg/L and 8.14. In autumn, the DO and pH range from 5.2 to 8.3 mg/L and 8.00 to 8.60, respectively, with averages of 7.1 mg/L and 8.31. The silicate (SiO3 -Si), phosphate (PO4 -P), and inorganic nitrogen (DIN) are the nutrients of planktons. In spring, SiO3 -Si and PO4 -P concentrations in surface waters are 6.08∼38.7 and 0∼1.96 μmol/L, with averages of 14.1 and 0.42 μmol/L, respectively. In summer, SiO3 -Si and PO4 -P concentrations in surface waters are 5.16∼47.63 and 0.11∼0.69 μmol/L, with averages of 20.08

1.1 Basic Status of China Marginal Seas Sea surface temperature ( )

Sea surface salinity ( )

34 (a) 32 30 28 26 24 1960

1970

1980 Year

1990

1997

14

9

(b)

13 12 11 10 1960

1970

1980 Year

1990 1997

Fig. 1.8. Long-term variation of annual mean (a) SSS and (b) SST of the Bohai Sea. The solid lines are from the averages of the respective annual means of the Bohai Sea and the dashed lines are their linear regressions (Lin et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

and 0.27 μmol/L, respectively. In autumn, SiO3 -Si and PO4 -P concentrations in surface waters are 7.35∼47.61 and 0.07∼1.34 μmol/L, with averages of 19.16 and 0.4 μmol/L, respectively (Cheng et al., 2004). DIN mainly includes nitrite (NO2 -N), nitrate (NO3 -N), and ammonium (NH4 -N). In spring, NO3 -N, NO2 -N, and NH4 -N concentrations in surface waters range from 0.85 to 22.23 μmol/L, 0.03 to 5.4 μmol/L, and 0 to 5.24 μmol/L, with average values of 8.65, 0.68, and 1.16 μmol/L, respectively. In summer, NO3 -N, NO2 -N, and NH4 -N concentrations in surface waters are 0.32∼5.78, 0∼8.21, and 0∼4.84 μmol/L, with averages of 3.86, 0.95, and 1.02 μmol/L, respectively. In autumn, NO3 -N, NO2 -N, and NH4 -N concentrations in surface waters are 2.72∼10.71, 0.08∼3.22, and 0∼4.34 μmol/L, with averages of 4.89, 0.93, and 0.84 μmol/L respectively (Cheng et al., 2004). 1.1.1.3 Tides In the Bohai Sea, the tidal movement is one of the prominent hydrodynamic processes. The tidal dynamics is characterized by a syntonic tide system deduced by the introduction of the tidal wave system of the northern Yellow Sea, which propagates through the Bohai Strait into the Bohai Sea. The largest part of the Bohai Sea exhibits a mixed semidiurnal tide. The tides are predominately semi-diurnal showing a tidal range of about 4 m with surface tidal currents reaching maximum values of 0.7 and 2 m/s during neap and spring tides, respectively. The M2 tide is the principal tidal constituent in the Bohai Sea. There are two amphidromic points, one close to the coast near Qinhuangdao and the other off the Huanghe River delta. The K1 tide has an amphidromic point at the southern part of the Bohai Sea. The maximum amplitude of the (M2+K1) tide is about 2 m. The maximum velocity of (M2+K1) tidal current is about 1 m/s. Fig. 1.9 (Jiang et al., 2000) shows

10

1 Introduction

the M2 tidal ellipses in the surface layer. There is a (left) rotating tidal flow entering the Bohai Sea from the Yellow Sea and propagating into the three inner bays almost as an alternating current (Feng et al., 1999). N 41

Tidal ellipses of M2 Layer: 1, 0~3 m d t =6 min barotropic of HAMSOM

40

Qinhuangdao Changxingdao Tanggu

39

38 25 cm/s

R Y.

. Longkou

50 cm/s 100 cm/s

37 118

119

120

121

122

E

Fig. 1.9. Tidal ellipses of the M2 in the surface layer (Jiang et al., 2000) (With permission from Elsevier’s Copyright Clearance Center)

In summer and autumn, typhoon surges usually occur in the southeast and south Chinese coastal zones, but there are some typhoon surges in the Bohai Sea. In winter there are strong wind surges in Bohai Bay and Laizhou Bay (Feng et al., 1999). In addition, two counter-clockwise semidiurnal (M2, S2) amphidromic systems are formed (Sun and Xi, 1988). In the Bohai Sea the maximum surface current velocity is 1∼1.5 m/s. In Laizhou Bay the flow is weaker and the maximum current velocity is less than 1 m/s. The currents are stronger in the northern part of the Bohai Strait where the maximum current velocity can reach 2.5 m/s. Wind and abundance structure also contribute to the residual currents. The general circulation in the Bohai Sea consists of an inflow through the northern section of the Bohai Strait and an outflow through the southern part. The vertical stratification occurs only in summer and can easily be eroded by strong wind events. 1.1.1.4 Circulation In the Bohai Sea, five water masses are recognized. They are the Bohai Sea and the Yellow Sea mixed water, the North Yellow Sea bottom cold water, the Bohai Sea central water, the Bohai Sea coastal water and the continental diluted water. There is a strong seasonal signal in temperature with a smaller


11

amplitude in the cold water at the bottom. Salinity exhibits a clear seasonality only for the Bohai Sea and the Yellow Sea mixed water. In winter, the distribution of water mass at the sea surface and on the bottom layer is basically identical (Fig. 1.10, S¨ undermanna and Feng, 2004). Gradual modification occurs from November to April at the sea surface and from December to March on the bottom layer. Distribution of the water mass at the sea surface and on the bottom layer in spring and autumn is transitional and patchy. The distribution of temperature and salinity and the circulation pattern in the Bohai Sea are displayed in Fig. 1.11 (S¨ undermanna and Feng, 2004). The “Yellow Sea Warm Current Extension”, like a jet, enters the Bohai Sea through the Bohai Strait, and moves westward along the central part until it meets the coast and there splits into two branches. One branch moves towards Liaodong Bay, forming a clockwise gyre, due to the merging of a current off the Huanghe River delta in a northeast direction and a current from the northeast in western Liaodong Bay, while the other current moves towards Bohai Bay, forming a counterclockwise gyre. 1.1.1.5 Wind Wind waves are a dominant feature in the Bohai Sea. They are important to the suspended particle matter (SPM) dynamics. Because there is a strong monsoon signal in the meteorological data, especially in the eastern part of the Bohai Sea, the wave direction changes accordingly. In winter, southerly waves prevail and in summer northerly waves dominate. The significant wave height is 0.3∼0.7 m in the near shore area of the Bohai Sea. In the Bohai Strait and the central part of the Bohai Sea it can reach 1.1 m (Qin et al., 1985). The East Asia Monsoon dominates the meteorology of this region. In winter, under the influence of Asian High Pressure and Aleutian Low Pressure, strong winds with a mean wind speed of 6∼7 m/s frequently blow over the Bohai Sea from the north. In summer, southerly winds with a mean speed smaller than 4∼6 m/s blow. The air temperature over the Bohai Sea reaches its lowest value of −4 to ∼0 ◦ C in January and highest value of 25 ◦ C in July. 1.1.1.6 Distribution Pattern of Sediments The sediments in the Bohai Sea are generally fine (Fig. 1.12, Jiang et al., 2004). They consist of soft clay mud (sediment with sizes smaller than 0.01 mm amounts to a portion of more than 70%), fine silt mud (sediment with sizes smaller than 0.01 mm amounts to a portion of 50%∼70%), coarse silt, and fine sand. In Liaodong Bay the coarse silt (0.1∼0.05 mm) and fine sand (0.25∼0.1 mm) dominate in the sediment. In Laizhou Bay the sediment consists of silt deposits, whereas in the central basin fine sand spreads widely. At the Laotieshan waterway (entrance to the Bohai Sea), the tidal flow is very strong, the area is often eroded and therefore the sediment particles are coarse there. The constituents of SPM in the Bohai Sea are mainly inorganic

12

1 Introduction

Fig. 1.10. The Bohai Sea water mass at the sea surface and on the bottom layer. (a) Winter, surface; (b) Winter, bottom; (c) Summer, bottom. B: Bohai Sea central water; BS: Bohai Sea coastal water; BY: Bohai Sea and Yellow Sea mixed water; F: continental diluted water; YC: North Yellow Sea bottom cold water (S¨ undermanna and Feng, 2004) (With permission from Elsevier’s Copyright Clearance Center)


13

N 40

35

30

25

20 105

115

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135 E

Fig. 1.11. Schematic diagram of the circulation in the Bohai Sea, Yellow Sea, and East China Sea. (a) Winter; (b) Summer (S¨ undermanna and Feng, 2004) (With permission from Elsevier’s Copyright Clearance Center)

Yingkou N 40

Tianjin

Qinhuangdao

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38

Hu an gh eR ive r

Dalian

Penglai

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Fig. 1.12. Sediment type distribution of the Bohai Sea. 1, Gravel; 2, Medium sand; 3, Fine sand; 4, Coarse silt; 5, Fine silty mud; 6, Silty clay mud; 7, Clay mud; 8, Shell; 9, Nodule (Jiang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

14

1 Introduction

particles from the land. Most of the particles are smaller than 0.01 mm in size (Qin et al., 1985). The sources of SPM are the adjacent rivers and resuspension. Atmospheric dust transported by the wind is a further source of SPM in the Bohai Sea (Qin and Li, 1982). The seasonal and regional variation of SPM concentration is evident, which is closely related to the flow and the sediment concentration of the rivers. 1.1.1.7 Riverine Discharge There are several rivers (e.g., Huanghe River, Haihe River, Liaohe River, and Luanhe River) which flux into the Bohai Sea with a total annual water discharge of 68.5×109 m3 /yr and annual suspended matter input of 1.1×109 t/yr. The Huanghe River is famous for its high concentration of sediments in the water. The average run-off is 42×109 m3 /yr and the sediment transport per year is 1.0×109 t, accounting for more than 50% of the fresh water and 90% of the sediment input into the Bohai Sea through rivers. But there is strong seasonal variation in both sediment and water discharge from the Huanghe River, whereas those from the Luanhe and Haihe Rivers are about 2.67×107 and 6×106 t/yr, respectively (Qin et al., 1985). The river discharges concentrate in July to October, the flood season in China (Li GX et al., 2004; Qin et al., 1985). After entering the Bohai Sea, the Huanghe River sourced suspended particle matter (SPM) is carried onto the western bank of Liaodong Bay, while most of the sediments settle down in estuary regions. Deposition flux decreases gradually with the distance from land, which has been demonstrated in the mineral distribution in surface sediments of the Bohai Sea (Qin et al., 1985). Sediments from the Luanhe River are transferred into Liaodong Bay along the western coast of the Bohai Sea, while those of the Liaohe River move along the eastern coast into the southern Bohai Sea. Most of the water and sediment discharge occur in the flood season (July to September). Although historical observation of nutrients in this region started in the late 1950s, including several national and international cooperative programs in the 1980s, most of these research activities were focused on distribution and variation of nutrients in the water column. Benthic metabolism and nutrient regeneration processes at the sediment-water interface are still poorly known in the study of the region. Decadal-average annual freshwater flux into the Bohai Sea was shown in Fig. 1.13 (Lin et al., 2001). Most of the rivers have been dammed for irrigation and municipal water supply; for example, over the last two decades for many days in the year the Huanghe River discharged no water into the Bohai Sea, because of the constant withdrawal of water upstream along the river. Moreover, sedimentation affects the estuaries, the coastal zone and the sea enormously. This effect is specifically important at the Huanghe River delta. Here each year the coastline extends towards the sea 150∼420 m and,

1000 800 600 400 200 0 200 400 600 800 1000

15

34

Sea surface salinity ( )

Fresh water flux ( 104 m3)

1.1 Basic Status of China Marginal Seas Sea surface salinity 32 30 28 26 Fresh water flux 24 1960 1970 1980 Year

1990

1997

Fig. 1.13. Annual variations of the (partial) net fresh water flux into the Bohai Sea and annual mean SSS of the Bohai Sea. The dashed lines are their respective linear regression representations (Lin et al., 2001) (With permission from Elsevier’s Copyright Clearance Center)

on average, 23 km2 of land is created. It should be pointed out that in recent years, owing to the dry weather on the middle and low reaches of the Huanghe River and the retaining and drawing of water from the Huanghe River, the runoff of the river decreased dramatically. For example, in 1997 the lower river was dry for as many as 226 days (Feng et al., 1999). 1.1.1.8 Biological Characteristics Long-term variations of SST and SSS may affect the ecosystems in the Bohai Sea. Limited observations show that the population of phytoplankton and zooplankton, benthos community biomass, fish community biomass, the index of species diversity, and the recruitment of penaeid shrimp all showed an outof-phase change with the variations in SSS and SST of the Bohai Sea (Table 1.1). (1) Chlorophyll a and primary production The concentration of chlorophyll a is used as the most convenient index of phytoplankton biomass. In 1982/1983, the annual cycle of chlorophyll a concentration in the Bohai Sea was of a double peak type, and two blooms occurred in spring and autumn. The chlorophyll a concentration ranged from 0.60 to 1.95 mg/m3 . An exception was in the Bohai Bay where the chlorophyll a concentration was low in March and increased after April until the maximum in August. The spring bloom spread out from Laizhou Bay and south of the central Bohai Sea in March into Liaodong Bay, and into the central Bohai Sea in April. Finally it spread into Bohai Bay in August. In May, high chlorophyll a concentration existed in Laizhou Bay and south of Bohai Bay, while low chlorophyll a concentrations were found in Liaodong Bay and north of the central Bohai Sea. High chlorophyll a concentrations were found along the coast of Bohai Bay and west of the Laizhou Bay, while the chlorophyll

16

1 Introduction Table 1.1. Biological population change in the Bohai Sea∗

Observation period Phytoplankton biomass (×104 cells/m3 ) Zooplankton biomass (mg/m3 ) Zooplankton density (ind./m3 ) Coscinodiscaceae (×104 cells/m3 ) Chaetoceros (×104 cells/m3 ) Mean catch per haul of benthos (kg/haul) Mean density per haul of benthos (ind./haul) Fish community biomass (t) Index of species diversity (Shannon-Weiner index, H ) Recruitment of penaeid shrimp (106 number of individuals) ∗

1959 188 205 8 129

1982∼1983 222 160 800∗∗ 20 90 22.91 2,683

1992∼1993 99 70 100 9 8 13.99 1,428

82,074 3.6092

72,120 2.5296

361

72

The data originate from Cheng and Guo (1998), Deng et al. (1999), Jin and Tang (1998),

Meng (1998) and Wang and Kang (1998).

∗∗

1985 data

a concentration was extremely low in the central Bohai Sea in September. In the year 1984/1985, high chlorophyll a concentration appeared in summer and autumn. The chlorophyll a concentration ranged from 0.5 to 4.0 mg/m3 . The highest chlorophyll a concentration occurred in spring and summer. In autumn, the chlorophyll a concentration was higher in east of the Laizhou Bay than in the other areas. The chlorophyll a concentration in the Bohai Strait was low during the whole year, except in autumn. The annual average net primary production was high in Laizhou Bay and the central Bohai Sea (>200 mg C/(m2 ·d)), while it was lowest in Bohai Bay (420 mg C/(m2 ·d)), while the low primary production was located in the southwest of the observed area (160 km3 and suspended materials of 1.1×109 t annually into the Bohai and Yellow Sea systems. The Yalujiang River is a major river directly running from the east coast of Liaodong Island into the Yellow Sea. It has a catchment area of 61,889 km2 and stretches for 790 km, with a water discharge of 25×109 m3 /yr, suspended materials of 113×104 t/yr, a rainfall of 1,050 mm and a sediment delivery of 5∼10 mg/(L·yr) (Qin et al., 1989). The Huanghe River-derived deltaic muddy sediments (9%∼15% of sediment discharge) are transported southward and accumulate in the central Yellow Sea. Echo-character mapping and sediment coring have revealed that the mud unit is up to 6 m thick and thins out southeastward. A thick clinoform mud deposit occurs at the eastern tip of the Shandong Peninsula, referred to as the Shandong mud wedge. It was mostly deposited early in the Holocene (prior to 9.0 cal. kyr) before the Huanghe River mouth shifted to the Bohai Sea, which is the reason why the Shandong mud wedge is not connected to the present Huanghe River delta. Large birdfoot-like sand bodies occur north of the Changjiang River mouth off the Jiangsu coast, which were formed prior to the southward shift of the river mouth by tidal-current winnowing. On the other hand, the sand bodies were deposited during the regressive phase of the sea level and were formed by tidal currents and longshore transportation of sands from the abandoned Huanghe River and modern Changjiang River deltas. Further north, along the Jiangsu coast, muddy sand, sandy mud, and mud were deposited by the old Huanghe River delta prior to 1855. These sediments have been partly winnowed by waves and tidal currents and acted as a source of the mud patch southwest of Jeju Island. The Changjiang River-derived sediments are mostly confined to the south and seasonally transported offshore by a plume event. The east of the Changjiang River mouth is covered with sandy sediments, either relict sands of the continental shelf or an active offshore tidal sand sheet. The Korean rivers discharge relatively small amounts of sediments into the eastern Yellow Sea. The fine-grained sediments, largely derived from the Geum River (discharge, 5.6×106 t/yr), are transported southward, forming a distinct mud belt, the Heuksan mud belt, in a water depth of 20∼70 m. Although it is known that fine-grained sediments largely originate from the Geum River, along with other small-scale rivers and coastal erosion, an integrated study of the origin and depositional processes of this mud belt is warranted. Besides, billions of tons of terrestrial materials are discharged annually through rivers (including the Huanghe, Aprock, Han, Keum, and others), tens of million of tons of mineral dusts (otherwise known as “yellow sand”) are deposited annually into surface seawaters from the atmosphere. It has been suggested that the atmospheric dust flux at the Yellow Sea may be comparable to the river input. The sea receives more than 60% of precipitation in the

34

1 Introduction

period from June to September, under the influence of the northeast Asian monsoon. 1.1.2.8 Biological Characteristics (1) Chlorophyll a and primary production The concentration of chlorophyll a is used as the most convenient index of phytoplankton biomass. The average value is 0.56 mg/m3 . It varies in different seasons and areas. The highest value is found in spring, with 0.66 mg/m3 . In spring, the concentration of chlorophyll a in surface waters ranges from 0.09 to 2.16 mg/m3 . The highest value is found in the Changjiang River Estuary. In summer, the concentration of chlorophyll a in surface waters ranges from 0.06 to 3.39 mg/m3 . The highest value is found in the Changjiang River Estuary and the coast of Jiangsu. In autumn, the concentration of chlorophyll a in surface waters ranges from 0.12 to 2.8 mg/m3 . The highest value is found in the southeast of the south Yellow Sea and the northeast of the north Yellow Sea. In winter, the concentration of chlorophyll a in surface waters ranges from 0.2 to 2.25 mg/m3 . The highest value is found in the north of the north Yellow Sea and the east of Jiaozhou Bay (Jin et al., 2005). The amount of primary production (PP) in different areas and seasons varies. The highest value is found in spring and summer and the lowest value is found in winter. In spring, PP ranges from 8.14 to 177.87 mg C/(m2 ·h). High primary production was observed in the middle of the north Yellow Sea. In summer, PP ranges from 10.85 to 261.65 mg C/(m2 ·h). High primary production was observed near Diandao Island. In autumn, PP ranges from 5.75 to 139.24 mg C/(m2 ·h). High primary production was observed in the north of the north Yellow Sea. In winter, PP ranges from 4.05 to 247.57 mg C/(m2 ·h). High primary production was observed near Changshan Islands. (2) Phytoplankton The phytoplankton of the Yellow Sea includes 63 species—diatoms 55 species, dinoflagellates 8 species, accounting for 87.3% and 12.7% of total, respectively. In spring, the dominant species include Coscinodiscus radiatus, Coscinodiscus asteromphalus, Melosira sulcata, Navicula, Pleurosigma, Ceratium, Ceratium tripos, Peridinium, etc. The phytoplankton biomass is 2.24×104 cells/m3 . Diatom biomass is 1.57×104 cells/m3 , accounting for 70.1% of total phytoplankton. In summer, the dominant species include Chaetoceros compressus, Chaetoceros lorenzianus, Cheatoceros subsecundus, Chaetoceros paradox, Cheatoceros debilis, Chaetoceros didymus, Chaetoceros curvisetus, Chaetocers teres, Chaetoceros affinis. The phytoplankton biomass is 20.17×104 cells/m3 . Diatom biomass is 18.73×104 cells/m3 , accounting for 90% of total phytoplankton. In autumn, the dominant species include Chaetoceros compressus, Chaetoceros castracanei, Chaetoceros lorenzianus, Chaetoceros affinis, Chaetoceros densus, Eucampia zoodiacus, Biddulphia regia, Thalassionema nitzschioides. The phytoplankton biomass is 7.92×104 cells/m3 . Diatom biomass


35

is 7.3×104 cells/m3 , accounting for 92.2% of total phytoplankton. In winter, the dominant species include Coscinodiscus asteromphalus, Coscinodiscus radiatus, Melosira sulcata, Thalassionema nitzschioides, Chaetoceros curvisetus, Chaetoceros castracanei, Rhizosolenia styliformis, Rhizosolenia styliformis, Chaetoceros densus. The phytoplankton biomass is 18.25×104 cells/m3 . Diatom biomass is 17.97×104 cells/m3 , accounting for 98.4% of total phytoplankton (Jin et al., 2005). (3) Zooplankton According to investigation during the four seasons of 1998 to 2000, zooplankton includes 67 species. In autumn, the zooplankton structure is most complicated, including 28 species. In spring, summer, and winter, Carapace species change little. It is found that Calanus sinicus, Labidocera bipinnata, Oithona similis, Themisto gracilipes, Euphausia pacifica, Sagitta crassa, etc. always exist in each season. In addition, the dominant species vary in different seasons (Table 1.2). In spring, Copepoda is the dominant species. In other seasons, the dominant species include Calanus sinicus Brodsky and Sagitta crassa Tokioka (Jin et al., 2005). Table 1.2. The dominant species of the Yellow Sea in each season (unit: ind./m3 ) (Jin et al., 2005) (With permission from Jin XS) No. 1 2 3 4 5 6 7 8 9

Species Spring Summer Autumn Winter Percentage (%) Calanus sinicus Brodsky 58.20 48.03 40.84 428 52.01 Sagitta crassa Tokioka 0.51 22.68 18.53 40.29 22.46 Sagitta enflata Grassi ND ND 4.48 1.80 1.72 Penilia avirostris Dana ND 21.15 ND ND 5.79 Oithona similis Claus 11.43 ND ND ND 3.13 Labidocera euchaeta ND 1.11 8.36 0.32 2.68 Giesbrecht Euphausia pacifica Hansen 1.26 1.67 3.31 1.88 2.22 Euchaeta concinna Dana ND ND 6.50 6.01 3.43 Parathemisto gaudichaudi ND 1.89 1.03 4.95 2.17 Guerin

Dominant species: 1>5>7>2 in spring, 1>2>4>9>7>6 in summer, 1>2>6>8>3>7>9 in autumn, 1>2>8>9>3>6 in winnter

The zooplankton biomasses vary in different areas and seasons. The spatial zooplankton biomass decreases from south to north; the average biomasses in the north, middle area and south are 28.8, 31.7, and 74.3 mg/m3 , respectively. The highest value is found in summer, followed by spring, and the lowest value is in winter. In spring, the average biomasses in the north, middle area, and south are 12.9, 14.3, and 134.7 mg/m3 , respectively. In summer, the average biomasses in the north, middle area, and south are 50.1, 45.4, and 27.1 mg/m3 , respectively. In autumn, the average biomasses in the north, middle area, and

36

1 Introduction

south are summer (206 species)>autumn (181 species)>winter (178 species). The benthos biomass also varies with seasons: spring (50.75 g/m2 )>autumn (35.35 g/m2 )>summer (32.64 g/m2 )>winter (29.94 g/m2 ). The sequence of abundance is: spring (359 ind./m2 )>winter (290 ind./m2 )>summer (186 ind./m2 )>autumn (165 ind./m2 ) (Table 1.3). Table 1.3. The benthos biomasses of the Yellow Sea in different seasons (Jin et al., 2005) (With permission from Jin XS) Season Polychaeta Mollusk Carapace Echinodermata Others Spring 11.07 8.54 3.81 18.99 8.60 Summer 10.49 3.19 6.50 6.41 6.07 Autumn 10.75 1.78 2.56 11.56 8.69 Winter 7.89 3.69 1.56 9.48 7.33 Average 10.05 4.30 3.61 11.61 7.67 Abundance Spring 202 57 70 24 5 (ind./m2 ) Summer 108 16 55 5 2 Autumn 130 7 13 11 3 Winter 131 72 37 11 38 Average 143 38 44 13 12 Biomass (g/m2 )

Total 50.75 32.64 35.35 29.94 37.17 359 186 165 290 250

(5) Fish During 1985∼2002, species diversity showed a decreasing tendency before 1992 and an increasing trend thereafter for the whole fish assemblage. In 1985, there were seven dominant species, accounting for 72.5% of the total catch, of which the snailfish Liparis tanakae dominated (34.2%). In 1986, the three dominant species accounted for 78.9% of the total catch by weight, of which the proportion of anchovy increased and amounted to 61.5%. In 1987∼1998, there were


37

usually 1∼3 dominant species, accounting for 81.4%∼94.1% of the total catch, of which anchovy dominated (58.8%∼94.1%) and small yellow croaker Pseudosciaena polyactis usually followed (9.3%∼22.2%). In addition, the cardinal fish Apogonichthys lineatus accounted for 11.1% of the total catch in 1991. In 1999∼2002 there were 4∼8 dominant species, accounting for 78.7%∼86.6% of the total catch. Compared with 1986∼1998, the proportion of anchovy decreased, amounting to 17.4%∼45.4%. Small yellow croaker also accounted for a relatively high proportion, ranging from 11.4% to 31.7%. The percentages of snailfish and anglerfish Lophius litulon amounted to 12.4%∼31.4% of the total catch. Some planktotrophic species, such as blackgill croaker Collichthys niveatus, silver pomfret Pampus argenteus, half-fin anchovy Setipinna taty, chub mackerel Scomber japonicus, long-tailed anchovy Coilia mystus, and gizzard shad Konosirus punctatus, accounted for relatively small proportions, varying at 2.6%∼9.6% (Table 1.4). According to investigation in 1997 to 2000, it was found that 124 species of fish exist in the Yellow Sea. The most dominant species is Engraulis japonicus (65.5 kg/h), followed by Trachurus japonicus (8.9 kg/h), then Pseudosciaena polyactis (3.2 kg/h), Trichiurus lepturus (1.5 kg/h), Pampus argenteus (1.4 kg/h). In spring, the number of species is 90, with an average abundance of 27.5 kg/h. The dominant species is Engraulis japonicus (15.7 kg/h), accounting for 57.0%. The second dominant species is Ammodytes personatus (3.7 kg/h), accounting for 13.4%. In summer, the number of species is 70, with an average abundance of 238.3 kg/h. The dominant species is Engraulis japonicus (225.8 kg/h), accounting for 94.8%. In autumn, the number of species is 74, with an average abundance of 93.4 kg/h. The dominant species is Trachurus japonicus (35.7 kg/h), accounting for 38.2%. The second dominant species is Scomber japonicus (21 kg/h), accounting for 22.5%. In winter, the number of species is 85, with an average abundance of 43.4 kg/h. The dominant species is Engraulis japonicus (114 kg/h), accounting for 26.2%. The second dominant species is Pseudosciaena polyactis (6.9 kg/h), accounting for 15.9%. (6) Trophic levels Mean TL of dominant species in the Yellow Sea declined from 3.61 in 1985∼1986 to 3.40 in 2000∼2001, or by about 0.14 per decade (Fig. 1.26). Dominant species in the Yellow Sea included four kinds of planktivorous species, four kinds of omnivorous species, one kind of benthivorous species, and no piscivorous species in 1985∼1986. In 2000∼2001 there was one kind of planktivorous, omnivorous, and benthivorous species, respectively, also no piscivorous species. In the Yellow Sea, the percentage of planktivorous species also increased from 50.70% in 1985∼1986 to 60.08% in 2000∼2001 and the percentage of omnivorous species decreased from 14.50% in 1985∼1986 to 7.41% in 2000∼2001 (Fig. 1.27).

Species Anchovy Small yellow croaker Snailfish Plaice Anglerfish Olive flounder Pacific cod Cardinal fish Blackgill croaker Silver pomfret Half-fin anchovy Chub mackerel Hairtail Long-tailed anchovy Smallhead hairtail Gizzard shad Total

1985 Engraulis japonicus 4.1 Pseudosciaena polyactis 6.5 Liparis tanakae 34.2 Cleisthenes herz ensteini 10.7 Lophius litulon 8.6 Paralichthys oliv aceus 5.0 Gadus macrocephalus 3.5 Apogonichthys lineatus Collichthys niveatus Pampus argenteus Setipinna taty Scomber japonicus Trichiurus lepturus Coilia mystus Eupleur ogrammus muticus Konosirus punctatus 72.5 4.7 9.0 4.4 7.0

12.1

5.8

7.5

4.7

3.4 9.6

6.0 2.6 78.9 93.0 88.0 93.7 81.4 94.1 86.7 87.6 82.9 78.7 80.7 82.2 86.6

11.1

1986 1987 1988 1990 1991 1992 1993 1994 1998 1999 2000 2001 2002 61.5 80.2 88.0 71.5 60.7 94.1 86.7 78.3 58.8 28.1 45.4 17.4 22.4 22.2 9.6 9.3 19.4 17.8 17.4 31.7 11.4 10.2 12.7 8.4 9.9 14.1 7.3 4.0 11.0 17.3

Table 1.4. Dominant fish species determined by Hill’s N2 and their percentages in the total catch by weight in the southern Yellow Sea over the period 1985∼2002 (Xu and Jin, 2005) (With permission from Elsevier’s Copyright Clearance Center)

38 1 Introduction


39

4.20 4.00

TL

3.80 3.60 3.40 3.20 3.00 1959~1960 1982~1983 1985~1986 1992~1993 1998~1999 2000~2001 Year Mean TL of dominant species accounting for 80% of total biomass Mean TL of dominant species accounting for 85% of total biomass Mean TL of dominant species accounting for 90% of total biomass

Fig. 1.26. Decadal variations of mean TL of the Yellow Sea and the Bohai Sea (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center) 20.2% 9.3% 14.5%

19.63% 50.7%

5.25% 7.41%

60.08%

7.63% 5.3% (a)

Planktivorous Benthivorous Omnivorous Piscivorous Invertebrates Others

(b)

Fig. 1.27. The feeding habit composition of dominant species accounting for 80% of total biomass. (a) 1985∼1986 in the Yellow Sea; (b) 2000∼2001 in the Yellow Sea (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

1.1.3 The East China Sea The East China Sea (ECS) is known for its broad continental shelf, rich natural resources, and tremendous river runoff from China, which is surrounded by several countries and has a complicated status, such as topography, chemical environment, hydrography, sediment pattern, riverine input, and biological characteristics. 1.1.3.1 Topography The East China Sea is one of the largest marginal seas of the continental shelf areas of the world, surrounded by Korea, Japan, Chinese mainland, and Taiwan region. Its size is about 1,300 km from north to south and 740 km from east to west (Fig. 1.28, Valle-Levinson and Matsuno, 2003). It lies over the broad shelf of the Northwest Pacific Ocean, with a surface area of 770×109 m2 and average depth of 370 m. More than 70% of the surface area of the East China Sea is covered by continental shelf. The ECS is bordered by the Okinawa Trough, with a maximum depth exceeding 2,000 m. Together with the

40

1 Introduction

Okinawa Trough, the Ryukyu island arc, and the Ryukyu trench, it makes up another Trench-Arc-Basin system in the West Pacific Ocean, which links northwards with the Japanese island arc and connects southwards with the Taiwan-Philippine island arc. The northwest shelf connects with the Yellow Sea and between them is the Changjiang River delta, with a water depth of about 50∼100 m.

N 35

30

25 120

125

130 E

Fig. 1.28. The East China Sea showing the water depth (Valle-Levinson and Matsuno, 2003) (With permission from Springer)

1.1.3.2 Hydrographical and Chemical Environment The SST of ECS ranges from 27 to 29 ◦ C in summer, but some cold eddies were found off northeast Taiwan and to the south of the mouth of the Changjiang River. SST anomalies at the center of these eddies were about 2∼5 ◦ C. The strongest front usually occurs in May each year and its temperature gradient is about 5∼6 ◦ C over a cross-shelf distance of 30 nautical miles. The Yellow Sea mixed with cold water also provides a contrast from China Coastal waters shoreward of the 50 m isobath, the cross-shore temperature gradient is about 6∼8 ◦ C over 30 nautical miles.


41

The seasonal variation of the evaporation minus precipitation (E–P) had a secondary effect on the SSS distribution (Fig. 1.29).

N 40

N 40

35

35

30

30

Salinity (

25

)

0~20 20~26 26~30 30~32 32~34 >34

25 120

125 130 E Jul. 28, 1998

120

N 40

N 40

35

35

30

30

25

125 130 E Aug. 1, 1998

25 120

125

130 E

120

Aug. 8, 1998

125

130 E

Aug. 11, 1998

N 40

N 40

35

35

30

30

25

25 120

125

130 E

Aug. 16, 1998

120

125

130 E

Sept. 7, 1998

Fig. 1.29. Temporal variations in horizontal distributions of salinity from July 28 to September 7, 1998 during the 1998 flood (Chen et al., 2006) (With permission from Elsevier’s Copyright Clearance Center)

42

1 Introduction

The dissolved oxygen (DO) and pH are important chemical parameters of seawaters. The average DO and pH are 6.7 mg/L and 8.22, respectively (Jin et al., 2005). The silicate (SiO3 -Si), phosphate (PO4 -P), and inorganic nitrogen (DIN) are the nutrients of planktons. In winter, the SiO3 -Si, PO4 -P, and DIN concentrations in surface waters are 3∼40, 0.1∼1.6, and 1∼25 μmol/L, respectively. In summer, the SiO3 -Si, PO4 -P, and DIN concentrations in surface waters are 5∼50, 0.2∼1.0, and 1∼50 μmol/L, respectively. 1.1.3.3 Tides The East China Sea continental margin is characterized by high tidal currents, frequent and intense storm events, and extremely high sediment supply relative to other margins (e.g., Atlantic Coast, Gulf of Mexico). Semidiurnal tides are dominant in the East China Sea, with the spring tidal range between ∼1.25 and 1.80 m. The tidal wave propagates from the northwestern Pacific Ocean through the straits between Taiwan and Kyushu as a progressive northeast-southwest-oriented wave front. Tides can also locally affect surface sediment distribution. Tides are particularly dominant in the East China Sea of the mouth of the Changjiang River and off southwestern Korea. In both instances tidal currents are sufficiently strong (1∼2 knot(s)) to erode and transport sediments. The intrusion of the Taiwan Warm Water (TWW) between the Changjiang Coastal Water (CJCW) and the Jiangsu Coastal Water (JCW) results in a seaward transition from turbid (CJCW) to clear (TWW) to turbid (JCW) waters east of the Changjiang River Estuary. As a result, Changjiang River sediments tend to be transported to the south by the CJCW, while the sediments to the east of the TWW are predominantly Huanghe River sediments transported south by the JCW. 1.1.3.4 Currents The East China Sea circulation is dominated by the northward flow of two loops of the Kuroshio Current (KC): the Taiwan Warm Water (TWW) in the west and the Yellow Sea Warm Water (YSWW) in the east. Both water masses are characterized by high salinity and warm water temperatures. In contrast, a southward flow in close to sea bottom water occurs from the flow of the Changjiang (CJCW) and Jiangsu Coastal Waters (JCW) along the Chinese coast, the Korean Coastal Waters (KCW) in the east, and the Yellow Sea Cold Waters (YSCW) in the north (Fig. 1.30, Lee and Chao, 2003). The coastal currents in particular appear as seasonally cold and brackish water masses. To the east of ECS, the Kuroshio Current, a strong western boundary current, flowing along the Pacific Margin of northeastern Asia, borders the shelf slope of the East China Sea. When passing through the ECS, it has

1.1 Basic Status of China Marginal Seas N 40

N

(a)

40

(b)

50 cm/s

50 cm/s

Korea

Korea

35

35

Shikoku Kyushu

Shikoku Kyushu Yangtze River

Yangtze River

30

30 China

China

25

25 120 N 40

43

125

130 E

120

N

(c)

40

125

130 E

(d)

50 cm/s

50 cm/s

Korea

Korea

35

35 Shikoku Kyushu

Shikoku Kyushu

Yangtze River

Yangtze River

30

30 China

China

25

25 120

125

130 E

120

125

130 E

Fig. 1.30. Model-derived depth-averaged surface currents in: (a) October, (b) December, (c) April, and (d) August. The depth average extends from sea surface to 830 m or bottom, whichever is shallower (Lee and Chao, 2003) (With permission from Elsevier’s Copyright Clearance Center)

a great effect on the ocean environment of the continental shelf area of the ECS. In addition, the Changjiang River dilution and the Kuroshio upwelling are two principal sources of materials of the ECS. 1.1.3.5 Wind Summer winds tend to be gentle and come from the south, except during major storms (typhoons). Resuspension by wind-generated waves is infrequent.

44

1 Introduction

Moreover, except in very shallow waters, the water column is stratified and therefore resuspended sediment tends to remain confined to nearbottom waters below the pycnocline. Resuspension of sediment is largely the result of storm activity and the subsequent transport of this sediment by regional currents. Graber et al. (1989) proposed that the present-day distribution of sediments in the East China Sea is largely related to storm-generated surface waves during the winter season. Furthermore, these authors suggest that erosion of sand along the Chinese coast is largely the product of surface wave action. 1.1.3.6 Distribution Pattern of Sediments Surface sediments in the ECS inner shelf are characterized by gray mud and generally contain 40%∼45% clay, 40%∼60% silt, and less than 5% sand (Fig. 1.31, Liu et al., 2006). Surface sediments are composed generally of well sorted, positively skewed silts, clayey-silts, and silty clays with median diameter (MD, φ) from 6.0 to 8.0. Clay mineral assemblages are dominated by illite (generally more than 70%), with small amounts of chlorite (12%), kaolinite (9%), and smectite (3%). The high illite concentrations and low smectite concentrations (1.5 mg/m3 ); there then exist some high value (>1.5 mg/m3 ) areas such as the one where the weak current speeds and Kureahio splits, others are in upwelling current areas such as the Taiwan northeast offshore, middle and north of Fujian offshore and Zhejiang offshore (Lu et al., 1997). (2) Phytoplankton During spring in the shelf water, diatoms were the most abundant taxa followed by chlorophytes, cryptophytes, chrysophytes, and prymnesiophytes. In summer, diatom became sharply less abundant towards the mid and offshelf (Fig. 1.34). It was found that the total abundance obvious seasonal variations from 1997 to 2000 in the East China Sea (23◦ 30 ∼33◦ N and 118◦ 30 ∼128◦ E). It peaked in autumn with a mean value of 211.91×104 cells/m3 , followed by summer (50.40×104 cells/m3 ) and winter (11.34×104 cells/m3 ). The lowest abundance occurred in spring (2.01×104 cells/m3 ). Over the 4 seasons excluding winter, the mean density of phytoplankton was over 100×104 cells/m3 . For the horizontal distribution, the abundance in summer and spring was

1.1 Basic Status of China Marginal Seas 4

47

(a)

3

2

Spring Summer

Chl a ( m g/L)

1

0 2

(b)

1

0 (c)

1

0

0

1

3 2 Total Chl a ( m g/L)

4

5

1.5 1.0 0.5 0.0

12 1 2 3 4 5 6 7 8 9 10 Month

55 50 45 40 35 30 25 20 15 10 5 0

100 80 60 40 20

Contribution (%)

Biomass (g C/m 2)

2.0

Total Chl a (mg/m 2)

Fig. 1.32. Relationship between total chlorophyll a and algal group specific chlorophyll a. (a) Diatoms; (b) Eukaryotic flagellates; (c) Procaryotic alage (Furuya et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

0

Fig. 1.33. Seasonal variation in average integrated carbon biomass of Synechococcus (◦), average integrated total chlorophyll a (), and Synechococcus contribution to total phytoplankton biomass (♦) from Dec. 1997 to Oct. 1998. Vertical bars represent standard error (Chiang et al., 2002) (With permission from Elsevier’s Copyright Clearance Center)

48

1 Introduction

Total Chl a (mg/m2)

100

(a)

(b)

80 60 40 20

Relative abundance (%)

0 100

Prochlorophytes

80

Cyanobacteria

60

Chlorophytes

40

Prymnesiophytes

Chrysophytes

Cryptophytes 20 0

Dinoflagellates Diatoms 1 2 3 4 5 6 7 8 9 10 11 Station PN

1 2 3 4 5 6 7 8 9 10 11 12 Station PN

Fig. 1.34. Horizontal variations in integrated total chlorophyll a (chlorophyll a plus divinyl chlorophyll a) amount with contribution of various algal classes in (a) spring and (b) summer. Stations PN-2 and PN-12 were not sampled during the spring cruise (Furuya et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)

higher in nearshore than that in offshore areas of the East China Sea. In autumn and winter, the abundance in the offshore of the north was higher than that in the nearshore, while in the south this was reversed. Over the 4 seasons, phytoplankton abundance showed a significant correlation with the water temperature. Eleven dominant species were observed in 4 investigated seasons, in which Chaetoceros lorenzianus and Thalassiosira subtilis dominated in winter, Chaetoceros lorenzianus and Noctiluca scientillans in spring, Chaetoceros pseudocurvisetus and Rhizosolenia alataf gracillima in summer, and only Chaetoceros socialis in autumn (Luo et al., 2007). (3) Zooplankton The seasonal mean value of total biomass of zooplankton was 65.32 mg/m3 from 1997 to 2000. Among them the total biomass in autumn (86.18 mg/m3 ) was greater than those in summer (69.18 mg/m3 ), spring (55.67 mg/m3 ), and winter (50.33 mg/m3 ); average value of diet biomass of zooplankton was 40.9 mg/m3 , which was about 60% of total biomass of zooplankton. Among them, total diet biomass of zooplankton in autumn (56.84 mg/m3 ) was greater than those in summer (46.10 mg/m3 ), winter (30.82 mg/m3 ), and spring (29.82 mg/m3 ). Total biomass distribution trends toward to similar with diet biomass (Xu et al., 2004).


49

Based on the results of autumn 2000 and spring 2001, the total 210 species with occurrence >2 were identified. There were 112 common species to occur in both of seasons. The most common species in autumn were Sagitta nagae, Calanus sinicus, Sagitta enflata, and Euckaeta concinna, and in spring were Calanus sinicus, Sagitta crassa, Sagitta nagae, Paracalanus parvus, and Euphausia pacififa. In autumn, the average abundance was 6,018 ind./m2 and the most abundant species were Calanus sinicus, Sagitta nagae, and Paracalanus aculeatus. In spring, the average abundance was 9,271 ind./m2 , and the most abundant species was Calanus sinicus, which accounted for 87% of total abundance (Zuo et al., 2005). The main dominant species of zooplankton were Calanus sinicus and Macrura larva in summer, and Calanus sinicus, Sagitta enflata, and Doliolum denticulatum in autumn. In summer 2003, the distribution of zooplankton was not uniform, and the higher biomass presented in the outer sea. The main dominant species were Calanus sinicus, Macrura larva, Labidocera euchaeta, and Sagitta larva. In summer 2004, the higher zooplankton biomass occurred in the north of the ECS. The main dominant species were Calanus sinicus, Macrura larva, and Themisto gracilipes. In summer 2005, the higher zooplankton biomass occurred in the north of the ECS. The main dominant species were Calanus sinicus and Invertebrate egg. In autumn 2003, the distribution of zooplankton also was not uniform, with the higher biomass in the north of the ECS. The main dominant species were Doliolum denticulatum, Calanus sinicus, Sagitta enflata, and Nannocalanus minor. In autumn 2004, the higher zooplankton biomass occurred in the north of the ECS. The main dominant species were Calanus sinicus, Sagitta enflata, Euchaeta marina, and Salpa fusiformis. In autumn 2005, the higher zooplankton biomass appeared in the Yellow Sea-East China Sea interface. The main dominant species were Calanus sinicus and Sagitta enflata (Li HY et al., 2007). (4) Benthos species The benthos species in the East China Sea includes 855 species. The dominant species mainly include Polychaeta, Mollusk, and Carapace. The average biomass and abundance are 21.36 g/m2 and 283 ind./m2 , respectively. The biomass and abundance vary with the change of seasons. The highest biomass is found in spring (41.27 g/m2 ) and the lowest value is found in winter (10.23 g/m2 ), while the highest abundance is found in autumn (461 ind./m2 ) and the lowest abundance is found in winter (146 ind./m2 ) (Jin et al., 2005). Based on the data obtained in the autumn of 2000 and spring of 2001, the secondary production of macrobenthos from the East China Sea was calculated with Brey’s empirical formula. The results showed that the mean abundance in autumn (2000) was 87 ind./m2 , lower than that in spring (2001), 138 ind./m2 ; the mean biomass in ash-free dry weight (AFDW) in autumn was 1.40 g/m2 , higher than that (1.25 g/m2 ) in spring; the mean annual secondary production in the study area was 1.62 g/(m2 ·yr) (AFDW), much lower than those in the Bohai Bay and the south Yellow Sea. The production of mac-

50

1 Introduction

robenthos should be affected by water temperature and water depth (Li XZ et al., 2005). (5) Fish On the basis of the data from bottom trawl surveys in 4 seasons of 2000 in the East China Sea and the Yellow Sea, studies of the seasonal characteristics of fish quantity distribution are conducted by the method of factor analysis. By R type analysis, the results show that there are 4 species, Raja kenojei, Coilia mystus, Muraenesox cinereus, and Collichthys niveatus, which are correlated closely in spring; 5 species, Engraulis japonicas, Pseudosciaena polyatis, Lophius litulon, Chelidonichthys kumu, and Saurida elongate, in summer; 5 species, Trichiurus haumela, Pampus nozawae, Erisphex potti, Pneumatophorus japonicas, and Pseudosciaena polyatis, in autumn; and 3 species, Trichiurus haumela, Champsodon capensis, and Acropoma japonicum, in winter. By the approbation of historic survey information, the main relationship among the 5 species in spring is the relation between prey and predators. By the promoted analysis, it was discovered that the water areas where correlative species of each season assemble together correspond to those areas where these species take physiology cycling migrations in each season. Based on Q type analysis, the synthesis of the predominant species in each season and their mainly distributed water area can be obtained. The results show that Trichiurus haumela and Pseudosciaena polyatis are the absolutely predominant species in the East China Sea and the Yellow Sea. Except for these two syntheses of the predominant species, the others are all small types of fish which have low value and quick growth characteristics (Liu and Cheng, 2008). 1.1.4 The South China Sea The South China Sea is the second largest marginal sea in the world, which resembles other subtropical oceans in a warm and oligotrophic condition, and has complex coastlines and bottom topography as well as a chemical environment, hydrography, sediment pattern, riverine input, and biological characteristics. At the same time, the South China Sea is the most sensitive to environmental changes. 1.1.4.1 Topography The South China Sea (SCS) is the largest semi-enclosed marginal sea off East Asia in the western tropical Pacific Ocean, spreading from the equator to 20◦ N and spanning zonally about 15◦ in longitude with a broad shelf and a deep basin and an area of about 3.5×106 km2 (Fig. 1.35, Liu QY et al., 2008). The size is about 3,330 km from north to the south and 1,670 km from east to west. SCS lies between the South China coast and the maritime continent, located between the Asian land mass to the north and west, the Philippine Islands to the east, Borneo to the southeast, and Indonesia to


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N 25

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120 E

Fig. 1.35. Bathymetry of the South China Sea. The 300-m isobath is indicated by the heavy solid line (Liu QY et al., 2008) (With permission from Springer)

the south. It includes the shallow Gulf of Thailand and connections to the East China Sea (through the Taiwan Strait), the Pacific Ocean (through the Luzon Strait), the Sulu Sea, the Java Sea (through the Gasper and Karimata Straits), and the Indian Ocean (through the Strait of Malacca). All of these straits are shallow except the Luzon Strait, the maximum depth of which is 1,800 m. The South China Sea has complex topography including the broad shallows of the Sunda Shelf in the south/southwest, the continental shelf of the Asian landmass in the north, extending from the Gulf of Tonkin to the Taiwan Strait, a deep, elliptically shaped basin in the center, and numerous reef islands and underwater plateau scattered throughout. The SCS basin is the deepest in the center with a maximum depth exceeding 5,000 m. The shelf, which extends from the Gulf of Tonkin to the Taiwan Strait, is consistently nearly 70 m deep and averages 150 km in width, the central deep basin is 1,900 km along its major axis (northeast-southwest) and approximately 1,100 km along its minor axis, and extends to over 4,000 m depth. The Sunda Shelf is the submerged connection between southeastern Asia, Malaysia, Sumatra, Java, and Borneo, and reaches 100 m depth in the middle. The center of the Gulf of Thailand is about 70 m deep. The Luzon Strait is rather wide, but a series of small islands cross its width. Other important connections are to the Sulu Sea through the Mindoro Strait and Balabac Strait, to the East China Sea through the Taiwan Strait, to the Java Sea through the Karimata Strait and to the Andaman Sea through the Malacca Strait, which is one of the most important marine transportation routes in the world.

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In general, the SCS can be divided into the northern continental margin, the central oceanic basin, and the southern continental margin. The Zhujiang River Mouth Basin (ZRMB) is largely distributed on the continental shelf of the northern SCS. 1.1.4.2 Hydrographical and Chemical Environment Fig. 1.36 (Chern and Wang, 2003) depicts the horizontal distribution of temperature in the second layer, 25∼50 m, averaged over a year. The 25 ◦ C isotherm separates the SCS into two regions. The cold water, 25 ◦ C, occurs in the southern SCS, including the Gulf of Thailand, the Luzon Strait, and the region of the anticyclonic eddy to the southwest of Taiwan region.

27

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4 100 102 104 106 108 110 112 114 116 118 120 122 E

Fig. 1.36. Horizontal distribution of annual mean temperature (◦ C) of the 25∼50 m layer (Chern and Wang, 2003) (With permission from Springer)

Fig. 1.37 shows the seasonal variation in the vertical temperature distribution at four different sites shown in Fig. 1.36. The temperature variation in the SCS mainly occurs in the upper eight layers, 0∼400 m. The thermal structure to the west of Luzon, station C, and in the southern SCS, station F, shows distinct seasonal variations, cold in winter and warm in summer. The seasonal change in the temperature on the continental slope to the southwest of Taiwan, station D, has quite a different pattern. The Kuroshio intrusion across the Luzon Strait in October increases the upper layer temperature at station D. The water temperature in this slope area shows a major drop only when the cyclonic eddy to the west of Luzon extends into this area after February


53

and then maintains a slow warm-up during the spring and summer period. To the east of Vietnam, station E, the warming and cooling of the upper ocean has a much shorter period. The upwelling associated with the southwest wind in summer cools the upper ocean and then the water temperature increases slowly during the northeast wind period. The temperature at station E drops again during March and April, consistent with the westward spreading of cold eddy water along the continental slope south of China (from station D to station E). During May and June, the apparent temperature increase at stations C, E, F and the mild warming at station D indicate that this is a warming period for the upper ocean in the SCS. Sta. C 2 4 6

Levels

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Fig. 1.37. Seasonal variation of vertical temperature distribution at four sites in Fig. 1.36. Vertical coordinate is the layer number and horizontal coordinate is the Julian day in a year. Shaded area denotes land mask. Contours are in ◦ C. The study stations were C (17◦ 57.53 N, 118◦ 46.60 E), D (21◦ 23.62 N, 118◦ 15.80 E), E (15◦ 11.72 N, 111◦ 14.16 E), and F (6◦ 56.64 N, 111◦ 11.79 E) (Chern and Wang, 2003) (With permission from Springer)

The dissolved oxygen (DO) and pH are important chemical parameters of seawaters. The average DO and pH values are 5.8 mg/L and 8.12 in northern waters, and 5.19 mg/L and 8.1 in middle and southern waters. The average DO is 7.78 mg/L (Jin et al., 2005). The silicate (SiO3 -Si), phosphate (PO4 -P), and inorganic nitrogen (DIN) are the nutrients of planktons. In winter, the SiO3 -Si, PO4 -P, and DIN concentrations in surface waters are Br>K. Vertical fluxes of major elements have a close relationship to SST. Na and other 8 elements decreased exponentially with SST, but K and Ba had no relationship. The order of sensitivities to SST was Fe>Br>Sr>Ca>Na>Cl>Mg>I>Al. The vertical flux changes of Fe and Br can react to SST variations (Song, 1997a). The vertical transferring flux of most of the measured rare elements in the Yongshu Reef Lagoon was higher than that in the Zhubi Reef Lagoon. The vertical transferring forms of rare elements were mainly in the carbonate form, but Ta, As, Th mainly in the ion-exchange form, Ag in iron-manganese oxides form, and Sb in the organic matter+sulphide form. None of the 18 rare elements was transferred mainly in the form of detritus silicate to the sea floor. This proved that the rare elements originating from the earth’s crust were redistributed in sinking particulates after they were brought into

1.2 Progress in Marine Biogeochemical Process Research in China

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the ocean. The relation between the fluxes and surface seawater temperature (SST) was also studied; the sensitivity of rare elements to SST was in the order of Rb>V>As>Ti>U>Zn>Sb>Hf>Ag>Cs (Song and Li, 1998). In the East China Sea, the content of PIC in the surface sediments is higher than that of POC. Although there is a considerable high vertical flux of particulate carbon (PC) in the bottom seawater at St. 410, the maintenance of PC in the surface sediments is almost impossible due to the low depositing rate, relatively high energy dynamic condition, and oxidative environment of the sea-floor in that area. The PC may be transferred from the seawater to the surface sediments during the blooming season (i.e., the summer) of marine organisms, but part of them will be transferred back to the seawater again. Possibly, surface sediments dominate in the dis-carbon process during the winter. Therefore, the budget of PC on the sea-floor sediment-seawater interface is in dynamic equilibrium, making the contribution of the carbon’s catch very limited in the middle continental shelf sand area. The cold eddy mud area and the Zhejiang coastal mud area are the PC sinks since the two areas have relatively high vertical fluxes of PC and the conditions for PC’s maintenance on the surface sediments are satisfied. The storage of carbon in the Zhejiang coastal mud area is much higher than that of the cold eddy mud because the former area has a much higher depositing rate. The PC sinks in the seawater controlled by the marine organism productuin and other carbon origins do not always coincide with the PC sinks of the sea-floor surface sediments in the East China Sea. 1.2.1.4 Biogeochemical Behavior near the Sediment-Water Interface The nutrients (NO3 -N, NO2 -N, NH4 -N, PO4 -P, SiO3 -Si) in sediment interstitial waters and their diffusion fluxed across the sediment-water interface are studied in the ecological system of the lagoon and outer-reef in the sea waters of Nansha Islands, the South China Sea. The main results show that: (1) The nutrient concentrations are high in interstitial waters of the seawaters and ΣN/P of interstitial waters is higher in the outer-reef than in the lagoon. The distribution of nutrients is different in the interstitial waters of the outer-reef. (2) The activity of sediment in the sea region of the Nansha Islands is higher than that in the East China Sea. A large number of nutrients diffuse from sediment to overlying water. H4 SiO4 is the main diffusion flux component of + nutrients in the outer-reef and NO− 3 or NH4 is the main diffusion flux component of nutrients in lagoons. The diffusion fluxes of H4 SiO4 and NO− 3 are higher in the outer-reef than in the lagoon. The nutrient diffusion characteristic is decided by the nutrient feature and sediment environment. (3) The high temperature in the sea waters of the Nansha Islands is the main reason for a large number of nutrients produced in sediment to be released to the overlying water. The apparent active energy decreases under high temperature and the sediment activity increases, so a large number of nutrients are produced

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in sediment and diffused from sediment to the overlying water (Song et al., 1997a; Song, 1999). HS− concentrations in the lagoon interstitial waters were much higher than those in the off-reef sediment interstitial waters. Concentrations in interstitial waters of lagoons without human activities and in the near-reef sediment interstitial waters were much higher than those in the off-reef sediment interstitial waters. Concentrations tended to increase with the depth of the off-reef sediment interstitial waters. HS− and S2− diffused from sediment to overlying seawater, but it was the opposite for SO2− 4 . The average diffusion flux in the lagoon was higher than that off-reef. The average diffusion flux in the lagoons was 61.34 μmol/(m2 ·d) for HS− and –0.41 μmol/(m2 ·d) for SO2− 4 . The average diffusion flux off-reef sediments was 14.96 μmol/(m2 ·d) for HS− and –0.35 2− μmol/(m2 ·d) for SO2− 4 . A –2 valence sulfur was controlled by the S+2e→S redox pair in sediment interstitial waters of the water region of the Nansha Islands. The Eh values calculated from the redox pair corresponded to the measured Eh values. Sulfur as a sub-stable form can exist in sediments and induce pyrite (FeS2 ) precipitation (Song and Li, 1996). A monograph “Chemistry of Sediment-Seawater Interface of China’s Seas”, which concerns the marine biogeochemical process, was published in 1997 (Song, 1997a). The monograph contains 7 chapters and the main contents are as follows: An outline of the Bohai Sea, the Yellow Sea, the East China Sea, the South China Sea and the feature of surface sediments in China’s seas are given in chapter 1. The contents and methods of studying the marine sedimentseawater interface are summarized in chapter 1 and chapter 2. The chemical features of overlying waters of marine sediments are described in chapter 3. The diffusion fluxes across the sediment-seawater interface of China’s seas are discussed using Fick’s Law. Two main conclusions should be paid much attention to: (1) In the location of volcanic activities, the diffusion fluxes of elements from sea bottom to water are very high, for instance in the Okinawa Trough the diffusion flux of Cl− from sediment to water is as high as 10.24 mmol/(m2 ·d). (2) In the coral reef ecosystem of the Nansha Islands, a large number of nutrients such as N, P, and Si are diffused to seawater, so it is apparent that the diffused nutrients are key factors for maintaining the high production of the coral reef ecosystem. The thermodynamic equilibria of elements near the sediment-water interface of China’s seas is elucidated in chapter 5. By analyzing Fe, Mn, and S systems in the sediment interstitial waters and focusing on the redox features of sediments, a series of new concepts and theories have been proposed by “Degree of Relative Equilibrium (DRE)” to quantitatively study the equilibrium degree of the redox pair in sediments and “Redox Degree (ROD)” to assess redox features of marine sediments. The redox pair of China’s Sea sediments are in quasi-equilibrium, and the redox interface is of 200 mV and ROD is 15. The reduction in lagoon deposits of the Nansha Islands is stronger than that in locations on the out-reefs, and the pure chemical standard “Grain Size La-


79

bel (GSL)” is also proposed to classify marine sediment types. All these new concepts, theories, and methods are the bases for studying chemical equilibria near the marine sediment-water interfaces. Early diagenesis models of elements near the sediment-water interface of China’s seas are discussed in chapter 6. The elements include P, Si, S, N, F, Cl, Br, and I. Vertical fluxes of biogenic elements, rare and rare-earth elements, and major elements are described in chapter 7. The elemental biogeochemical processes were systematically studied, which are a key problem in oceanographic research into global climate changes and the frontiers of oceanographic research in the world. 1.2.1.5 Function of Small Organisms in Marine Biogeochemical Cycling If three dissolved organic phosphorus (DOP) compounds available as nutrient sources for the experimental culture of three algae were studied, results indicated that these compounds could be utilized by algae, and that dissolved inorganic phosphorus (DIP) was first to be taken up when various forms of phosphorus (DIP and DOP) co-existed. Dicrateria zhanjiangensis’ uptake of sodium glycerophosphate was faster than that of D-ribose-5-phosphate. The increase in sodium glcerophosphate had little effect on the maximum uptake rate (Vmax ) of Chlorella sp., but increased the semi-saturation constant (Ks ) remarkably; the photosynthesis rates (PR) of Dicrateria zhanjiangensis and Chlorella sp. were rarely affected by using various forms of phosphorus in the culture experiments. The possible DOP pathways utilized by algae are discussed (Hong et al., 1995). Microzooplanktons including Foraminifera, pelagic Molluscs, Copepoda, and Copelata are major resident members of the coral reef lagoons in the Nansha Islands. Microzooplanktons are resident members with a mean density of 4,246 ind./m3 in these lagoons. They are an important source of food and nutrients for the coral reef lagoons. The carbon, nitrogen, phosphorus, hydrogen, and calcium contents and atomic ratios of these elements (C:N:P:H) of the major taxonomic groups in the microzooplankton were measured (Zhang and Li, 1997). The contents and composition of the nutrients elements of the carbon, nitrogen, phosphorus, silicium, and calcium of zooxanthellae from the coral reef lagoons in the Nansha Islands were reported firstly. The average contents of C, N, P, Si, and Ca in the cells are 522.1, 87.4, 3.68, 0.17, and 3.40 mg/g dry wt, respectively. The average atomic ratio of these biologically active elements C:N:P:Si:Ca is 366:53:1:0.051:0.71. It is remarkably different from the stoichiometric Redfield ratio. The enrichment factors of C, N, P, and Si in the cells relative to the ambient water are 2.24×104 , 3.45×106 , 4.96×105 , and 7.79×103 , respectively (Li PC et al., 1997). It was reported that the fact probably relates to the biological species community in the coral reef lagoon. With the ball-jar process, the in situ test for corals and algae in coral reef lagoons of the Nansha Islands was conducted in May 1990. It was found that

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the coral will finally reduce the oxygen, but the algae will increase the oxygen much more than the chlorophyll in the waters. The productivity of four algae (Turbinaria sp., Padina sp., Boodla sp., Caulerpa sp.) was estimated in different weather conditions: sunny, overcast, cloudy, to be 3.48, 0.35, and 0.86 mg/(g·h), respectively. In addition, if there are algae of 150 g/m2 of reef, and the average sunshine time is 10 h/d, the productivity will reach 1,300 mg/(m2 ·d) (Wu and Li, 1997). The four parts described above show that marine chemistry research in China at the end of the 20th century takes much interest in biogeochemical processes in such interfaces as the seawater-air, seawater-particulate, seawater-sediment, and seawater-organisms interfaces. The theoretical research is closely related to the application of resources and environments. Chinese marine chemists will pay much attention to new methodologies and new discoveries in marine chemistry research in China’s seas in the 21st century. 1.2.2 Progress in Biogeochemical Processes of Marine Carbon Cycles since 2000 in China The biogeochemical process of the marine carbon cycle is one of the key links controlling global change. With the development of the key international plans of JGOFS, GLOBEC, SOLAS, etc. in the past ten years, the carbon cycle study has made great progress. It may be said that the biogeochemical process of the marine carbon cycle has been understood more systematically than ever before. In particular, the marine biological pump process and the mechanism of CO2 absorbed in oceans have been quantitatively recognized and understood. This part focuses mainly on the progress of biogeochemical processes of marine carbon cycles after the year 2000 in China. It includes 3 parts: the CO2 fluxes and processes between atmosphere and seawater; carbon and its biogeochemical cycles; functions of sediment and soils around estuaries in marine cycles. 1.2.2.1 CO2 Fluxes and Processes Between Atmospheres and Seawaters The aquatic ecosystem, especially the ocean, is in total a huge CO2 reservoir. According to a recent estimation, human behavior contributes 5.5×109 t of CO2 to the atmosphere annually, of which about 2.0×109 t is absorbed by the ocean, accounting for 35% of the total discharge, and about 0.7×109 t is absorbed by the terrestrial ecological system, accounting for 13%. It is shown that the ocean and land hold about half of the CO2 from human activity, and the other half of the CO2 is emitted into the atmosphere. It is clear that the ocean could weaken the greenhouse effect from CO2 , and play an important role in regulating the levels of atmospheric CO2 and hence global climate. The study of biogeochemical processes of marine carbon cycling has become the


81

key issue in studying the marine carbon cycle and global climate change, and it also will be an important study in international oceanography in the 21st century. and HCO− Carbon in the oceans mainly exists in the forms of CO2− 3 3. The total dissolved CO2 (TCO2 ) in most seawaters is about 2 mmol/kg and about ten times that of the dissolved organic carbon (DOC), and much higher than the particulate organic carbon (POC). The ocean’s role in regulating the uptake capacity of CO2 and the carbon exchange between atmosphere and ocean depends on the mixed layer carbonate chemistry, the advection transfer of carbon dissolved in seawaters, the CO2 diffusion across the waterair interface, the various biological processes and settling of organic carbon from biological production, the dissolving and settling of carbonates around the sediment-seawater interface, etc. Many models have been established and developed in order to evaluate the CO2 sink in the oceans. The net ocean sink is estimated to be in a range from 1.2 to 2.4 Gt C/yr based on the box model and the general circulation model, which is generally accepted as 2.0 Gt C/yr. CO2 in the atmosphere is driven by biological pump into the ocean. In the marine ecosystem CO2 is then changed into OC due to biological carbonates of biological photosynthesis in the mixed layer, and is further transferred from the surface to the deep layer, which is the main process of marine carbon cycles (Fang et al., 2001). In order to gain a deeper insight into the global carbon cycle, the first thing is to study the variations of CO2 in the surface water and the differences of PCO2 (ΔPCO2 ) between the sea and the air. The changes in total dissolved CO2 (TCO2 ) in the surface water in the tropical Pacific (10◦ S, 20◦ N, 120◦ E, 90◦ W) during the El Ni˜ no and the La Ni˜ na events have been numerically simulated using a 3D global ocean carbon cycle model with biological pump. The results showed that the changes in the total dissolved CO2 and the partial pressure difference between the sea and the air (ΔPCO2 ) in the northwest Pacific (0∼20◦ N, 120∼150◦ E) and in the central and east equatorial Pacific no events, (10◦ S, 10◦ N, 150◦ E, 90◦ W) were noticeable. During the El Ni˜ the changes in TCO2 in the surface water increased in the northwest Pacific and decreased in the central and east equatorial Pacific; there were opposite changes in both regions during the La Ni˜ na events (Xing and Wang, 2001). A 3D global ocean carbon cycle model with the ocean biological pump was developed. In this model, the atmosphere is represented as a well-mixed box of CO2 , where CO2 from the surface water is exchanged. The carbon cycle model has been numerically integrated for 1,200 years and finally reached a quasiequilibrium state. Under the quasi-equilibrium state condition of the model, the computed TCO2 , alkalinity, the dissolved oxygen concentration in seawaters, the distribution of new production and the differences in PCO2 between the sea and the air are close to the observed results. CO2 absorbed by the sea is 42% and 7% with and without the ocean biological pump, respectively, which shows that there are significant effects due to the ocean biological pump on the capacity of the ocean to absorb CO2 in the air. A 3D ocean carbon

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cycle model and a simple terrestrial biosphere model were used to simulate the anthropogenic CO2 uptake by the ocean and terrestrial biosphere under the Intergovernmental Panel on Climate Change (IPCC) scenarios to predict the atmospheric TCO2 levels in future. It was estimated that the anthropogenic carbon emissions must be reduced in order to stabilize CO2 in the atmosphere at various PCO2 levels ranging from 350×10−6 to 750×10−6 . All the stabilization scenarios require a substantial future reduction in emissions (Jin and Shi, 2001). Song (2004) reported that the relation between partial pressure of CO2 water (PCO ) and temperature (T ) in the surface water was obtained from the simu2 water =6.62T +221.03. lated laboratory experiments, which showed the formula PCO 2 water The relative error between the estimated PCO2 and the measured values is lower than 4.5%. The air-sea flux seasonal distributions and strength of source/sink of CO2 in the East China Sea were obtained for the first time based on the data of surface seawater temperatures and partial pressure of the atmosphere. The seawater could take in CO2 from the atmosphere in the Bohai Sea, the Yellow Sea, and the East China Sea and the flux values are higher in winter than those in spring. In summer, the situation is reversed and CO2 is released into the atmosphere. In autumn, the seawaters can take in CO2 in the Bohai Sea and the northern Yellow Sea, but release CO2 into the atmosphere in the East China Sea and the southern Yellow Sea. The minimum and maximum of air-sea flux of adsorbed CO2 appear in autumn in the northern Yellow Sea (5.3 g C/(m2 ·yr)) and in winter in the Bohai Sea (106.0 g C/(m2 ·yr)), respectively, and the minimum and maximum of released CO2 appear in summer in the northern Yellow Sea (–1.9 g C/(m2 ·yr)) and the East China Sea (–18.8 g C/(m2 ·yr)), respectively. The annual mean fluxes from seawater to air are 36.8, 35.2, 21.0, and 3.5 g C/(m2 ·yr) in the Bohai Sea, the northern Yellow Sea, the southern Yellow Sea, and the East China Sea, respectively (the Yellow Sea flux is 23.7 g C/(m2 ·yr)), the East China Sea is the net sink of atmospheric CO2 in spring and winter, which can take in 7.69 and 13.56 million tons of carbon, respectively, and is the source of the release of CO2 into the air with 4.59 million tons of carbon. The Bohai Sea and the northern Yellow Sea are the sink of atmospheric CO2 and can take in 0.27 million tons of carbon. The southern Yellow Sea and the eastern China Sea are the source of CO2 , which releases into the air 3.24 million tons of carbon in autumn. As a result, the net carbon sink strength of the East China Sea is 3.24 million tons of carbon in autumn. The annual mean sink strength of atmospheric CO2 in the seas east of China is 13.69 million tons of carbon. In conclusion, in the past 4 years the studies into exchanges of carbon between air and water in Chinese marginal seas have made some progress, especially on exchanges of CO2 . Various ocean carbon cycle models from different points of view have been proposed and have been applied when studying the carbon cycles between air and water.


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1.2.2.2 Carbon and Its Biogeochemical Cycles in Seawaters The presence of CO2 in the marine environment is a crucial link for the global carbon cycle, and plays a predominant role in the exchange and flow of carbon between the atmosphere, hydrosphere, ecosphere, and lithosphere. The research into the transportation and sink of CO2 in the marine environment, which includes the adsorption and transportation ability of CO2 by the marine environment and the cyclic mechanism of CO2 in the marine environment, has been an important research content in international marine science nowadays. The marginal sea and continental slope, especially estuaries and bays impacted greatly by human activities, have played a significant role in the whole carbon cycle. In recent years, the research in these regions has made great progress. Now our national study focuses mainly on CO2 exchange between sea water and the atmosphere, colorful dissolved organic matter (CDOM), inorganic carbon in sediment in typical regions, such as Jiaozhou Bay, Zhujiang River Estuary, and Changjiang River Estuary (Song et al., 2008). The research in Jiaozhou Bay in June and July 2003 showed that the average dissolved inorganic carbon (DIC) of surface water is 2,066 μmol/L in June and 2,075 μmol/L in July. In the outside bay, the average DIC is 1,949 μmol/L for surface water and 2,147 μmol/L for sea bottom water. In June, the DIC values of inner Jiaozhou Bay is higher than that of the outside bay, but the result is reversed in July. The concentration is the highest in the northeast and decreases to a minimum toward the west. The total trend of vertical distributions is for a gradual increase from the surface to the bottom, which has some relation to the particulate N and P. The average CO2 seawateratmosphere flux in June in Jiaozhou Bay is 0.55 and 0.725 mol/(m2 ·yr) in July. In addition, Jiaozhou Bay is the source of atmospheric CO2 in both of the two months. The total CO2 flux from seawater into the atmosphere is 62 and 81 t in June and July, respectively (Li CH et al., 2004). As for the north part of the South Sea, it is the source of atmospheric CO2 and the average seawater-atmosphere flux in summer is 7 mmol CO2 /(m2 ·d) and 1∼3 mmol CO2 /(m2 ·d) in spring and autumn. The PCO2 of surface seawater is influenced greatly by the seawater temperature. CDOM is the substance with optical activity. It plays an important role in the marine cycle, which has distinctive optical characteristics. However, up to now, the biochemical structure and the biochemical substance of the complicated compound are not clear, and the possible constitutes may be amino acids, sugar, glucidamin, fatty acid, carotene, and phenol. In Jiaozhou Bay, the total fatty accounts for 86.90% of CDOM, total sugar is 5.82%, free amino acids is 7.22%, and glucidamin is 0.06%, but the percentages of the carotene and phenol are small, and several orders lower than those of the total fatty, amino acids, and total sugar. The CDOM in Jiaozhou Bay is mainly land-derived; the biochemical substances are further diffused and transported under the influence of river freshwater and inner bay circulation. From the research on CDOM and DOC in the Zhujiang River Estuary in 1999, it is

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revealed that the CDOM content in freshwater is the highest and in seawater is the lowest, which indicates that river water is the primary source of the CDOM in the Zhujiang River Estuary. However, the CDOM in this region is lower than that of the other estuaries in the world, and the CDOM does not show the conservative mixture behavior. This behavior cannot be explained by the removal effect (such as flocculation and photodegeneration), and it may be the result of the different constitute and optical characteristics of various water masses. In another way, DOC concentrations vary little with salinity change, and as a result the CDOM and DOC distributions in the Zhujiang River Estuary are different. The difference between CDOM and DOC shows that the contribution of CDOM to DOC is mobile, so it is infeasible to evaluate the DOC concentration in the Zhujiang River Estuary by remote sensing techniques. The research into DOC in the Zhujiang River Estuary shows that the adsorption coefficient at 355 nm in the Zhujiang River Estuary is lower than that of America and Europe due to the fact that the Zhujiang River Estuary is impacted greatly by human activities. The CDOM produced by land-derived human activities is apparently more than that of natural plant degradation. Additionally, the location of CDOM fluorophore varies with the source, and in this way it can decide the constitute characteristics and trace the sources of CDOM. It is especially important to study carbon cycles in the East China Sea, which is a marginal sea with its coastal regions affected significantly by human activities. It includes a typical continental shelf, continental slope, and semi-deep-sea regions. Studies into DOC and POC along a cross-shelf transect in the southern East China Sea showed that the DOC concentrations were higher (>85 μmol/L) in the inner shelf and slope waters but lower (ca. 65 μmol/L) around the shelf break, where the Kuroshio upwelling occurred. Such a distribution pattern showed a little temporal variation. The coastal water contained less colloidal organic carbon (COC) than the oligotrophic slope water, suggesting a lower production rate and/or a higher breakdown rate in the coastal water. The POC distribution showed a decreasing trend from the inner shelf to the slope with a local maximum at the shelf break, where POC was enriched due to the enhanced primary productivity induced by upwelling. The average POC content corresponded to about 1/10 of the average DOC content. There was a maximum POC in the mid-depth over the slope, which indicated the lateral transport of POC going offshore from the shelf. The net POC exports of DOC and POC from the shelf were estimated to be 414 and 106 Gmol C/yr, respectively (Chen, 1999). Heterotrophic bacterial biomass, production, and turnover rates were investigated in transect across the continental shelf of the southern East China Sea during spring and autumn. In the coastal and upwelling areas, bacterial biomass was 350∼200 mg C/m2 , production was 28∼329 mg C/m2 , and the averaged turnover rates were 0.09∼0.22 d−1 , which were at least 2-fold those in the Kuroshio waters. Production and turnover rates were positively correlated with primary production (90∼2,133 mg C/m2 ) and POC (1,415∼4,682 mg C/m2 ). Dissolved


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organic carbon from the non-phytoplankton and allochthonous sources might play a significant role in supporting bacterial carbon demand in the shelf area of the East China Sea. The southern East China Sea continental shelf region is characterized by relatively low organic carbon concentration with a fast sedimentation rate. The organic carbon concentration ranged from 0.3 to 0.6 wt% and the sedimentation rate from 0.2 to 0.7 cm/yr. In addition, the normal marine S/C ratios were observed. Up to 96% of pyrite-sulfur was reoxidized before its final burial. The sulfate reduction rate and the pyrite-sulfur burial rate increased linearly with the increase in the organic carbon burial rate, which indicated that the organic carbon deposition controlled the pyrite formation in the East China Sea continental shelf sediments. The organic carbon utilized by the sulfate reduction and its burial represented a significant but a relatively small fraction of the primary production in the studied East China Sea region. According to carbonate and the related parameters, the low-temperature, low-salinity water mass in the summertime northern East China Sea originates from the Yellow Sea Cold Water, which is formed farther northward. There is no apparent annual variation in the carbonate parameters in the Kuroshio east of the shelf break. The partial pressure of CO2 calculated from the pH, TA or TCO2 data in this study shows that the surface water in the shelf area is undersaturated with CO2 in spring and summer. Taking the above data, combined with the other data collected in different seasons into consideration, it is shown that the shelf area of the East China Sea is indeed a net sink for atmospheric CO2 , and that it absorbs as much as 0.013∼0.030 Gt C/yr (Wang et al., 2000b). The vertical fluxes and the molar ratios of carbon of the suspended particulate matter in the Yellow Sea were studied based on the analysis of the suspended particulate matter. It was shown that most of the particulate organic matter in the Yellow Sea water column comes from marine life rather than the continent, and that the vertical fluxes of POC in the Yellow Sea are much higher than those in other seas around the world. There was high primary production in this region (Wang et al., 2002). The average carbon biomass of Sybechococcus in the East China Sea ranged from 0.09 g C/m2 (early spring) to 0.90 g C/m2 (autumn). The upward flux of nitrate into the euphotic zone in the South China Sea was calculated by the coupled Ra-nitrate approach, and further converted into a new production of 4.4 mmol C/(m2 ·d) based on a Redfield ratio of 6.6 for C:N. The 234 Th-238 U disequilibrium and the measured ratio of POC to particulate 234 Th yield a POC export flux of 5.7 mmol C/(m2 ·d), and it is consistent with the new production calculated by nutrient budget. Based on the 234 Th-228 Ra disequilibrium, POC export flux was estimated to be 1.7 mmol C/(m2 ·d), significantly lower than the derived new production. The discrepancy can be caused by the uncertainty of the DOC transport or accumulation of the data obtained in different ways and over different seasons and durations. Donghu Lake is a typical shallow eutrophic lake along the Changjiang River’s middle reaches. The mean concentrations of DOC were (15.11±3.26), (15.19±4.24), (14.27±3.43), (13.31±3.30) mg/L in four stations during 1996∼

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1997, respectively. The DOC concentrations of the study area were very similar to those in other lakes along the Changjiang River’s middle reaches. The mean POC of the whole lake was 5.01 mg/L due to the large amount of organic detritus from both local origin and allochthonous origin. A significant linear relation was found between POC and chlorophyll a at all of the four stations, which presumably shows that phytoplankton and its exudates and metabolic products are the main contributors to the POC in the water column. DOC/POC ratio (mean value of 4.40) indicated that the organic detritus would be the most important component of the particulate organic matter. phytoplankton is also a factor dominating the particulate organic matter in Donghu Lake (Liu et al., 2000). Han (1998) studied the component of OC and carbon cycle in Daya Bay and the Zhujiang River Estuary. They classified OC into the dissolved OC (DOC), the particulate OC (POC), and the sedimentary OC (SOC). It is found that both the East China Sea and the Taiwan Strait are reservoirs of CO2 in winter, and the Taiwan Strait is a weaker source of CO2 in spring. Research into DOC and COC in the Zhujiang River Estuary shows that COC accounts for 3%∼32% of DOC, the maxima are lower than those of low salinity regions (3.5 mg C/(m2 ·yr)). When organic carbon flux increased to above 3.5 in the southern South China Sea during the Last Glacial Maximum and in the northern South China Sea during the early Holocene, a group of detritus feeders such as Bulimina aculeate and Uvigerina dominated over the others. However, the suspension feeders such as Cibicidoides wuellerstorfi and Chilostomella ovoidea gradually became more important than detritus feeders as soon as the organic carbon flux decreased to 2.5∼3.5 mg C/(m2 ·yr). During the LGM, the high organic carbon flux and the increased abundance and accumulation rates of B. aculeate and U. peregrina in the southern South China Sea were mainly caused by the enhanced NE winter monsoon-driven upwelling and the associated productivity, and partly by the increased input of terrigenous nutrients as a result of the lowered sea level. However, during the first part of the Holocene, around 10,000 a B.P., the remarkably increased abundance and accumulation rates of B. aculeate and U. peregrina especially in the northern South China Sea, together with the high organic flux, point to increasing productivity, probably driven by a maximum intensity in the SW summer monsoon (Jian et al., 1999). Carbon biogeochemical processes in the special environment have also been studied. For example, the hydrothermal vent communities are quite different from the typical deep-sea communities in many aspects, whose food chain structure is chemosynthetic bacteria, which feed on reduced inorganic chemicals like hydrogen sulfide in order to synthetize OC to provide energy for macroanimals by means of endosymbioses. The DOC and POC at 12 stations in the Yantai Sishili Bay in May, August, November of 1997 and March and May of 1998 were investigated. The DOC concentrations varied from 1.14 to 5.35 mg/L. The average values at all stations in each cruise varied from 1.52 to 2.12 mg/L. The POC concentrations varied from 0.049 to 1.411 mg/L. The average values of POC in each cruise varied from 0.159 to 0.631 mg/L. Horizontal distribution of DOC was influenced by several factors, such as terrestrial input, organism activity, temperature, aquiculture environment. The higher POC concentration occurred along the coast. The vertical distribution of DOC and POC changed obviously in spring and summer, but not in autumn or winter. The DOC concentration was the highest in summer and POC in spring; both DOC and POC were the lowest in winter. The seasonal variation of DOC was consistent with that of primary productivity. The seasonal variation of POC was consistent with that of Chl a. There was a significant seasonal variation trend of C/N ratio in the dissolved organic matter, but the C/N ratio of particulate organic matter had no significant trend (Zhao et al., 2001).


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Carbon biogeochemistry is a science that studies the correction and role of carbons that affect lives and their environments by exploring carbon’s transfer and recycling. Biogeochemical abundance, currents, coupling, and field are its four elementary ideas. The transfer and recycling of CO2 , CH4 , N2 O, etc. have been paid close attention to because of abnormal global climates. Li (2001) adopted a DNDC model to study the biogeochemical factors and processes associated with C and N cycling, and to simulate the interactions among global climate change, human activities, and terrestrial ecology. DNDC represents the denitrification and decomposition which are two main reactions that cause C and N transfer from soil to atmosphere. This model has been used to forecast fertility of the soil and the emission of greenhouse gases in some countries. There are some reports about biogeochemical cycling models of C and N abroad, but so far there is no such systematic or comprehensive model which can simulate the C cycle and N cycle in the atmosphere-ocean-land system. To develop such a comprehensive model we need to set up a multidisciplinary project in which all scientists from marine chemistry, biology, geology, and physics can make a joint study. A theoretical study for imitating the carbon cycling test in a laboratory has also been conducted. Wang et al. (2000b) found that pH can affect adsorption of DOC on goethite. The adsorption percentage of DOC shows a maximum at pH 5∼6, and above 50% at pH 8.1. It shows that the adsorption can affect distributions of DOC in seawater. A simple dynamical box-model was constructed in order to test the seasonal variation features of phytoplankton, zooplankton, DIN, DIP, DOC, POC, as well as dissolved oxygen (DO) in the northern part of Jiaozhou Bay in 1995. The annual variations of the phytoplankton production show two high value periods (Mar. to Apr. and July to Aug.) and two low value periods (May to July and after Oct.). DOC shows the common features: it is high in summer and low in winter (Wu and Yu, 1999). Their model equations describing the cycles of phytoplankton and OC in the ocean are as follows: (1) Phytoplankton (P: mg C/m3 ) dP/dt = B1 − B2 − B3 − B4 − B6 − B7 , (2) Zooplankton (Z: C mg/m3 ) dZ/dt = B4 − B8 − B9 − B10 , (3) Particulate organic carbon (POC: mg C/m3 ) d[POC]/dt = B6 + B8 + B10 − B12 − B13 − B14 , (4) Dissolved organic carbon (DOC: mg C/m3 ) d[DOC]/dt = B2 + B13 − B15 + QDOC , where B1 stands for photosynthesis; B2 , external secretion; B3 , exhalation; B4 , zooplankton assimilating; B6 , natural death; B7 , settling; B8 , dejection; B9 , excretion; B10 , natural death including assimilation; B12 , POC decomposed by bacterium into inorganic matter; B13 , POC decomposed by bacterium into DOC; B14 , POC settling; B15 , DOC decomposed by bacterium into inorganic

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matter. QDOC is the discharge of terrestrial wastewater and the dissolution in sediment. The methods of getting colloid organic carbon (COC) analyzed and separated have also been studied at this stage. The cross flow-over filter technology (CFF) is a common means that separates COC from the total dissolved organic carbon (TDOC). COC and TDOC are usually determined by high temperature combustion (HTC) and the UV/persulfate method (Wang et al., 2000a). It may be found from the above review that the main aspect of marine carbon cycling studies in China in recent years is the carbon geochemistry in the China Seas, which includes the forms, transfers, distributions, and changes of carbon, the biological productions, the models of carbon cycling, etc. So far, comprehensive or systematic studies of carbon cycles in China marginal seas, especially the process studies of carbon cycles in the carbon biology-chemistrydynamics system, have not been reported and this should be studied in depth in the coming years. 1.2.2.3 Carbon and Its Biogeochemistry in Marine Sediments Marine sediment is the primary part of the marine environment. Carbon in the sediment exchanges unceasingly with water, organisms, the atmosphere, and rivers that empty into the sea. The gas carbon in the atmosphere will transform into dissolved carbon in the water body by a complicated marine biogeochemical progress, and then particulate carbon finally becomes sediments by deposition. However, under suitable conditions, the reverse process will happen. Therefore, carbon in marine sediments plays a very important role in the carbon cycle. Carbon in sediments includes mainly organic carbon and inorganic carbon, and the main study is of organic carbon at present. The organic carbon in sediments is mainly land-derived and autogenetic, but the percentage is not the same in different sea areas. According to the spatial distribution of OC, TN, δ 13 C, and δ 15 N in sediments, the land-derived particulate organic matter accumulation was faster than that of marine organism debris in the Zhujiang River Estuary and its adjacent South Sea sediments, and the contribution of marine organic matter was 31%∼67%; as far from the estuary the contribution of marine organic matter was higher (Hu et al., 2005). Organic carbon derived from algae was lower than 0.06% in the estuary, but higher than 0.57% in the inner shelf. Carbonate is an important composition of inorganic carbon in marine sediments. Up to now, the research into inorganic carbon concentrates mostly on the source, distribution, dissolution, and precipitation of carbonate in sediments. For example, in the western South China Sea, the contents of carbonate in the north and mid-southern areas are high, but low in the middle and southeast areas. The distribution characteristics are controlled by terrigenous material supply and are in close relationship with the extent of the shelf and the gradient of the slope. The contents of carbonate are highest in the area


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of water depth between 400 m and 600 m. The average content in the area with a water depth of 500∼600 m is as high as 44.37%. The dissolution of carbonate intensifies in a water depth of more than 1,300 m, and the contents decrease distinctly. Carbonate concentration becomes stable near a depth of 3,500 m, which suggests that 3,500 m should be a CCD of the sea area (Li XJ et al., 2004). However, the role of inorganic carbon in sediments for the marine carbon cycle has not been paid enough attention to. In order to evaluate the contribution to the marine carbon cycle, the inorganic carbon in sediments is divided into several forms by Li XG et al. (2004a) who studied them in detail in Jiaozhou Bay and the Changjiang River Estuary, for example. The characteristics of different inorganic carbon in sediments are obvious, that is NaCl formIEF-N (13.0%)>IMOF-N (1.4%)>CF-N (1.0%). The nontransferable nitrogen accounted for 69.15%, of which about 49% was caused by sediment particulate (Ma et al., 2003). The nitrogen in different grain sizes in the South Yellow Sea sediment is divided into four forms. They are ion exchangeable form of nitrogen (IEF-N), weak acid extractable form of nitrogen (WAEF-N), strong alkaline extractable form of nitrogen (SAEF-N), and strong oxidant extractable form of nitrogen (SOEF-N). Among them, the content of SOEF-N was the highest and IEF-N was the primary form of transferable inorganic nitrogen, which was also the easiest participant in the cycle. When all forms of transferable nitrogen of the same grain sizes can take part in a cycle, their contributions to the nitrogen cycle followed the order: SOEF-N>IEF-N>SAEF-N>WAEF-N. For different grain size sediments, the absolute contents for different transferable forms of nitrogen in fine sediments are the highest and the lowest in coarse sediments. If the proportions of each grain in the sediments are the same, the contents of transferable nitrogen in fine sediments can occupy 60% of the total transferable nitrogen, which is two times that in medium size sediments and almost seven times that in coarse sediments. Thus, fine sediment has the highest potential contribution to nitrogen cycling. In addition, the relative contents of organic transferable nitrogen increase with a finer grain size, while those of inorganic nitrogen decrease (L¨ u et al., 2004b). Grain size is also important for the nitrogen distribution in the southern Yellow Sea sediments. The contents of various forms of nitrogen were higher when the fine grain size sediment increased. Nutrient burial efficiency was the highest in fine grain size sediment. Moreover, the highest burial efficiency of TN was 30.21%, indicating that an excess of 70% nitrogen in the southern Yellow Sea surface sediments could be released to take part in biogeochemical recycling. The nitrogen released from sediments could supply 6.54% nitrogen for the new primary productivity of the southern Yellow Sea (L¨ u et al., 2005b). However, the ecological function of different nitrogen in different grain sizes has great differences. Generally, the various transferable nitrogens in fine grain sizes sediment had close relations with phytoplankton and benthos, and the transferable nitrogen of medium and coarse sediments was mainly correlated with zooplankton. In the four transferable nitrogens of different grain sizes in the southern Yellow Sea surface sediment, SOEF-N and SAEF-N had close correlation with the growth and breeding of phytoplankton and promotion of productivity. For the inorganic nitrogen compounds, chlorophyll a, the total phytoplankton abundance and the primary productivity had a positive relationship with the contents of the two inorganic existence forms of nitrogen (NH4 -N and NO3 -N), which indicated that the transferable NH4 -N and NO3 -N could accelerate the phytoplankton growth and promote the primary productivity. Moreover, they were the primary forms absorbed by phytoplankton. Also, NO3 -N was more


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useful than NH4 -N and the finer the grain size the more obvious the function. On the whole, NH4 -N and NO3 -N cannot obviously improve the growth of zooplankton or benthos. This was because that they could not be assimilated by zooplankton or benthos directly but carried into the food chain. No matter what form of nitrogen, its ecological role was easily realized when these nitrogens are transformed into inorganic form (L¨ u et al., 2004a). Finegrained components are the predominant composition in the research area in the northern Yellow Sea core sediment, and their structure and characteristics directly affect the form, content and distribution of nitrogen in natural sediments, so that the vertical distribution of nitrogen in natural sediments is very similar to that in fine sediments. The grain size of sediments has an important role in the early digenesis of nitrogen. The finer the sediments are, the smaller the decomposition rate of organic nitrogen will be, i.e., the decomposition rate of organic nitrogen is the lowest in fine sediment, so organic nitrogen is easily enriched in fine-grained sediments. The burial fluxes of various forms of nitrogen are different because of the varying sedimentation rates in different sampling stations. The higher the sedimentation rate, the greater the burial flux of nitrogen (L¨ u et al., 2005a). 1.2.3.2 Biogeochemical Cycle of Phosphorus Phosphorus is not only one of the essential components for the growth and breeding of phytoplankton, but also the fundamental element for marine primary productivity and the food chain. Sediment is one of the important sources of phosphorus in seawater; moreover, it is a buffer for the phosphorus in overlying water. Therefore, it is of great significance for the dynamic cycle, the transformation at the sediment-seawater interface, and the subsequent digenesis to study phosphorus and its forms. Phosphorus in sediments includes mainly organic phosphorus (OP) and inorganic phosphorus (IP), and IP is the primary form. However, the percentage of IP in different regions is different, for example the IP percentage of total phosphorus (TP) was more than 60% (Li XG et al., 2005) for Jiaozhou Bay sediments and was over 50% for Bohai Bay (Zhao ZM et al., 2005). It cannot reflect fully the biogeochemical behavior of phosphorus in sediments if phosphorus was divided only into inorganic and organic phosphorus. Therefore, phosphorus in sediments is generally divided into 5 forms: adsorption form (Ad-P), iron-bound form (Fe-P), calcium-bound form (Ca-P), detrital form, and organic form (OP). The contents of total phosphorus (TP), organic phosphorus (OP), and the iron-bound phosphorus (Fe-P) are primarily controlled by the source matter. Ad-P and OP belong to bioavailable phosphorus and their cycle in sediments is dependent on iron-oxides. Calciumbound phosphorus (Ca-P) in sediments mainly comes from marine plankton. Song (2000b) divided the transferable phosphorus in natural sediments into ion-exchange form, carbon-bound form, iron-manganese oxide form, organic matter-sulphide form. Only these phosphorus forms can take part in the geo-

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chemical cycle. For the Bohai Sea sediment, the transferable phosphorus accounted for 19.2% of total phosphorus, and the organic matter-sulphide form was the primary form of transferable phosphorus accounting for 10.7% (Song et al., 2003). The spatial distribution of phosphorus in sediments is primarily controlled by source matter and the hydrodynamic condition, which has clear regional characteristics. The P distribution presented clear spatial and seasonal characteristics, which were dominated by the resources of P input, sediment texture, and a number of biogeochemical processes in different hydrodynamic and environmental conditions of the Changjiang River Estuarine and coastal zone (Gao et al., 2003). The phosphorus combined by apatite in the Nansha Islands sea area has the same source as dissolved phosphorus in seawaters and can be utilized by organisms when it returns to the biogeochemical phosphorus recycle; therefore, the phosphorus was mainly controlled by biological action. Total phosphorus and organic phosphorus contents were correlated observably to FeO and apatite combined phosphorus was correlated to CaO. The phosphorus source and its means of input were controlled by the precipitation of detrital particles in water. The total phosphorus in the Zhujiang River Estuary sediments had a general tendency to increase from sea bottom to surface and the maximums mostly emerged at the surface or at a layer of about 10 cm depth. The vertical profiles of organic phosphorus were similar to those of total phosphorus with the maximums mostly emerging at a depth of 5∼10 cm. The contents of Fe-P and Al-P were higher and decreased downward, but the trend was not obvious. TP, OP, Fe-P, and Al-P have a distinct synchronous effect on their sedimentation process, especially total phosphorus and organic phosphorus (Yue and Huang, 2005). Sediment is one important source and sink of phosphorus, and its adsorption and release are always the key points that scientists are focusing on. For the East China Sea, the fluxed PO3− 4 and total dissolved phosphorus (TDP) were removed from overlying water to sediment. The dissolved phosphorus diffuses from seawater to sediment in most stations near land. Moreover, the closer the distance to land, the larger the exchange flux. For the whole of the Yellow Sea and the East China Sea, the exchange fluxes of PO3− 4 are negative; that is, sediment absorbs dissolved phosphorus from water and the sediment 3− is a sink of PO3− 4 (Qi et al., 2006). The PO4 absorbed from water every year accounted for 67% of Changjiang River inputs in the region (Qi et al., 2003). In Jiaozhou Bay, PO4 -P was transferred from sediment to seawater at most stations and the exchange rates usually ranged from 0.1 to 90 μmol/(m2 ·d). By considering the area percentage of different patterns out of the total area of Jiaozhou Bay, the exchange flux of PO4 -P from the sediments to seawater in Jiaozhou Bay was estimated as 9.76×106 mmol/d, which occupied 24% of the river input. The exchange flux can provide (9±3)% of phosphate required by phytoplankton in Jiaozhou Bay (Jiang et al., 2003). The PO3− 4 exchange flux of Laizhou Bay of the Bohai Sea was 6.7∼6.8 μmol/(m2 ·d). The biological activity influenced the phosphate exchange greatly.


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In order to understand the phosphorus release behavior in sediments, considerable simulation in the lab and investigation in the field have been done. Studies showed that the adsorption capacities of different sediment samples have “stable pH scope”, and that this scope is consistent on the whole, which is between 6.5 and 9.5. The maximum adsorption capacity of different sediment samples occurs at salinity 6‰ with a range of 5∼7. The phosphate adsorption capacities on most of the sediments increase with the increase in temperature, showing the characteristics of endothermic reactions, while some samples show the characteristics of exothermic reactions (Li M et al., 2005). Under a static environment, phosphorus released from sediment reaches its highest level after vibration for about 10 min, and then maintains stability after 3 h. Adsorption kinetics could be fitted to both the Elovich equation and the two-constant-rate equation. The most released quantity is closely related to the composition of the sediment. Those sediments consisting mainly of silty and muddy components are higher in phosphorus released than those dominated by sandy composition. In the adsorption test, Fe-P is the most active one, with a releasing ratio higher than other types of phosphorus, followed by Ad-P and OP. The varieties of dissolved inorganic phosphate (DIP) and dissolved organic phosphate (DOP) were the same under static conditions. In the lower vibration frequency (60 times/min), DOP and DIP have a similar variable trend with those in quiescence. However, in the higher vibration frequency (120 times/min, 150 times/min), the variety of DOP’s concentration has an opposite trend to that of DIP’s concentration. The concentration of DIP is increased with the increase in the vibration frequency, but the concentration of DOP is reduced with the increase in the vibration frequency. The adsorption of sediment to phosphorus can reach a percentage of over 85% of balanced quantity after 4 h. The adsorption almost did not increase as the time extended beyond 12 h. The influence of temperature on the adsorption balance time and balanced adsorption quantities is not obvious. The suspended sediment content is the most important factor, and the influences of pH, temperature, and salinity on adsorption quantity are similar. Adsorption happens on fine grain size sediment, the higher the fine particle sediment contents are, the higher the adsorption quantity is. The study of the phosphorus adsorption in the Changjiang River Estuary sediments showed that the sediments were buffers to phosphate and the balance time of phosphorus adsorption (desorption) was about 6 h. The saturated adsorption quantity of phosphorus was about 600 μg/g and the saturated desorption was about 126.37 μg/g. The apparent adsorption heat of fine sediments (ΔH ) was 47.59 kJ/mol. The adsorption of phosphate was in accordance with Freundlich’s isothermal equilibrium and it was a heat absorption reaction. The contents of sand and mud in sediments had some influence on adsorption and the adsorption would decrease with the increase in sand or mud content. In general, the increase in pH favors adsorption. Moreover, the increase in salinity does not favor adsorption but favors desorption. The research shows that the release of phosphorus in the sediments of the Changjiang River Estuary

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is small. The re-release of phosphorus is slow after great adsorption by the sediments. The balancing point of adsorption and release of phosphorus in the sediments is 0.038∼0.085 μg/ml by measurement, which is higher than that in sea bottom water. So the Changjiang River Estuary sediments are inclined to release phosphorus to overlying water (Li and Yang, 2004). In addition, the research on sediment phosphorus can supply information about the paleoenvironment. The information about the environment and biogeochemistry of phosphorus extracted from the South Sea shelf slope sediments showed that the supply of terrigenous phosphorus for the sea was steady overall. The change in phosphorus contents in different depths was the result of climate and environmental changes. The vertical distribution of phosphorus had an opposite trend to calcium carbonate and Cd in sediment. The ebb and flow between CO2 and PO3− 4 in seawater calculated by chemical balance indicated that the accumulation of phosphorus in marine sediments was related to the atmospheric CO2 change, and the decrease in sediment phosphorus accumulation and the increase in calcium carbonate contents may be one of the key factors leading to a decrease in CO2 in the atmosphere in the ice age. 1.2.3.3 Source and Characteristics of Biogenic Silicate Silicate is a very important nutrient in the ocean. Unlike other major nutrients such as phosphate and nitrate or ammonium, which are needed by almost all marine plankton, silicate is an essential chemical only for certain biota such as diatoms, radiolarian, silicoflagellates, and siliceous sponges. However, this biology is one of the most important producers in marine. The estimation shows that diatoms contribute more than 40% of the entire primary production. Therefore, silicate cycling has received significant scientific attention in recent years and many scientists have studied silicate behavior in marine environments. Biogenic silicate is the amorphous content extracted by chemical methods, which is named as biogenic opal or opal in brief. The concentration of dissolved silicate in the world ocean is about 70.6 μmol/L and the net input of dissolved silicate from land to ocean is (6.1±2.0)×1012 mol (calculated by Si) every year, and the primary contribution (about 80%) comes from river. Silicic phytoplankton is the essential constitute of marine primary productivity and their reliquiae is the main source of biological silicate in bottom sediment. The accumulation of biogenic silica (BSi) in sediments can reflect the influence of the nutrient changes on the growth of diatom and other phytoplankton; moreover, it also records the occurrence and development of the eutrophication process. At the same time, the biogenic silica accumulation in sediment can reflect the long time and spatial changes in overlying water primary productivity. In the East China Sea and the Yellow Sea surface sediments, the BSi contents ranged from 0.21% to 0.70%, which was in accordance with the primary productivity. For the Changjiang River Estuary, the accumulation of BSi had a close relation with Chl a and the primary productivity


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of waters and the change in the BSi content in the sediments recorded the annual changes in the nutrient fluxes of N, P, Si transported by the Changjiang River and its runoff in the last 20 years (Ye et al., 2006). Silica in sediments, which is mostly paid attention to, is the part that can be dissolved and takes part in the biogeochemical cycle under natural conditions. Song et al. (2003) divided silica in natural grain size sediment into ion-exchange form, carbonate-bound form, iron-manganese oxide form, and organic matter-sulfide form, and thought that only these forms can take part in the biogeochemical cycle. In the Bohai Sea sediment, the content of the transferable form of silicon accounted for 0.12% of total silica, and the carbonate-bound silica was the primary form accounting for 0.05% of total silica. In the East China Sea and the Yellow Sea, SiO2− 3 will transform from released from sediment sediment to seawater, and the contribution of SiO2− 3 to the primary productivity was 13% and 10%∼18%, respectively. Compared with river input and atmospheric deposition, the SiO2− 3 contribution of sediment was 90% and 86%, which indicated that sediment was the main source of SiO2− 3 (Qi et al., 2006). The regeneration of silica in sediments is the main supply source for the ocean, and the silica supplied to the world’s ocean from biogenic silica dissolution in sediment is four times that from rivers. However, for the marginal sea, sediment silica behavior is different from that in the ocean due to the shallow water depth, high primary productivity, and considerable terrigenous inputs. Long-term research on Jiaozhou Bay nutrients showed that the nutrients are characteristic of high nitrogen and low phosphorus and silica. In the past four decades the nitrogen has increased unceasingly but the silica contents decreased continually and, as a result, silica may have become the primary possible limiting factor for phytoplankton growth in Jiaozhou Bay. Studies of the biogenic silica in Jiaozhou Bay sediments showed that the biogenic silica in three surface sediments samples was 1.58%, 1.44%, and 1.48%, respectively, higher than those of the East China Sea and the Yellow Sea (biogenic silica contents were lower than 1% (Zhao YF et al., 2005)). The BSi/TN ratios in Jiaoxhou Bay sediments were much greater than 1 and BSi/TP ratios were >16 too, which were the opposite of the ratios of BSi/TNSO2− >CH O. China marginal sea sedimentary environments had 2 4 been systematically researched and it was found that the sedimentary redox environments not only related to Eh, organic carbon, the radio of Fe3+ /Fe2+ , but also related to the grain sizes of sediments and the sulfur system in interstitial waters. Hereby, a new function—redox degree (ROD) was introduced to appraise the redox environments of sediments. The magnitude of ROD quantificationally reflected the degree of redox reactions. The analysis of ROD shows that the sedimentary reduction properties in the northern Huanghe River Estuary are stronger than those in the South Yellow Sea and the Okinawa Trough, etc. The reducing property of sediment is stronger than that of seawater; the reducing property in regions far offshore is stronger than that close inshore.

1.3 Functions of China Marginal Sea Sediments in Cycles of Biogenic Elements

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The S system is one of the main systems for regulating sedimentary redox processes. It participates in a series of diagenesis such as complexation, exchange adsorption, precipitation. The existing forms of S are regulated by environmental pE and pH. S in the East China Sea sediment interstitial waters − mainly exists in the form of SO2− 4 , it amounts to 99% of total S, HS amounts 2− to only 1% of total S, the others are a little H2 S, S , etc. Sulfate is reduced to − 0 sulfide through microbial processes in anoxic circumstances. SO2− 4 / HS (Sx ) is one of the main redox electrode pairs which regulate marine environments. The greatest reduction rate of sulfate occurs in surface layer sediment interstitial waters, and it is regulated by the content of organic matter in sediment and influenced by the season. S with –2 state mainly results from the diffusion from sedimentary interstitial waters to overlying waters, the diffusing flux of HS− in the Huanghe River Estuary regions reached 8.95 μmol/(m2 ·d) (Qiu et al., 1999). Behavior of OC in sediments is mainly controlled by depth and redox environment. OC from sulfur mineralization accounts for 60%∼80% of average net primary production. Phosphorus is controlled by pH and dissolved oxygen of water columns. The more the phosphorus released, the higher the pH and DO. Silicon is mainly controlled by some inorganic dissolution. For example, it can precipitate in the formation of meerschaum mixing with Mg(OH)2 , and decrease enormously in dissolved silica concentration in interstitial waters. Only a little N in sediments is buried, and the others are regenerated through − + mineralization and are released in the forms of NO− 3 , NO2 , NH4 , N2 or N2 O via early diagenesis (Song, 1997b). 2− 3− Song (1997b) studied the early diagenesis models of NH+ 4 , SO4 , PO4 , and H4 SiO4 in East Sea China sediment interstitial waters. The decomposition, precipitation, and adsorption of ammonium, dissolved silicate, phosphate, and sulfate in interstitial waters were ubiquitous, and they could be treated with the first dimension kinetic reaction model. The dissolution of silicate and reduction of sulfate occurred in the upper surface layer (0∼10 cm). Profiles of silicate in interstitial waters showed a decreasing trend with depth. Profiles of sulfate showed that the further the distance was, the higher were the concentrations of sulfate. Alkalinity and ΣPO3− 4 in sedimentary interstitial waters increased with depth, suggesting that the origin of NH+ 4 came from the decomposition of bottom sediments and the release of the decomposed productions, not from the diffusion or reduction of NO− 3 in overlying waters. The decomposition of organic matter depended upon the activity of organic matter in sediment and the efficient actions of electron acceptors in special sedimentary circumstances, which included O2 , NO− 3 , MnO2 (or Mn2 O3 , MnOOH), Fe2 O3 (or FeOOH), SO2− 4 , and C. Daya Bay is rich in biological resources. Most of its sediments are soft mud, but contents of nutrients are not high. Daya Bay has high primary production and is prone to bring about red tide. The determined contents of NH+ 4, HPO2− , and H SiO were arranged in the order: interstitial waters>overlying 4 4 4 waters>bottom waters. No significant change had been found for NO− 2 and

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NO− 3 , and no significant change had been found in the concentration profiles − 2− of NO− 3 , NO2 , HPO4 , and H4 SiO4 in sedimentary interstitial waters, but the + profiles of NH4 concentrations decreased with depth in the exponential function. The content of NH+ 4 in sedimentary interstitial waters was 2∼3 numeric − orders greater than those of NO− 2 and NO3 . It suggested that the surface sediment of Daya Bay was in a reducing state. The high concentration of NH+ 4 suggested the recycling utilization ratio of N was very high, and NH+ 4 might have been assimilated before it was converted. The concentration of NH+ 4 in Daya Bay interstitial waters was higher than that in the East China Sea (Qiu et al., 1999). The transferable phosphorus in the natural grain size sediments was in the range of 58.5∼69.8 mg/g in Huanghe River Estuary regions, accounting for 9.1%∼11.0% of the total phosphorus. In comparison, the phosphorus in totally-ground sediments was 454.8∼529.2 mg/g, accounting for 9.1%∼11.0% of total phosphorus. It suggests that the transformed P in totally-ground sediments mostly could not participate in the biogeochemical recycling. It was found from the investigated results in situ from benthos that the biomass of big or small benthos correlated well to the non-detrital state P in natural grain sizes of sediments. Bad correlations were found between the biological mass and the phosphoric content in totally-ground sediments. It showed that the transformed P was the P of the non-detrital state in natural grain size sediments, and not the P in the totally-ground sediments, which had been taken for granted before (Song, 2000c). Accordingly, the recycling of biogenic elements in China marginal sea sediments is correlated with pH, ROD, temperature, water dynamics, and biological activity in sedimentary circumstances. In addition, it is very important to know whether the means of study adopted and its results can really reflect the biogeochemical processes and their contributions to marine changes or not. 1.3.3 Contribution of Settling Particles to Biogenic Element Recycling in China Marginal Seas The sedimentation of marine settling particles is an important source of sediment on the seabed. It plays an important role in sediment-seawater interface processes. It is significant that we research the regulation of settling particles on material transfer, transforming, and depositing. Fluxes of some biogenic elements in China marginal sea settling particles are listed in Table 1.9. The settling fluxes of particle carbons in the euphotic zone of the southern Taiwan Strait regions were 73.76 (5 m) and 98.50 (60 m) mg/(m2 ·d), respectively, the annual fluxes in these regions were 2.45×109 kg C/yr, the total removal rates in the euphotic zone were about 2.5×109 kg C/yr, and total carbons in the euphotic zone of the region were 6.48×1010 kg C/yr. Accordingly, the retention period of total carbon in the euphotic zone was estimated to be about 26 years, which showed that the recycling rate of OC in the region was rather quick. It was not rich in inputs of exterior nutrients in Luoyuan


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Table 1.9. The fluxes of biogenic elements of settling particulates (mg/(m2 ·d)) Regions Nansha coral reef lagoons Xinyi Lagoon Zhubi Lagoon Zhubi Lagoon Yongshu Lagoon Nansha Islands Taiwan Strait Luoyuan Bay Xiamen Bay East China Sea

Depth (m)

5 24 16 5 80 25 50 60 5 5 5

POC

463.40 53.38 90.16 220.10 193.50 99.25 73.80 98.50 5,054 856 213.80

TPC

402.3 575.7 2,735

PON TPN POP TPP

8.5 5.6 32.1

9.8 13.8 45.9

C:N:P

50.4: 6.3: 1 2.98 4.1 199.1: 9.6: 1 1.15 3.07 119.4: 14.9: 1 4.82 12

5.64 47.70

89.4:11.1:1

20.10

Bay regions, whereas about 91% of N, 97% of P, and 68% of Si for biological activities came from inner nutrient recycling in the seawater. The contents of POC decreased successively from suspended matter to settling particulates and to surface sediments, 15%∼40% of POC in suspended matter had participated in recycling in the settling process from water to sediment. The annual average content of PP in Luoyuan Bay waters was (0.342±0.249) μmol/L, and this amounted to about 40% of TP. The distribution of PP was mainly controlled by resuspension and continental inputting processes in winter, and was mainly controlled by biological processes in the waters in summer. Upwelling played an important role in the recycling of biogenic elements of settling particles in the East China Sea continental shelf, sometimes it was the main contributor, and the biological activity was less. About 80% of surface sediments were constituted of suspended matter, so upwelling also influenced the interfacial behavior of biogenic elements. Upwelling increased the provision of nutrients, and also increased biological production. 25%∼40% of primary production came from seabed sediments in the processes, and ultimately led to 76%∼83% of PP to regenerate. Strong upwelling occurred at the near-shore in summer and existed at the south edge of the Taiwan bank throughout the year. Suspended matter from 0∼10 m upwelling water layer on the south edge of the Taiwan bank mainly consisted of biological particles in summer. The PP accounted for 50% of TP in Xiamen Harbor and Jiulong River Estuary regions in spring, and the correlativities between PP and TP were very good. Transformations among different forms of P mainly occurred in solution, and were also influenced by biological activities (Hong, 1994). Resuspension of sediment had a significant influence on the compositions of setting particles in the East China Sea continental shelf regions. Resuspensions of sediments from storm, tides, bottom currents, and benthos irrigation were greater in the East China Sea continental shelf regions than those in the Changjiang River Estuary. There were significant differences in the material vertical fluxes in the East China Sea continental shelf regions from other regions and depths. The vertical transferring of C in the regions mainly depended on POC, which accounted for 98% of TC in surface layer

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waters and 68% of TC in the bottom waters. POC/PON ratios in planktons and suspended particles both were close to 10.6, much lower than that in sediments. The particle inorganic carbons (PIC) in planktons and suspended particles accounted for 0.20%∼1.9% of PC. PIC/TP value in bottom layer suspended matter was much greater than that in surface layer suspended matter. Therefore, resuspension of bottom layer matter had a significant influence on the composition, contents, and fluxes of settling particles in the East China Sea. The C in seawater of sediment came from the CO2 exchanged across the seawater-air interface, and then the DIC was translated into DOC and POC through photosynthesis of phytoplankton and production of zooplankton, which constituted the main vertical transferring matter and C deposits on the East China Sea continental shelf. The settling particles in the Nansha Islands coral reef ecosystems mainly came from biological origins, their setting rates were greater than those in near-shore regions, and it reflected the higher biological production in the regions. C:N:P was 123:10:1 in the particles on average, having a greater difference from the Redfield ratio (106:16:1), and was higher than that in Luoyuan Bay (89.4:11.1:1). About 86%∼89% of OC in lagoon setting particles could be decomposed and released, about 11%∼14% of which might be buried. The flux of OC was in direct proportion to TC, and was in inverse proportion to water depth. In contrast with this, the fluxes of ON were in inverse proportion to TN, and were in direct proportion to the water depth. The content of PP in the coral reefs was about 3.13×1018 g, which accounted for 96.2% of TP, and played an important role in the recycling of P in the ocean. About 86%∼89% of OC in the setting particle had been released before sinking to the seabed, which played an important role in the recycling of OC and in maintaining higher coral reef biological production. The flux of OP was between OC and ON. The recycling rates of setting PP in lagoons were rather quick, 35 d in the Zhubi Reef and 7 d in the Yongshu Reef. POP in setting particles of Nansha coral reef lagoons accounted for 37.9%∼72.6% of TP. The releasing ratios of setting PP could be estimated by the contents of TP and POP of setting particles and through surface sediments of lagoons. The releasing ratio of OP was 97.5% in Zhubi Reef regions, which accounted for 90.7% of TP. Therefore, 97.5% of OP was released into waters for recycling. The releasing ratio of POP was 81.8% in the Yongshu Reef region. Accordingly, 2.5%∼18% of OP was buried and because of its burial aggradation IP could also be released to a certain extent. It mainly came from the regeneration and dissolution of solid calcium phosphate in bones by bacterial activity. The Nansha Islands coral reef ecosystems were the exporter of organic particles and nutrients. In conclusion, 15%∼40% of OC and 82%∼97.5% of OP had been released into sea waters for recycling before biogenic elements in the setting particles reached the seabed in China marginal seas. The researches of biogenic elemental recycling processes in China’s seas vertical setting particles are not systemic. It is necessary to systematically study the recycling and controlling mechanisms of biogenic elements. Considering the past research in China’s


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seas, biological processes and water dynamical processes must be two important factors. 1.3.4 Contributions of China Marginal Sea Sediments in the Recycling of Biogenic Elements Terrestrial detrital organic matter and suspended particles are the main sources of marine sedimentary biogenic elements, and they are brought into the ocean by river runoffs, biological activities, and living activities, and ultimately reach the seabed. The biogenic elements carry through mass transfer mainly by molecular diffusion across the sediment-seawater interface. Molecular diffusion results from a series of chemical reactions and processes, such as mineralization of organic matter, dissolution and precipitation of minerals, adsorption, and exchange of matter, which alter the composition of interstitial waters, and result in a concentration gradient in interstitial waters. Recycling of biogenic elements between sediments and interstitial waters is carried through endlessly along the gradient. The physical chemical environments of sediments play an important regulating role in the recycling. The mass exchanges between interstitial waters and overlying waters are important material conditions to maintain the ecosystem in surface sediments and in bottom waters. The diffusing fluxes of biogenic elements across sediment-seawater interfaces mainly depend on the diffusion of the concentration difference caused by the concentration gradient near the interfaces. The net diffusing fluxes of biogenic elements across the sediment-water interface in some of China’s sea regions can be estimated by the First Fick Law, and the results are listed in Table 1.10. Table 1.10. Diffusion fluxes across the sediment-seawater interfaces in China’s seas Regions East China Sea

HCO− SO2− S2− NH+ NO− 3 4 4 2 −0.634 ∼ −2.055 ∼ −0.022 ∼ −34.081 ∼ 0.387 1.214 0.141 0.193 −0.66 2.24 1.410

NO− 3

HPO2− H4 SiO4 4 −1.199 ∼ −324.9 ∼ 1.938 148.800

Liaodong Bay Nansha Islands Lagoon (1993) −0.39 0.70 630.26 30.22 103.86 9.13 255.10 Lagoon (1994) −0.43 14.70 197.61 4.42 212.86 6.71 151.63 Off-reef (1993) −0.36 0.36 619.77 19.82 345.15 8.35 2, 637.70 Off-reef (1994) −0.33 0.22 485.3 14.81 585.05 15.78 1, 627.32 Xiamen Bay 720 7.74 2800 Okinawa Trough −4.488 1.131 2.209 Outer-Wenzhou −6.640 0.133 111.90 South Yellow Sea −1.321 −0.102 −55.91 Luoyuan Bay 0.37 314.80 Daya Bay 302 ∼ 0.06 ∼ 1.82 2.53 47.96 Unit: HCO− and SO2− are in mmol/(m2 ·d); S2− is in pmol/(m2 ·d); the others are in 3 4 μmol/(m2 ·d). “−” stands for the flux direction from overlying waters to sediment

The diffusing behavior of nutrients depends upon their inner characteristics and the environment of the regions. The whole oceanic annual input

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fluxes of PO4 -P, SiO4 -Si, NH4 -N, and Mn2+ to overlying waters are 3.8×1011 , 1.1×1013 , 1.0×1013 , and 1.4×1011 g/yr, amounting to 54%, 2.2%, 4.8%, and 40% of total river inputting fluxes, respectively. Nutrients released from organic matter decomposition in bay sediments of China’s seas contribute 80% of N and 200% of P that plankton needs in the bays. Due to shallower water depth, stronger seasonal changes in temperatures and biogeochemical actions, stronger changes in early diagenesis processes near interfaces, and quicker production or adsorption of nutrients from sediments in the Bohai Sea, the Yellow Sea, and the East China Sea, the directions of nutrient diffusions are different in China marginal sea sediments. Most of the East China Sea continental shelf surface sediments were covered with sandy sediments. Content of H4 SiO4 in surface sediments was 24 times greater than that of surface layer waters, and 23 times greater than that of bottom layer waters. The value of PO4 -P was 19 times and 3 times greater, respectively. The annual releasing fluxes of P and Si into seawaters from sediments in the Bohai Sea were 1.02×107 kg P/yr and 1.91×108 kg Si/yr, respectively, accounting for 86.4% of recycling TP and 31.7% of recycling TSi, respectively. Accordingly, the processes near sediment-seawater interfaces in the Bohai Sea played an important role in the recycling of nutrients, especially for P (Song, 2000b). Liaodong Bay lies in the northern part of the Bohai Sea. Ammonium and sulfide diffused from sediments to overlying waters, SO2− 4 diffused from overlying waters to sediments, and this reflected their different chemical diagenesis processes (Song, 1997b). All the diffusion directions of nutrients near sediment-seawater interfaces in Nansha Islands regions were from sediments to overlying waters, their fluxes were greater than those in the Bohai Sea, the Yellow Sea, and the East China Sea regions. Due to the high water temperature in Nansha Islands regions year after year, the apparently active energy of nutrients diffusing and releasing from sediments decreased, and the sediment activities increased. This resulted in the production of a great deal of nutrients in sediments and the diffusion from sediments to the overlying waters. HS− concentrations in Nansha lagoon interstitial waters were much higher than those in the outer-reef sediment interstitial waters. The concentrations in interstitial waters of the lagoons from human activities were higher than those in interstitial waters of lagoons without human activities, and were higher in the near-reef sediment interstitial waters than in the off-reef sediment interstitial waters. HS− and S2− diffused from sediments to overlying seawaters, but it was the opposite for SO2− 4 . The average diffusing fluxes in the lagoons were higher than those in the outer-reef. H4 SiO4 was the greatest diffusing + composition in the outer-reef nutrient compositions, and NO− 3 and NH4 were the greatest diffusing compositions. The diffusing flux of H4 SiO4 across the outer-reef sediment-water interfaces was 10.5 times greater than that in the lagoons, and that of NO− 3 was 3 times greater. All the diffusion directions of S2− , HS− , H4 SiO4 were from sediment to + overlying waters in the East China Sea, and the opposites were SO2− 4 , NH4 ,


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PO3− 4 , namely from overlying water to sediments. The different diffusing directions reflected their different early diagenesis processes in the sediments (Song, 1997b). The diffusions of HCO− 3 from seawater to sediment in the outer-Changjiang River Estuary and the Okinawa Trough regions showed that HCO− 3 must be transferred in the surface layer sediment. The diffusions of HCO− 3 from sediments to seawaters in the northern Taiwan Strait were due to their short distances from the bank, high contents of organic matter in sediments, low pH value, the dissolution of CaCO3 and the oxidation of organic matter. The diffusions of NH+ 4 were from sediments to seawaters in the outer-Changjiang River Estuary, the Okinawa Trough, and the northern Taiwan Strait, which reflected the fact that the transfer of NH+ 4 was due to its adsorbtion in clay minerals in the East China Sea sediments. There was rather a lot of H4 SiO4 in sediments diffused to seawater in the Okinawa Trough and the Taiwan Strait because of their low pH in sediments. HPO2− 4 could be transferred through adsorption or through forming apatite in the Okinawa Trough and the southern Taiwan Strait. HPO2− 4 could diffuse from sediment to seawater in the Okinawa Trough sediments, which was due to the characteristics of volcanic activity and oceanic sedimentation in the regions. There was much more organic matter from biological activities in the sediments of the Changjiang River Estuary − near-shore, which could decompose and release more NH+ 4 and HCO3 to enter interstitial waters, and then be diffused into overlying waters, which provided more chemical substances for overlying waters than in the off-shore regions. The transfer of Si existed in the sediments of the outer-Changjiang River Estuary. The diffusions of SO2− 4 across the sediment-water interface were from overlying waters to sediments in the regions of Nansha Islands, the Okinawa Trough, the Huanghe River Estuary, and the South Yellow Sea, but also in outer-Wenzhou. It was due to the stronger reduction of SO2− in surface 4 sediments that led to the consumption and diffusion of SO2− 4 . The diffusion directions of S2− and HS− were from sediments to overlying waters, which reflected the stronger reduction of the two elements in the sediments than that in the overlying waters. S2− from the reduction of SO2− 4 diffused into seawaters. Most of S2− was produced by chemical diagenesis in initial stages when S2− was buried in sediments. Diffusion of S2− into overlying waters showed that S2− in the surface sediments could be dissolved and freed from reduction. The magnitude of transferring fluxes of HS− reflected the reductions of sediments to a certain degree. The sedimentary reduction could be listed below according to the diffusion fluxes of HS− : Huanghe River outer-shore region>Huanghe River near-shore region>southern Yellow Sea region>outerWenzhou region>Okinawa Trough region, which mainly depended upon the input of terrigenous matter, especially of terrigenous organic matter (Song, 1997b). Luoyuan Bay belongs to a sub-tropical region, and is a typical semi-closed bay. Annually P and Si diffusing into overlying waters from the bay sediments are 6.61×102 g P/yr and 4.95×105 g Si/yr, respectively. The average fluxes

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of P and Si into the ocean from the river were about 7.13×106 g P/yr and 1.97×109 g Si/yr, so the diffusions accounted for about 10% of P and 25% of Si in the input fluxes, respectively. The diffusing fluxes of Si in Luoyuan Bay sediments were less than those in Xiamen Bay sediments, and greater than those on the East China Sea continental shelf and in the outer-Changjiang River. The diffusion fluxes of P in Luoyuan Bay sediments were less, and this was due to the content of P in its interstitial waters being only 1.0 μmol. The content was comparable with the contents in Xiamen Bay sediments, and was tens to millions of times less than those in the world’s oceans sediments. The diffusing fluxes of P into overlying waters in the western Xiamen Bay were 0.9 μmol/(m2 ·d). The diffusing fluxes of P across sediment-seawater interface were 0.026 mg/(m2 ·d) in Luoyuan Bay and 0.24 mg/(m2 ·d) in Xiamen Bay, respectively. The fluxes of PP determined from the sampling implements under the euphotic zones were 5.64∼47.7 mg/(m2 ·d). Accordingly, most of the P released participated in recycling in the sinking processes. PP that reached the seabed was strongly adsorbed as solid, only about 0.5% of which diffused into waters in the form of solute through early diagenesis processes, even though some P had been released through mineralization of organic matter. From the Huanghe River Estuary to the Bohai Sea, the fluxes of dissolved P were 1.6×106 kg/yr, the fluxes of dissolvable P were 2.81×108 kg/yr, and the fluxes of buried P were 5.88×108 kg/yr. The direct contributions of dissolved P to the Bohai Sea were less than the release of dissolvable P from suspended particles and from sediments for higher suspended matter transportation and lower efflux. About 2/3 of TP transferring from the Huanghe River Estuary sank and was buried in marine sediments. OP released from the decomposition of river organic matter was the main source of dissolved P and dissolvable P in the region (Li and Yu, 1999). The nutrients supplied from sediments to seawaters in Daya Bay were more important than those in the other regions. The cultivating regions of Daya Bay were covered with soft mud, the vertical concentration distributions of − 2− NO− 3 , NO2 , H4 SiO4 , HPO4 had no changes in the interstitial waters of the regions, and the distribution of NH+ 4 decreased distinctly with depth. The vertical distribution trend of NH+ 4 was similar with that in the East China Sea, but the content of NH+ 4 in Daya Bay sedimentary interstitial waters was greater than that in the East China Sea. The diffusing fluxes into seawater of 2− NH+ 4 , HPO4 , and H4 SiO4 in Daya Bay sediments were much less than those in Xiamen Bay and the Nansha Islands, but greater than those in many near− shore regions. The diffusing directions of NO− 3 and NO2 were from overlying waters to sediments, which showed the enrichment of organic matter near the surface sediment interfaces in the cultivating regions. The contents of H4 SiO4 in the interstitial waters of Daya Bay were higher than those in other regions, but changed much less than those in other regions. In conclusion, the differences in biogenic element diffusing fluxes near sediment-seawater interfaces in different regions are great, and their directions are not all the same. It reflects the differences in their early diagene-


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sis processes, interfacial geological circumstances, and biological and physical chemical environments. 1.3.5 Influences of Biological Productions in China Marginal Sea Sediments on the Recycling of Biogenic Elements Of all marine organic matter, life particle organic matter accounts for 2%, non-life particle organic matter accounts for 9%, and dissolved organic matter accounts for 89%. Life activities are the most active factors in the ocean, and almost involve all the geochemical processes. It is the 2% of the life organic particles in the ocean that regulate the existence of 89% of dissolved organic matter. 1.3.5.1 Influences of Planktons on the Recycling of Biogenic Elements Planktons make an important contribution to marine primary production. Marine biological productions are initiated from the photosynthesis of phytoplanktons, which utilize sunlight and CO2 to synthesize 40%∼50% of global primary productions. 2∼20 μm micro-phytoplanktons and those less than 2 μm account for a great proportion of marine primary producers, and their contributions sometimes exceed the diatoms—the traditional primary producers, especially in the tropical ocean. The diatoms with tens of μm were the main producers of marine primary production in the past, especially in the spring when algal bloom occurs. The primary production estimated according to chlorophyll a in the Nansha regions was 406 mg/(m2 ·yr), and the proportion of the vertical fluxes of organic carbon in primary production was also estimated. The proportions were 17.7% in the Nansha Zhubi Reef and 54.2% in the Yongshu Reef. It showed that 17.7%∼54.2% of primary productions in the lagoons sank to the seabed through biological particles, and 45.8%∼82.3% of OC participated in recycling in waters. The values were lower than those in Luoyuan Bay, and higher than those in general sea regions (Song, 1997b). The nutrient concentrations of surface layer waters were 0.2 μmol/L, lower than those of bottom waters, and 0.4 μmol/L higher than those of offshore regions and, moreover, increased with depth. The regenerating rates of phosphates between 200 m and 600 m in depth were 6.7∼20 μmol/(L·yr), and the average sinking rates were 15∼80 d (Zhu et al., 1999). The influence of planktons on the recycling of biogenic elements was also embodied in seasonal changes in the contents and the recycling of biogenic elements. The fluxes of amino acids and organic matter in the northern South China Sea setting particles showed intense seasonal changes, and the increase and decrease of planktons such as diatom, coccolith, and foraminifer were similar to the changes in total fluxes. The siliceous and calcareous planktons were the main contributors of organic matter. The organic matter was assembled and rapidly settled through mutual actions between organic particles

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and inorganic particles, decomposed and dissolved by microbes, and led to the particles returning back to the waters for recycling. Nutrients had an important effect on the growth rates of pelagic diatoms, whereas the biological production processes of pelagic diatoms also had an important effect on the recycling of biogenic elements. The concentrations of nutrients in marine ecosystems not only influenced the total biomass, but also led to changes in the composition, structure, and characteristics of particles. Some studies showed that waters would have become eutrophic when IN>0.2 mg/L, IP>0.045 mg/L, and COD>0.2∼0.3 mg/L. The optimal nutrient ratios (N:P:Fe:Si) for the growth of Cocconeis scutellum var. parva, Amphora coffeaeformis, and N. millis were 50:0.25:0.5:2.0, 2.5:1.0:1.0:2.0, and 5.0:0.15:0.5:2.0, respectively. The primary production in estuary regions reached 500∼1,000 g/(m2 ·yr), in contrast to 100 g/(m2 ·yr) in continential shelf regions. The primary production had an important influence on the transfer of OC and on the other biogenic elements. The different microbial and plankton activities, different biogeochemical processes across particle-water interfaces, and different circumstances leading to the changes in forms and the transforms of phases of the biogenic elements C, N, P in the Changjiang River Estuary, further altered the surface compositions, properties of particles, and the components of the medium, and consequently influenced the stability of the colloidal particle. The biogeochemical process was an important factor and also one of the main mechanisms in regulating the flocculations of particles in the Changjiang River Estuary regions (Lin et al., 1995). About 50% of particle organic matter had been used up before they reached the 5 cm waters of the upper seabed in the Yongshu Reef and Zhubi Reef. The consuming rate was greater in the Yongshu Reef than that in China marginal sea continental shelf. The main source of organic matter in the Yongshu Reef was marine planktons (Duan et al., 1998). Upwelling in the Taiwan Strait regions led to net nutrient fluxes, higher nutrient consumption rates, and net production rates of particle organic matter and dissolved oxygen. POC in planktons provided 25% of total POC. POC from the aggradation in euphotic zones was about 2×10 g C/(m2 ·s), accounting for 35% of the primary production (Hong, 1994). In conclusion, plankton is very important in the recycling of setting particle, its production processes influenced the releases, burials, and transfers of biogenic elements in China’s seas. 1.3.5.2 Benthos Influences on the Recycling of Biogenic Elements At present, studies on the recycling of biogenic elements in protozoans and minitype animals are not enough, but studies on the predations of several macrobenthos have been very distinct. Most of the macrobenthos prey on suspensions or sediments, they incept the adsorded sediments (for instance,


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echinus species), and another species prey on deep-water particles (for instance, star worms). The benthos near marine sediments include microbes (such as bacteria and epiphytes), protozoans, minitype animals, macrobenthos, etc. Their influences on the recycling of biogenic elements in marine sediments are mainly in: (1) influencing the evolvement of waters and the transfers of nutrients; (2) influencing the stability and transfer of biogenic elements in sediments; (3) influencing the recycling of global biogenic elements. The benthos near the sediment-seawater interfaces sometimes irrigate their own caves by taking oxygen and food near sediment-seawater interfaces, thereby facilitating the mixing and exchanging of biogenic elements between surface interstitial waters and overlying waters (Song, 2000a). Preying processes were the basis for the study of benthos ecological functions and biogeochemical processes. Funguses could decompose ligncellulose and chitin, and bacteria could decompose particle OC, and most of the particles were diatoms and the animal residues. Hydrolyzing bacteria started this process and created dissolved organic carbons (DOC). DOC could be oxi4+ dized to CO2 by oxidants (O2 , NO− , Fe3+ , SO2− 3 , Mn 4 , etc.) in sedimentary circumstances, and could be decomposed by zymosis too. The most typical bacteria were the sulfate deoxidizing bacteria. Mn2+ , Fe2+ , and SO2− 4 could be oxidized by some bacteria during the oxidizing processes of OC. Because the processes provided less energy, the production was also limited. However, it was the process that finished the recycling of C, N, S, Fe, and Mn in sediments, and further formed their oxides. Benthos influenced and regulated the recycling of N, for example the irritations of benthos strongly worked on the contents and its vertical distribution + of NH+ 4 . In general, the concentrations of NH4 in interstitial waters increased with depth up to a depth of 5∼10 cm or 10∼15 cm while, reversely, the + production rates of NH+ 4 decreased with depth. NH4 from surface sediments diffused into overlying waters prior to that from deep layer waters, and the benthos irritations accelerated the process. The water depth where the benthos located was in direct proportion to the acting time when the sediments were buried. Thereby, in spite of the lower production rate of NH+ 4 in deeper waters, there was much time for the releasing and accumulating of NH+ 4 from the decomposition of organic matter. Biological processes reduced the gradients between sediment interstitial waters and their overlying waters, further − reducing the ionic diffusing fluxes of NH+ 4 and HCO3 calculated by gradient means. On the other hand, due to overestimating the quantity of NH+ 4 that entered overlying waters, the flux calculated by gradient means increased. It − further uncovered the relations between distributions of NH+ 4 and HCO3 and benthos animal mass (Song, 1997a; 2000a). The biogeochemical regeneration and transfer of P were two significant controlling factors in primary production, and the biological activities played an important role in the transfer of phosphoric forms. The releasing of P was especially important in estuaries and near-shore circumstances. The regeneration of nutrients in near-shore sediments evidently provided what primary

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production needed in the overlying waters. Planktons per se have certain depositing functions to N and P in waters. Due to the decompositions of biological detritus and excretions in the waters under water columns, nutrients such as P and others gained regeneration. Of the nutrients, the regeneration rates of active P commonly were 3.88 μg/(L·d), and the net biological utilization rates were 3.10 μg/(L·d), so the biological inputting and outgoing processes of P basically were in a state of balance (Hong, 1994). Through surveying in situ and laboratory cultivation, the greatest rates of biological adsorptions and excretions of P in Xiamen Bay and the Jiulong River Estuary in spring were 0.191 h−1 and 0.063 h−1 , respectively. The distributions of alkaline phosphatase (ALPase) activities in western Xiamen Harbour sediments and Hongkong Victoria sediments had a certain relation to the degree of pollution indicated by the microbial characteristic factors, and had a notable relation to the anaerobic circumstances of sediments (Hong, 1994). The fluxes of nutrients in the Minjiang River Estuary were about 3.3×1011 g for silicate, 7.7×1011 g for phosphate, 4.6×1010 g for DIN, respectively. The average primary production was about 192.4 mg/(m2 ·d) (8.59∼685 mg/(m2 ·d)). The photosynthesis of planktons was related to the nutrient mass, and C:N:P: Si=106:16:1:15. The consumption of nutrients accounted for 14.5% N, 53.6% P, and 1.9% Si of the total river inputs (Chen, 1997). The recycling of S in sediments was related to biological activities, and the existing forms of S were related to its environment. The reduction in sulfate and the oxidation of sulfide were the two main processes in which S was metabolized in sediments, and the bacteria played a key role in the processes. It was estimated that benthos played a main role in the accumulation of S in sediments by information recorded on the fossils. Such a function might be carried out by regulating dissolved oxygen and the content of unstable carbons. In the light of the whole world, S was not a limiting element, but it played an important role in the recycling and in the bacterial evolution. The distributions of S2− were mainly controlled by bacteria, the ratios of S2− /S2− 2 increased from estuary to continental shelf. The oxidation of S from S2− to S0 was also carried out by S bacteria, a kind of oxidizing bacteria, so the ratio in the profiles of sediments could be used as a measurement for of S2− /S2− 2 estimating the settling rates in modern oceanography (Qiu et al., 1999). It is obvious that biological production influences the releasing, burying, and transferring of biogenic elements. It is also an important controlling factor for the recycling of biogenic elements in sediments. Planktons and benthos participate in the biogeochemical processes of biogenic elements in marine sediments, and also influence the oceanic environments and global marine changes. In conclusion, the functions of China marginal sea sediments in the recycling of biogenic elements are very different in different regions, and are controlled by contents of organic matter, biological irritations, physical chemical environments, geology, water dynamics, etc. The redox environments of biogenic elements and the ecological characteristics near the sediment-seawater

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interfaces are two main factors regulating the recycling. In the future, studies should be concentrated in particular on the distributions, spatial and temporal variabilities, transferring mechanisms of biogenic elements in sediments, and their influences on ecological environments.

References Bao G (1992) The geochemical characters of some major elements in sediments of Hangzhou Bay and its adjacent sea areas. Acta Oceanol Sin 14(5):69-74 (in Chinese with English abstract) Bao XW, Wang XQ, Wu DX (2004) Hydrologic characteristics of the end of summertime in 2000 and early wintertime in 2001 in the Bohai Sea. Acta Oceanol Sin 26(1):14-24 (in Chinese with English abstract) Cai LS, Fang JG, Dong SL (2004) Preliminary studies on nitrogen and phosphorus fluxes between seawater and sediment in Sungo Bay. Mar Fish Res 25(4):57-64 (in Chinese with English abstract) Cai YY, Han WY, Wu LX (1997) Study on the organic carbon of the sea water in coral reef lagoons of the Nansha Islands. In: Studies on the Chemistry and Biology in Coral Reef Lagoons of the Nansha Islands. China Ocean Press, Beijing, pp.33-37 (in Chinese) Chen B, Duan JC, Mai BX, Luo XJ, Yang QS, Sheng GY, Fu JM (2006) Distribution of alkylphenols in the Pearl River delta and adjacent northern South China Sea, China. Chemosphere 63(4):652-661 Chen CTA (2008) Distributions of nutrients in the East China Sea and the South China Sea connection. J Oceanogr 64(5):737-751 Chen CTA, Liu CT, Pai SC (1994) Transport of oxygen, nutrients and carbonates by the Kuroshio current. Chin J Oceanol Limnol 12(3):220-227 Chen J, Lian F, Wiesner MG (1998) Estimations of primary production and export production in the South China Sea based on sediment trap experiments. Chin Sci Bull 43(7):583-586 Chen JF (1999) Vertical changes of particular organic carbon fluxes in South China Sea and indicator of early decomposition. Sci Chin 29(4):372-378 (in Chinese with English abstract) Chen L, Zhu W, Wang W, Zhou X, Li W (1998) Studies on climate change in China in the recent 45 years. Acta Meteorol Sin 12(1):257-271 (in Chinese with English abstract) Chen M, Huang YP, Chen FZ (1997a) Particle Dynamics in Euphotic Zone. IV. Isotope Marine Chemistry of Nansha Islands Waters. China Ocean Press, Beijing, pp.145-153 (in Chinese) Chen M, Huang YY, Chen FZ (1997b) Particle Dynamics in Euphotic Zone. II. Isotope Marine Chemistry of Nansha Islands Waters. China Ocean Press, Beijing, pp.123-133 (in Chinese) Chen ST (1997) Biogeochemical behavior of nutrients and their fluxes in the Minjiang River Estuary. Chin J Oceanol Limnol 15(2):150-155 Chen ST, Ruan WQ (1995) Chemical characteristic and estimation on the vertical flux of N, P, Si in upwelling area of Taiwan Strait. Acta Oceanol Sin 14(3):383392

128

1 Introduction

Chen SY, Tian ZL, Long AM, Wu YH (2005) Barium as a tracer to study the paleoproductivity variation of Nansha Islands sea area. Acta Oceanol Sin 27(4):55-60 (in Chinese with English abstract) Chen YLL (2005) Spatial and seasonal variations of nitrate-based new production and primary production in the South China Sea. Deep Sea Res I 52(2):319-340 Chen ZL, Liu J, Xu SY, Wang DQ, Zheng XM (2005). Impact of macrofaunal activities on the DIN exchange at the sediment-water interface along the tidal flat of Yangtze River Estuary. Environ Sci 26(6):43-50 (in Chinese with English abstract) Cheng J, Guo X (1998) Distribution and dynamic variations of species and quantity of benthos in the Bohai Sea. Mar Fish Res 19(1):31-42 (in Chinese with English abstract) Cheng SB, Liu M, Liu HL, Xu SY (2006) Distribution of Polychlorinated Biphenyls (PCBs) in suspended particulate matters from the Yangtze Estuarine and coastal areas. Environ Sci 26(1):110-114 (in Chinese with English abstract) Cheng XS, Qin XG, Xu R, Wang JH (2004) Determination of trace lead in sediment samples by flow-injection hydride generation atomic-fluorescence spectrometry using a slurry sampling technique. Mar Environ Sci 23(2):72-74 (in Chinese with English abstract) Chern CS, Wang J (2003) Numerical study of the upper-layer circulation in the South China Sea. J Oceanogr 59(1):11-24 Chiang KP, Kuo MC, Chang J, Wang RH, Gong GC (2002) Spatial and temporal variation of the Synechococcus population in the East China Sea and its contribution to phytoplankton biomass. Cont Shelf Res 22(1):3-13 Dai MH, Wei JF, Zhai WD (2001) An overview on the study of carbon biogeochemistry on the South China Sea. J Xiamen Univ (Nat Sci) 40(2):545-551 (in Chinese with English abstract) Deng J, Zhuang Z, Zhu J (1999) Study of dynamic characteristics of egg abundance and recruitment of penaeid prawn (Penaeus chinesis) in the Bohai Bay. Zool Res 20(1):20-25 (in Chinese with English abstract) Duan Y, Luo BJ, Xu YQ, Ma LH (1996) Composition and geochemical significance of biomarkers in marine sediments from Nansha Islands waters, the East China Sea. Oceanol Limnol Sin 27(3):258-263 (in Chinese with English abstract) Duan Y, Song JM, Cui MZ, Luo BJ (1997) Organic geochemical studies of sinking particulate material in China Sea area (II): Geochemical significance of compositional feature of keton, aldehyde and alcohol compounds. Chin Sci Bull 42(22):1894-1898 Duan Y, Song JM, Cui MZ, Luo BJ (1998) Organic geochemical studies of sinking particulate material in China Sea area (I): Organic matter fluxes and distributional features of hydrocarbon compounds and fatty acids. Sci Chin (Ser D) 41(2):208-214 Duan ZH, Xiao HL, Dong ZB (2002) Estimates of total CO2 output from desertified sandy land in China. Atmos Environ 35(34):5915-5921 Fang GH, Kwok YK, Yu KJ, Zhu YH (1999) Numerical simulation of principal tidal constituents in the South China Sea, Gulf of Tonkin and Gulf of Thailand. Cont Shelf Res 19(7):845-869 Fang JY, Piao SL, Zhao SQ (2001) The carbon sink: The role of the middle and high latitudes terrestrial ecosystems in the northern Hemisphere. Acta Phytoecol 25(5):594-602 (in Chinese with English abstract)

References

129

Feng S, Li F, Li S (1999) An Introduction to Marine Science. Higher Education Press, Beijing, p.503 (in Chinese) Furuya K, Hayashi M, Yabushita Y, Ishikawa A (2003) Phytoplankton dynamics in the East China Sea in spring and summer as revealed by HPLC-derived pigment signatures. Deep Sea Res II 50(2):367-387 Gao XJ, Chen ZL, Xu SY, Liu LY (2003) Environmental geochemical characteristics of phosphorus in intertidal sediments of the Yangtze estuarine and coastal zone. Acta Sci Circum 23(6):711-715 (in Chinese with English abstract) Graber HC, Beardsley RC, Grand WD (1989) Storm-generated surface waves and sediment resuspension in the East China and the Yellow Seas. J Phys Oceanogr 19(8):1039-1059 Guan BX (1986) Evidence for a counter-wind current in winter off the southeast coast of China. Chin J Oceanol Limnol 4(4):319-332 Han JB, Ma DY, Yan QL, Wang JY, Xu DY, Chen SM, Chen HX, Chen HX, Yan JC (2003) Biotoxicity of Zinc in the marine sediment to amphipod Grandidierella japontca. Environ Sci 24(6):101-105 (in Chinese with English abstract) Han WY (1998) Marine Chemistry of South China Sea. China Ocean Press, Beijing, pp.1-289 (in Chinese) Han WY, Lin HY, Cai YY (1996) Study on the carbon flux in the South China Sea. Acta Ocean Sin 15(1):85-92 Hao YM, Li CL, Luo XJ, Mai BX, Sheng GY, Fu JM (2006) Free and bound PAHs in a sediment core from Nam Van artificial lake of Macao. Environ Sci 27(2):235-240 (in Chinese with English abstract) Hao YM, Mai BX, Li CL, Sheng GY, Fu JM, Wang ZS, Deng YH (2004) Primarily study of alkylphenols in sediment core from Nam Van artificial lake of Macao. Environ Sci 25(4):129-133 (in Chinese with English abstract) Hong HS (1994) Marine Biogeochemistry Research papers (1986∼1993). Xiamen University Press, Xiamen, pp.2-125 (in Chinese) Hong HS, Wang DZ (2001) Studies on biogeochemical process of biogenic elements in the Taiwan Straits. J Xiamen Univ (Nat Sci) 40(2):535-544 (in Chinese with English abstract) Hong HS, Wang HL, Huang BQ (1995) The availability of dissolved organic phosphorus compounds to marine phytoplankton. Chin J Oceanol Limnol 13(2):169176 Hu JF, Peng PA, Mai BX, Zhang G, Yin KD (2005) Sources and relative contribution of organic matter in surficial sediments of Zhujiang River mouth. J Trop Oceanogr 24(1):15-20 (in Chinese with English abstract) Huang BQ, Hong HS, Wang DZ, Lin XJ, Zhang F, Liu Y (2002) Size fraction of phytoplankton biomass and productivity and photosynthetic carbon flow in Taiwan Strait. J Oceanogr Taiwan Strait 21(1):23-30 (in Chinese with English abstract) Huang F, Yang YX, Hung J (2008) Monsoon-ocean coupled modes in the South China Sea and their linkage with the eastern Indian Ocean-western Pacific Warm Pool. J Ocean Univ Chin 7(1):35-42 Jia CX, Liu GS, Xu MQ, Huang YP, Zhang J (2003) Radionuclides and minerals in surface sediments of Jiaozhou Bay. Oceanol Limnol Sin 34(2):490-498 (in Chinese with English abstract)

130

1 Introduction

Jian ZM, Wang LJ, Kienast M (1999) Late quaternary surface paleoproductivity and variations of the East Asian Monsoon in the South China Sea. Quat Sci 1:32-40 (in Chinese with English abstract) Jiang FH, Wang XL, Shi XY, Zhu CJ, Han XR (2003) Benthic exchange rates and fluxes of PO4 -P at the sediment-water interface in Jiaozhou Bay. Mar Sci 27(5):50-54 (in Chinese with English abstract) Jiang WS, Pohlmann T, S¨ undermanna J, Feng SZ (2000) A modelling study of SPM transport in the Bohai Sea. J Mar Syst 24(3-4):175-200 Jiang WS, Pohlmann T, Sun J, Starke A (2004) SPM transport in the Bohai Sea: Field experiments and numerical modelling. J Mar Syst 44(3-4):175-188 Jin X, Shi GY (2001) The limit to anthropogenic carbon emissions under IPCC scenarios. Clim Environ Res 6(1):125-131 (in Chinese with English abstract) Jin X, Tang Q (1998) The structure, distribution and variation of fishery resources in the Bohai Sea. J Fish Sci Chin 5(3):18-24 (in Chinese with English abstract) Jin XS, Zhao XY, Meng TX, Cui Y (2005) Biological Resources and Inhabitable Environment of Yellow and Bohai Seas. Science Press, Beijing, pp.1-445 (in Chinese) Kuo NJ, Zheng QA, Ho CR (2000) Satellite observation of upwelling along the western coast of the South China Sea. Remote Sensing Environ 74(3):463-470 Lee HJ, Chao SY (2003) A climatological description of circulation in and around the East China Sea. Deep Sea Res II 50(6-7):1065-1084 Li CH, Jia XP, Cai WG (2004) Diversity of marine zooplankton in the north of South China Sea. J Fish Sci Chin 11(2):139-146 (in Chinese with English abstract) Li CH, Zhang SZ, Wu LX (1997) Studies on the chemical composition of zooxanthellae from coral reef lagoons of the Nansha Islands. In: Studies on the Chemistry and Biology in Coral Reef Lagoons in the Nansha Islands. China Ocean Press, Beijing, pp.71-77 (in Chinese) Li CS (2001) Biogeochemical concepts and methodologies development of the DNDC model. Quat Sci 21(2):89-99 (in Chinese with English abstract) Li GX, Yue SH, Zhao DB, Sun YT (2004) Rapid deposition and dynamic processes in the modern Yellow River mouth. Mar Geol Quat Geol 24(3):29-36 (in Chinese with English abstract) Li GX, Han XB, Yue SH, Wen GY, Yang RM, Kusky TM (2006) Monthly variations of water masses in the East China Seas. Cont Shelf Res 26(16):1954-1970 Li HY, Li JS, Ding FY, Cheng JH (2007) Distribution characreistics of Nemopilema nomurai (Scyphozoa: Rhizostomeae) and zooplankton in East China Sea. Chin J Ecol 26(12):1974-1980 (in Chinese with English abstract) Li M, Wei HP, Wang GQ, Ni JR (2005) The influence of environmental factors on the adsorption behavior of phosphate by suspended sediments from Changjiang Estuary. J Basic Sci Eng 13(1):19-25 (in Chinese with English abstract) Li PC, Song JM, Wu LR (1997) The vertical fluxes of sinking particulate organic carbon, nitrogen and five forms phosphorus in coral reef lagoons of the Nansha Islands. In: Studies on the Chemistry and Biology in Coral Reef Lagoons of the Nansha Islands. China Ocean Press, Beijing, pp.55-63 (in Chinese) Li RS, Yang H (2004) Adsorption and release of phosphorus in sediments of the Changjiang Estuary. Mar Environ Sci 23(3):39-42 (in Chinese with English abstract)

References

131

Li XB, Chen CQ, Shi P, Zhan HG, He QJ (2006) Estimation of primary production of South China Sea from 1998 to 2002 by remote sensing and its spatio-temporal variation mechanism. J Trop Oceanogr 25(3):57-62 (in Chinese with English abstract) Li XG, L¨ u XX, Sun YM, Li N, Yuan HM, Zhan TR, Song JM (2003) Relation of active iron and redox environments in the sediments of Bohai Sea. Mar Environ Sci 22(1):20-24 (in Chinese with English abstract) Li XG, Li N, Song JM (2004a) Determination for the different combined-form inorganic carbon in marine sediments. Chin J Anal Chem 32(4):425-429 (in Chinese with English abstract) Li XG, Song JM, L¨ u XX, Yuan HM, Zhan TR, Li N (2004b) The expression of redox environments in sediments outside of the Yellow River Estuary. Mar Sci Bull 23(4):25-31 (in Chinese with English abstract) Li XG, Song JM, Yuan HM, Li FY, Sun S (2005) High contents of biogenic silicate in Jiaozhou Bay sediments-evidence of Si-limitation to phytoplankton primary production. Oceanol Limnol Sin 36(6):92-98 (in Chinese with English abstract) Li XJ, Chen F, Liu J, Huang XH (2004) Distribution and its dissolution of carbonate in seafloor surface sediment in the western South China Sea. Geochimica 33(3):254-260 (in Chinese with English abstract) Li XZ, Wang JB, Wang HF, Zhang BL (2005) Secondary production of macrobenthos from the East China Sea. Chin J Appl Environ Biol 11(4):459-462 (in Chinese with English abstract) Li XZ, Li BQ, Wang HF, Wang SQ, Wang JB, Zhang BL (2007) Macrobenthic community characters of Zhubi Reef, Nansha Islands, South China Sea. Acta Zool Sin 53(1):83-94 (in Chinese with English abstract) Li Y, Yu JJ (1999) Geochemical characteristics of phosphorus near the Huanghe River Estuary. Chin J Oceanol Limnol 17(4):359-365 Li Y, Yu, ZM, Song XX (2006) Application of principal component analysis (PCA) for the estimation of source of heavy metal contamination in marine sediments. Environ Sci 27(1):137-141 (in Chinese with English abstract) Liang T, Wang LJ, Zhang CS, Ding LQ, Ding SM, Yan X (2003) Contents and their distribution pattern of rare earth elements in water and sediment of intertidalite. J Chin Rare Ear Soc 23(1):68-74 (in Chinese with English abstract) Lin CL, Su JL, Xu BR, Tang QS (2001) Long-term variations of temperature and salinity of the Bohai Sea and their influence on its ecosystem. Prog Oceanogr 49(1-4):7-19 Lin HY, Han WY, Wu LX (1997) Preliminary study on the carbon cycle in the Zhubi reef lagoons of the Nansha Islands. In: Studies on the Chemistry and Biology in Coral Reef Lagoons of the Nansha Islands. China Ocean Press, Beijing, pp.11-15 (in Chinese) Lin YA, Tang RY, Li Y, Dong HL, Guan XW, Chen YZ (1995) The relations between the biogeochemical characters of biogenic elements and their flocculations and sedimentations in Changjiang Estuary. Acta Oceanol Sin 17(5):65-72 (in Chinese with English abstract) Liu JP, Li AC, Xu KH, Velozzi DM, Yang ZS, Milliman JD, DeMaster DJ (2006) Sedimentary features of the Yangtze River-derived along-shelf clinoform deposit in the East China Sea. Cont Shelf Res 26(17-18):2141-2156

132

1 Introduction

Liu KK, Chao SY, Shaw PT, Gong GC, Chen CC, Tang TY (2002) Monsoonforced chlorophyll distribution and primary production in the South China Sea: Observations and a numerical study. Deep Sea Res I 49(8):1387-1412 Liu QY, Kaneko A, Su JL (2008) Recent progress in studies of the South China Sea circulation. J Oceanogr 64:753-762 Liu WC, Wang R (1997) Study on particulate organic carbon in the East China Sea. Oceanol Limnol Sin 28(1):43-47 (in Chinese with English abstract) Liu XJ, Xu XQ, Xie P (2000) Seasonal changes of dissolved and particulate organic carbon in Donghu Lake, China. Chin J Oceanol Limnol 18(4):372-377 Liu Y, Cheng JH (2008) A study on seasonal changes of the fish quantity distribution in the East China Sea and the Huanghai Sea by factor analysis. Acta Oceanol Sin 30(4):123-130 (in Chinese with English abstract) Liu ZL, Hu DX, Chu ZX (2008) Observation of currents in the southern Yellow Sea in the summers of 2001 and 2003. J Ocean Univ Chin 7(1):17-26 Lu B, Chen RH, Wang ZP, Zhu C, Vetter W (2005) The distribution pattern of persistent organic pollutants and their molecule stratigraphic records in the sediments from the Arctic Ocean. Acta Oceanol Sin 27(4):167-173 (in Chinese with English abstract) Lu BW, Wang R, Wang WQ (1997) Surface layer chlorophyll a concentration in different waters in the East China Sea in spring. Ocean Sci 5:53-55 (in Chinese with English abstract) Lu TS, Ji MH (1996) Studies on dissolved amino acids in seawater of Jiaozhou Bay. Oceanol Limnol Sin 27(2):124-130 (in Chinese with English abstract) L¨ u XX, Song JM, Yuan HM, Li XG, Zhan TR, Li N, Gao XL (2004a) The potential ecological roles of nitrogen in the surface sediments of the South Yellow Sea. Acta Ecol Sin 24(8):1635-1642 (in Chinese with English abstract) L¨ u XX, Song JM, Yuan HM, Li XG, Zhan TR, Li N, Gao XL (2004b) Geochemical characteristics of nitrogen in different grain size sediment from the southern Huanghai Sea. Acta Oceanol Sin 27(1):64-69 (in Chinese with English abstract) L¨ u XX, Song JM, Li XG, Yuan HM, Zhan TR, Li N, Gao XL (2005a) Geochemical characteristics and early diagenesis of nitrogen in the northern Yellow Sea sediments. Acta Geol Sin 79(1):119-123 (in Chinese with English abstract) L¨ u XX, Song JM, Yuan HM, Li XG, Zhan TR, Li N, Gao XL (2005b) Nutrient distribution and controlled factors at the surface sediment of the Changjiang Estuary and adjacent sea area. Mar Environ Sci 24(3):1-5 (in Chinese with English abstract) Luo MB, Lu JJ, Wang YL, Shen XQ, Chao M (2007) Horizontal distribution and dominant species of phytoplankton in the East China Sea. Acta Ecol Sin 27(2):5077-5085 (in Chinese with English abstract) Luo XJ, Chen SJ, Mai BX, Zeng YP, Sheng GY, Fu JM (2005) Distribution and sources of polycyclic aromatic hydrocarbons in sediments from rivers of Pearl River delta and its nearby South China Sea. Environ Sci 26(4):129-134 (in Chinese with English abstract) Ma CH, You K, Li FQ, Zhang MZ (2006) A Study on the relationship of the fish biodiversity and the faunal distribution in the South China Sea. Period Ocean Univ Chin 36(4):665-670 (in Chinese with English abstract) Ma HB, Song JM, L¨ u XX, Yuan HM (2003) Nitrogen forms and their functions in recycling of the Bohai Sea sediments. Geochimica 32(1):48-54 (in Chinese with English abstract)

References

133

Meng T (1998) The structure and variation of fish community in the Bohai Sea. J Fish Sci Chin 5(2):16-20 (in Chinese with English abstract) Naimie CE, Blain CA, Lynch DR (2001) Seasonal mean circulation in the Yellow Sea–A model-generated climatology. Cont Shelf Res 21(6-7):667-695 Ni JY, Maggiulli M, Liu XY, Wang FG, Zhou HY (2005) Pore water distribution and quantification of diffusive benthic fluxes of silicate, nitrate and phosphate in surface sediments of the equatorial northeastern Pacific. Geochimica 34(6):587594 (in Chinese with English abstract) Ni JY, Liu XY, Chen QJ, Lin YA (2006) Pore-water distribution and quantification of difusive benthic fluxes of nutrients in the Huanghai and East China Seas sediments. Acta Oceanol Sin 25(1):90-99 Qi J, Li FY, Song JM (2005) Distribution of 210 Pb activity of sediments in the Yellow and Bohai Seas. Geochimica 34(4):351-356 (in Chinese with English abstract) Qi XH, Liu SM, Zhang J, Chen HT (2003) Nutrients regeneration speed of sediment in harmful algae blooms (HAB) area of East China Sea. Chin J Appl Ecol 14(7):1112-1115 (in Chinese with English abstract) Qi XH, Liu SM, Zhang J (2006) Sediment-water fluxes of nutrients in the Yellow Sea and the East China Sea. Mar Sci 30(3):9-15 (in Chinese with English abstract) Qin Y, Li F (1982) Study of SPM in the Bohai Sea. Acta Oceanol Sin 4(2):191-200 (in Chinese with English abstract) Qin YS, Zhao YY, Chen LR, Zhao SL (1985) The Geology of the Bohai Sea. China Ocean Press, Beijing, pp.3-200 (in Chinese) Qin YS, Zhao YY, Chen LR, Zhao SL (1987) The Geology of the East China Sea. Science Press, Beijing, pp.1-290 (in Chinese) Qin YS, Zhao YY, Chen LR, Zhao SL (1989) The Geology of the Yellow Sea. China Ocean Press, Beijing, pp.1-289 (in Chinese) Qiu JW, Wang ZD, Gao HJ, Peng YH (1999) Diffusive fluxes of nutrients across sediment-seawater in cultural sea area of Daya Bay. Trop Oceanol 18(3):83-90 (in Chinese with English abstract) Song JM (1997a) Chemistry of Sediment-Seawater Interface of China Sea. China Ocean Press, Beijing, pp.1-222 (in Chinese) Song JM (1997b) Biogeochemical process of major elements in sinking particulate of Nansha coral reef lagoons, South China Sea. Acta Oceanol Sin 16(4):557-562 Song JM (1999) Progress in the studies of marine biogeochemical process in China. In: 1995-1998 China National Report on Physical Sciences of the Oceans and on Hydrologic Science for the XXIInd General Assembly of IUGG. Birmingham, UK, July 1999. China Meteorological Press, Beijing, pp.73-84 (in Chinese) Song JM (2000a) Functions of marine sediments speciles diversity in biogenic element cycling. Mar Sci 24(4):22-26 (in Chinese with English abstract) Song JM (2000b) Transferable phosphorus in sediments of the Huanghe River Estuary adjacent waters. Mar Sci 24(7):42-45 (in Chinese with English abstract) Song JM (2000c) Marine Chemistry in China. China Ocean Press, Beijing, pp.1210 (in Chinese) Song JM (2004) Biogeochemistry of China Seas. Shandong Science and Technology Press, Jinan, pp.1-591 (in Chinese)

134

1 Introduction

Song JM, Li PC (1996) –2 valence sulfur of lagoon and off-reef sediment environment in the South China Sea. Oceanol Limnol Sin 27(6):597-603 (in Chinese with English abstract) Song JM, Li PC (1998) Vertical transferring process of rare elements in coral reef lagoons of Nansha Islands, South China Sea. Sci Chin (Ser D) 41(1):42-48 Song JM, Li PC, Wu LX (1997a) Diffusion fluxes of N, P, Si across sediment-water interface in the coral reef ecosystem of the Nansha Islands. In: Studies on the Chemistry and Biology in Coral Reef Lagoons of the Nansha Islands. China Ocean Press, Beijing, pp.84-92 (in Chinese) Song JM, Zhan BQ, Li PC (1997b) Dissolved oxygen distribution and O2 fluxes across the sea-air interface in the East China Sea waters. Chin J Oceanol Limnol 14(4):228-232 Song JM, Ma HB, L¨ u XX (2002) Nitrogen forms and decomposition of organic carbon in the southern Bohai Sea core sediments. Acta Oceanol Sin 21(1):125133 Song JM, Luo YX, L¨ u XX, Li PC (2003) Forms of phosphorus and silicon in the natural grain size surface sediments of the southern Bohai Sea. Chin J Oceanol Limnol 21(3):286-292 Song JM, Ma HB, Li XG, Yuan HM, Li N (2004) Geochemical characteristics of adsorbed inorganic nitrogen in the south Bohai Sea sediments. Oceanol Limnol Sin 35(4):315-322 (in Chinese with English abstract) Song JM, Xu YF, Hu WP, Ni LY (2008) Biogeochemistry of Carbon in China’s Seas and Lakes. Science Press, Beijing, pp.1-553 (in Chinese) Sun WX, Xi PG (1988) The results of the 3D nonlinear tidal boundary problem and its analysis. J Shandong College Oceanogr 18(2):51-52 (in Chinese with English abstract) Sun YM, Song JM (2002) Functions of China’s marginal sea sediments in the cycles of biogenic elements. Mar Environ Sci 21(1):26-33 (in Chinese with English abstract) Sun YM, Song JM (2003) Progress on biogeochemical processes of marine carbon cycles in China (1998-2002). In: 1999-2002 China National Report on Geodesy and Geophysics for the 23rd General Assembly of IUGG. China Meteorological Press, Beijing, pp.116-128 (in Chinese) S¨ undermanna J, Feng SZ (2004) Analysis and modelling of the Bohai Sea ecosystem: A joint German-Chinese study. J Mar Syst 44(3-4):127-140 Tian T, Wei H, Su J, Zheng CS, Sun WX (2003) Study on cycle and budgets of nutrients in the Yellow Sea. Adv Mar Sci 21(1):1-11 (in Chinese with English abstract) Valle-Levinson A, Matsuno T (2003) Tidal and subtidal flow along a cross-shelf transect on the East China Sea. J Oceanogr 59(55):573-584 Wang BD, Zhan R, Xu MD (2002) Molar radios of C, N, P of particulate matter and their vertical fluxes in the Yellow Sea. Chin J Oceanol Limnol 20(1):90-96 Wang CH (1995) Geochemistry of Sediments in East Sea Seabed. China Ocean Press, Beijing, pp.59-72 (in Chinese with English abstract) Wang DQ, Chen ZL, Wang J, Xu SY, Yang HX, Chen H, Yang LY, Hu LZ (2006) Denitrification, nitrous oxide emission and adsorption in intertidal flat, Yangtze Estuary, in summer. Geochimica 35(3):271-279 (in Chinese with English abstract)

References

135

Wang J, Kang Y (1998) Study on population dynamics of phytoplankton in the Bohai Sea. Mar Fish Res 19(1):43-52 (in Chinese with English abstract) Wang JT, Zhao WH, Tan LJ, Zhang ZB (2000a) A comparison of HTC method and UV/persulphate method to determine colloidal organic carbon in sea water. Acta Oceanol Sin 22(2):67-70 (in Chinese with English abstract) Wang JT, Zhao WH, Zhang ZB (2000b) Adsorption of natural dissolved organic carbon in seawater on needle ferric mineral. Oceanol Limnol Sin 31(3):309-313 (in Chinese with English abstract) Wang P, Min Q, Gao J (1985) A preliminary study of foraminiferal and ostracod assemblages in the Huanghai Sea. In: Marine Micropaleontology of China. China Ocean Press, Beijing, pp.115-132 (in Chinese) Wei H, Tian T, Zhou F, Zhao L (2002) Numerical study on the water exchange of the Bohai Sea: Simulation of the half-life time by dispersion model. J Ocean Univ Chin 32(4):519-525 (in Chinese with English abstract) Wei H, Sun J, Moll A, Zhao L (2004) Phytoplankton dynamic in the Bohai Sea– Observations and modelling. J Mar Syst 44(3-4):233-251 Wei ZX, Li CY, Fang HG, Wang YX (2003) Numerical diagnostic study of the summertime circulation in the Bohai Sea and the water transport in the Bohai Strait. Adv Mar Sci 21(4):454-464 (in Chinese with English abstract) Wu LX, Li HY (1997) Contribution of alga to the coral reef lagoon’s primay productivity of the Nansha Islands by determining oxygen with Bell-Jar process. In: Studies on the Chemistry and Biology in Coral Reef Lagoons of the Nansa Islands. China Ocean Press, Beijing, pp.78-83 (in Chinese) Wu LX, Wang HK, Li PC, Song JM (1997) The vertical fluxes of sedimentary particles in coral reef lagoons of the Nansha Islands. In: Studies on the Chemistry and Biology in Coral Reef Lagoon of the Nansha Islands. China Ocean Press, Beijing, pp.43-48 (in Chinese) Wu QH, Jia HL (2005) The grain-size distribution of organochlorine pesticides in sediments. J Guangzhou Univ (Nat Sci Ed) 4(1):40-45 (in Chinese with English abstract) Wu ZM, Yu GY (1999) A pelagic ecosystem model and simulation of the northern part of Jiao Zhou Bay. I. A simulation study on the pelagic ecosystem seasonal variations. J Ocean Univ Chin 29(3):429-435 (in Chinese with English abstract) Xing RN, Wang ZG (2001) Numerical modeling of the response of tropical Pacific surface water to event. Acta Meteorol Sin 59(3):308-316 (in Chinese with English abstract) Xu BD, Jin XS (2005) Variations in fish community structure during winter in the southern Yellow Sea over the period 1985∼2002. Fish Res 71(1):79-91 Xu JR, Wang YS, Yin JP, Wang QJ, Zhang FQ, He L, Sun CC (2005) Transformation of dissolved inorganic nitrogen species and nitrification and denitrification processes in the near sea section of Zhujiang River. Acta Sci Circum 25(5):686692 (in Chinese with English abstract) Xu ZL, Chao M, Chen YQ (2004) Distribution characteristics of zooplankton biomass in the East China Sea. Acta Oceanol Sin 26(3):93-101 (in Chinese with English abstract) Yan J, Li J (1997) The climate change in the last 100 years over the East China Sea and its adjacent region. Acta Oceanol Sin 19(6):121-126 (in Chinese with English abstract)

136

1 Introduction

Yang GP, Zhang ZB, Liu LS, Liu XT (1996) Study on the analysis and distribution of dimethyl sulfide in the East China Sea. Chin J Oceanol Limnol 14(2):141-147 Yang QS, Mai BX, Fu JM, Sheng GY, Luo XJ, Lin Z (2004a) Fluxes of persistent organic pollutants from Humen tidal channel of pearl river to Lingdingyang Estuary. Sci Geogr Sin 24(6):704-709 (in Chinese with English abstract) Yang QS, Ou SY, Xie P, Mai BX, Fu JM, Sheng GY (2004b) Distribution and seasonal changes of polycyclic aromatic hydrocarbons (PAHs) in surface water from Humen tidal channel. Acta Oceanol Sin 26(6):37-48 (in Chinese with English abstract) Yang SY, Jung HS, Lim DI, Li CX (2003) A review on the provenance discrimination of sediments in the Yellow Sea. Earth Sci Rev 63(1-2):93-120 Yao QZ, Zhang J, Qin XG, Dong LX (2006) The behavior of selenium and arsenic in the Zhujiang (Pearl River) Estuary, South China Sea. Estuar Coast Shelf Sci 67(1-2):170-180 Ye XW, Liu SM, Zhao YF, Zhang J (2006) The distribution of biogenic silica in the sediments of the East China Sea and the Yelow Sea and its environmental signification. Chin Environ Sci 24(3):265-269 (in Chinese with English abstract) Yuan HM, L¨ u XX, Li XG, Li N, Sun YM, Zhan TR, Song JM (2003) Geochemical factors of organic carbon in natural grain size sediments of Bohai Sea. Environ Chem 22(2):115-120 (in Chinese with English abstract) Yuan YC, Liu YG, Liao GG, Lou RY, Su JL, Wang KS (2005) Calculation of circulation in the South China Sea during summer of 2000 by the modified inverse method. Acta Oceanol Sin 24(1):14-30 (in Chinese with English abstract) Yue WZ, Huang XP (2005) Distribution characteristics of phosphorus in core sediments from Zhujiang River Estuary and its environmental significance. J Trop Oceanogr 24(1):21-27 (in Chinese with English abstract) Zhang B, Tang Q, Jin X (2007) Decadal-scale variations of trophic levels at high trophic levels in the Yellow Sea and the Bohai Sea ecosystem. J Mar Syst 67(34):304-311 Zhang H, Sun F (1996) Dynamics analysis for flux of carbon dioxide across seasurface. Acta Oceanol Sin 14(4):447-455 Zhang JS, Shen ZL (1997) A study of change in nutrient structure of Jiaozhou Bay. Oceanol Limnol Sin 28(5):535-541 (in Chinese with English abstract) Zhang SZ, Li CH (1997) The nutrient role of microzooplankton in the lagoons ecosystem from coral reefs of the Nansha Islands. In: Studies on the Chemistry and Biology in Coral Reef Lagoons of the Nansha Islands. China Ocean Press, Beijing, pp.64-70 (in Chinese) Zhang ZB, Wang JT, Liu LS (1997) Dissolved organic carbon in the East China Sea in autumn. Chin J Oceanlo Limnol 15(2):137-144 Zhao WH, Wang JT, Jiao NZ, Zhao ZX (2001) Dissolved and particulate organic carbon in Yantai Sishili Bay aquiculture waters. Chin J Oceanol Limnol 19(2):175-185 Zhao YF, Liu SM, Ye YW, Zhang J (2005) The analysis of biogenic silica in the sediments of the East China Sea and the Yellow Sea. Period Ocean Univ Chin 35(3):423-428 (in Chinese with English abstract) Zhao ZM, Zhang L, Zheng BH, Guo KQ, Qin YW, Wang Y (2005) Spatial distribution of phosphorus and nitrogen in core sediments of Bohai Bay. J Northwest Sci Tech Univ Agric For (Nat Sci Ed) 33(4):107-111 (in Chinese with English abstract)

References

137

Zhou FX, Wang ZT (1999) Phase discontinuity of the SST interannual oscillation between the South China Sea and its adjoining west Pacific region. Oceanol Limnol Sin 30(6):679-686 (in Chinese with English abstract) Zhou H, Zhang ZN, Liu XS, Tu LH, Yu ZS (2007) Changes in the shelf macrobenthic community over large temporal and spatial scales in the Bohai Sea, China. J Mar Syst 67(3-4):312-321 Zhu CJ, Shi XY, Li T (1999) The distribution and regeneration of phosphate in the East China Sea. In: Hu D, Tsunogai S (eds.) Margin Flux in the East China Sea. China Ocean Press, Beijing, pp.219-227 (in Chinese) Zhu MH, Ding YS, Zheng DC, Tao P, Qu YJ, Cui Y, Gong WM, Ding DW (2005) Bioavailability and distribution of the familiar heavy metals around rhizosphere sediment of Suaeda heteroptera. Mar Environ Sci 24(3):21-24 (in Chinese with English abstract) Zuo T, Wang R, Chen YZ, Gao SW, Wang K (2005) Net macro-zooplankton community classification on the shelf area of the East China Sea and the Yellow Sea in spring and autumn. Acta Ecol Sin 25(7):1531-1540 (in Chinese with English abstract)

2 Biogeochemical Processes of the Bohai Sea

Abstract: In this chapter the biogeochemical processes in the Bohai Sea are described. The main contents deal with the distributions, transformation, and their controlling factors of carbon, nitrogen, phosphorus, silicon, and pollutants such as heavy metals and persistent organic pollutants (POPs) in the seawaters and sediments. In the Bohai Sea, human activity and riverine input are the most important controlling processes in the variation of biogenic elements and pollutants. The Bohai Sea is a semi-closed marine area of the NW Pacific Ocean, with a surface area of 77×109 m2 and an average depth of 18.7 m. The area with a depth of less than 30 m constitutes 95% of the total area of the sea (Fig. 2.1, Mao et al., 2008). The water depth of the Bohai Sea is shallow at 10∼20 m

Fig. 2.1. Bathymetry of the Bohai Sea. Boundaries between different parts of the Bohai Sea are indicated by solid lines. Isobaths are in meters (Mao et al., 2008) (With permission from Elsevier’s Copyright Clearance Center)

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in coastal areas, including Liaodong Bay, Bohai Bay, Laizhou Bay, central Bohai, and the Bohai Strait. The maximal water depth of 70 m is found in the northern part of the Bohai Strait. The rivers discharge into the Bohai Sea with various amounts of water and sediment loads, notably the Liaohe River, Shuangtaizihe River, Luanhe River, and Huanghe River (Yellow River).

2.1 Change Processes of Carbon in the Bohai Sea The continental shelf zones are usually active in biological production as summarized in IGBP and may have a significant role in the net absorption of atmospheric CO2 , although it comprises only 7.6% of the surface area of the world oceans. In general, the coastal oceans tend to absorb CO2 in winter, when the water cools, and in spring, as a consequence of biological processes. In summer and fall, the processes of warming, respiration of marine organisms, and decomposition of organic matter release CO2 back into the atmosphere. Finally, direct and indirect human perturbations to the continental margins (e.g., pollution, eutrophication) are large and have dire consequences for marine ecosystems. Unfortunately, owing to the diversity and therefore complexity of the shelf systems, their precise roles in the carbon cycle have yet to be quantified with any degree of certainty. There is still, in fact, no consensus on the simple question posed by Land-Ocean Interaction in the Coastal Zone project (LOICZ) in its first report: “Are continental shelves carbon sources or sinks?” 2.1.1 Partial Pressure of CO2 in Sea Water The surface water CO2 partial pressure (PCO2 ) at grid stations in the Bohai Sea in August, 2006 and in Laizhou Bay in July, 2005, was used to discuss the distribution of the PCO2 (Fig. 2.2, Zhang and Zhang, 2008). 2.1.1.1 Distribution of PCO2 in Surface Waters Fig. 2.3 (Zhang and Zhang, 2008) shows the distribution of PCO2 in the Bohai Sea in summer. The magnitude of PCO2 ranged from 313 to 1,118 μatm, and the mean was 537 μatm. PCO2 values in the central area were a minimum and a maximum of 313 and 621 μatm, respectively, with an average value of 435 μatm. Bohai Bay, Liaodong Bay, and Laizhou Bay all showed a higher level of PCO2 and acted as a source of CO2 . The region wholly acted as a source of CO2 except for the area along the western and central shore of the sea (38.5◦ ∼40◦ N, 119◦ ∼120.5◦ E) and the east of Liaodong Bay (39.9◦ ∼40.1◦ N, 120.7◦ ∼121.2◦ E) which acted as a sink of CO2 and the sum of the sink area accounted for one fifth of the whole area. Estuaries are known for significant supersaturation of CO2 with respect to the atmosphere. The value of PCO2 in the Huanghe River Estuary and

2.1 Change Processes of Carbon in the Bohai Sea

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N 40.5

39.5

38.5

37.5 117.5

118.5

119.5

120.5

121.5

122.5

E

Fig. 2.2. Sampling stations in the Bohai Sea (Zhang and Zhang, 2008) (With permission from Zhang LJ) N 40.5

39.5

38.5

37.5 117.5

118.5

119.5

120.5

121.5

122.5

E

Fig. 2.3. Horizontal distribution of PCO2 (in μatm) in the Bohai Sea in summer (Zhang and Zhang, 2008) (With permission from Zhang LJ)

Laizhou Bay (622∼950 μatm) was much higher than that in other areas and gradually decreased from the estuary to the east, which showed a distinct gradient distribution. The highest value of PCO2 existed near the estuary with the salinity lower than 24‰. The mechanism by which estuarine systems can sustain such high levels of PCO2 remains unclear. Abril et al. (2000) indicated that heterotrophic activity and acidification due to nitrification within the estuarine zone were major factors in the total estuarine emission to the atmosphere, while the excess CO2 transport by rivers followed by ventilation in the estuary was a minor one. Cai and Wang (1998) believed that the combined effects of pelagic and benthic respiration, photodegradation, and the mixing of seawater and acidic river water were insufficient to sustain the high PCO2 values and then high water-to-air fluxes in the estuaries they studied, and they suggested that the CO2 input from organic carbon respiration in tidally flooded salt marshes controlled the CO2 concentration. This explanation is consistent with a subsequent mass balance study of biogenic gases (Cai et al., 1999). Therefore, PCO2 distribution pattern in the estuary results from

142


the combination of various processes: the production/degradation/export of organic carbon, the production/dissolution/export of carbonates, the input of dissolved inorganic carbon by vertical mixing processes and/or freshwater runoff and the thermodynamic effects related to both water temperature variations and water mass mixing (Song, 2004; Song et al., 2008). In general, PCO2 in the Bohai Sea showed an uneven distribution which fully reflected the impact of riverine input on Bohai Sea waters although the whole Bohai area is not very large. 2.1.1.2 Relationship between PCO2 and Temperature, Salinity and Chl a Temperature and salinity are two important controlling factors of PCO2 in seawaters. Fig. 2.4 showed that, in the central area, the surface water salinity was between 31.0‰ and 31.6‰, while it was below 30‰ in Laizhou Bay and Liaodong Bay due to the effect of the freshwater discharge. The whole Bohai Sea can be divided into two parts according to the salinity distribution. One part is that with salinity higher than 31‰ (S >31‰) and another part is that with salinity lower than 31‰ (S 31‰ area, PCO2 showed a significant positive correlation with salinity (R 2 =0.705, n=28; Fig. 2.5a). This may be due to low PCO2 in the midwest area where intense phytoplankton bloom can consume significant amounts of dissolved CO2 and high PCO2 in the old Huanghe River subaqueous delta. In the S 31‰ area, PCO2 all showed positive correlation with temperature both in the sink area (R 2 =0.594, N 40.5

39.5

38.5

37.5 117.5

118.5

119.5

120.5

121.5

122.5

E

Fig. 2.4. Horizontal distribution of salinity (‰) in the Bohai Sea in summer (Zhang and Zhang, 2008) (With permission from Zhang LJ)


143

n=13; Fig. 2.6a) and in the source area (R 2 =0.520, n=17; Fig. 2.6b). The correlation coefficient in the source area was higher than that in the sink area, which indicated that PCO2 in the source area were more inclined to be influenced by temperature than in the sink area. In the S 31‰, source area; (c) Salinity31‰ area, PCO2 showed a significant negative correlation with salinity except for a maximum of Chl a in station D2 (R 2 =0.738, n=31; Fig. 2.7). In the sink area, the content of Chl a was high, with a maximum of 2.4 μg/L, and the transparency was higher than the average level for the Bohai Sea (4 m). In this situation, the photosynthesis of phytoplankton is strong and can adsorb more CO2 in surface water. y = 182.95 x +800.88 R 2=0.738

700

500

2

P CO ( m atm)

600

400 300 200 1

2

3

Chl a (mg/m 3)

Fig. 2.7. Relationship between PCO2 and Chl a in the Bohai Sea in summer (Zhang and Zhang, 2008) (With permission from Zhang LJ)

2.1.2 Riverine Sources and Estuarine Fates of Particulate Organic Carbon in Seawaters The scientific database of organic carbon regimes for large world rivers has been greatly enhanced in the last 10∼20 years. Indeed, the organic materials transported by large rivers can exert a significant impact on the carbon cycle and ecosystems in coastal and shelf regions. The organic material carried by small- and middle-size rivers, however, may affect the marine ecosystems and environment on a more regional scale. The Luanhe River and Shuangtaizihe River are two major freshwater sources from North China draining into the Bohai Sea. The long-term averaged water discharge, sediment load, and the general features of these rivers and estuaries are summarized in Table 2.1 (Zhang et al., 1998a). Zhang et al. (1998a) reported the following POC results in suspended particulates from the Luanhe River Estuary and Shuangtaizihe River Estuary.


145

Table 2.1. Drainage area, water discharge, sediment load and description of estuarine features for the Luanhe River and Shuangtaizihe River (Zhang et al., 1998a) (With permission from Elsevier’s Copyright Clearance Center) Water Sediment Drainage Estuarine features discharge load area (km2 ) (×109 m3 /yr) (×106 t/yr) Luanhe 54,412 6 20 The estuary has several shoals and sand-bars. The tidal range is 1∼2 m with currents of 0.5∼1 m/s. The tide is limited to 5∼10 km inland from the river mouth Shuang- 57,104 5.1 9.1 The estuary consists of two taizihe branches that join at the upper estuary. The tidal range is 3∼4 m on average, with currents of 1.0∼2.0 m/s. The area influenced by the tides is limited to 30∼40 km inland from the river mouth River

2.1.2.1 Particulate Organic Carbon In the Luanhe River Estuary, the organic carbon in suspended sediments shows an overall decrease with increasing chlorinity, indicating a rather simple dilution of riverine POC (1.3%∼2.4%) by local organic poor particle populations, although the data show considerable scatter (Fig. 2.8, Zhang et al., 1998a). A similar distribution has been found for the absolute concentration of POC (mg/L), which reduces from ∼2 mg/L in the river to 0.5 mg/L in the lower estuary (Fig. 2.8). It is difficult, however, to determine the real marine POC end-members since the sampling was ended at a chlorinity of 14‰∼15‰. Clearly, the POC levels in weight percentage in the Shuangtaizihe River Estuary remain low and relatively stable, varying between 1.1% and 1.6% over the whole salinity range sampled, except for one sample collected upstream of the Raoyanghe River which has a POC value of 6.7% indicating a different freshwater organic pool with low total suspended matter of 50 mg/L before joining the main stream (Fig. 2.9, Zhang et al., 1998a). A plot of absolute POC concentration in solution (i.e., mg/L) against chlorinity indicates a rapid removal of particulate organic materials at early stages of mixing between fresh and marine waters (Fig. 2.9). However, a value of 5∼10 mg/L for POC is typical in the lower estuary with chlorinity 10‰∼15‰, corresponding to a concentration of suspended sediments well above 100 mg/L. The absolute POC concentration (i.e., mg/L) in the Shuangtaizihe River Estuary can be up to a factor of 100 higher than in the Luanhe River Estuary (comparing Fig. 2.9 with Fig. 2.8).

146

2 Biogeochemical Processes of the Bohai Sea 2.5

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Fig. 2.8. Distribution of POC in the Luanhe River Estuary. (a) POC (%) vs chlorinity; (b) POC (mg/L) vs chlorinity; (c) POC (%) vs turbidity; (d) POC (mg/L) vs turbidity (Zhang et al., 1998a) (With permission from Elsevier’s Copyright Clearance Center)

Fig. 2.9. Distribution of POC in the Shuangtaizihe River Estuary. (a) POC (%) vs chlorinity; (b) POC (mg/L) vs chlorinity; (c) POC (%) vs turbidity; (d) POC (mg/L) vs turbidity (Zhang et al., 1998a) (With permission from Elsevier’s Copyright Clearance Center)


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2.1.2.2 Relationship between POC and Suspended Matter The plots of POC in weight-percent of suspended matter vs total suspended matter concentrations in the estuaries demonstrate a general reverse relationship between these two parameters, at least at the low end of the turbidity range (Figs. 2.8, 2.9). However, the absolute concentrations of POC (mg/L) in these two estuaries show a strongly positive relationship with suspended matter loads (Figs. 2.8, 2.9). The average POC to suspended sediment ratio is 0.013 for the Shuangtaizihe River Estuary and 0.015 for the Luanhe River Estuary. The correlation coefficient (R2 ) for the relationship between POC and total suspended matter (TSM) is 0.99 for the Shuangtaizihe River and 0.60 for the Luanhe River, which again indicates that POC distribution is regulated by the concentration of suspended sediments in the Bohai Sea estuaries. 2.1.2.3 Source of POC in the Estuary The POC can be reduced to 1.5%∼2.5% or lower while the suspended sediment concentrations exceed 50∼100 mg/L (Figs. 2.8 and 2.9). A value of 1.5%∼2.0% is typical of organic carbon concentrations in soils (1.3%∼1.8%) from drainage areas of these rivers. We hypothesize, however, that in low turbidity waters, photosynthesis can be an important contributor to the observed POC. The datasets obtained show that at a concentration greater than 50∼100 mg/L for suspended sediments, photosynthesis is probably strongly reduced, and the POC in the river is hence regulated by the organic materials supplied from soil erosion, with in situ primary production being in a state of radiation limitation (Figs. 2.8 and 2.9). While photosynthetic carbon may play a role in regulating POC in the estuary, there are still no existing datasets or other evidence to examine this hypothesis. The positive POC-TSM relationship in these estuaries suggests presumably a common terrigenous source for most of the POC. Further offshore, the POC value of ∼2 mg/L indicates a rather important input from the marine pool. The distribution of POC clearly illustrates distinctive processes and events taking place in both estuaries, with a simple dilution of the terrigenous organic pool by organic poor particles in the Luanhe River and a rather stable POC level irrespective of chlorinity in the Shuangtaizihe River. In both estuaries, the POC remains low and somewhat stable when the amount of suspended sediments is superior to 100 mg/L, presumably indicating a radiation limitation for in situ photosynthesis at these turbidity levels. 2.1.3 Inorganic Carbon in Liaodong Bay Sediments of the Bohai Sea The ocean is evidently a major sink of carbon dioxide and plays an important role in the global carbon cycle. However, the carbon flux between seawater and sediment in the coastal seas is still poorly understood (Song, 2003; Li

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et al., 2004a). Knowledge of the carbonate mineral dissolution in sediment during the processes of diagenesis, lithification, and evolution is of central importance to develop an insight into the carbon flux (Morse and Arvidson, 2002). The pattern of calcium carbonate accumulation rates can be used to decipher the Pliocene-Pleistocene history of biogenic production and its relationship with global and local changes in oceanic circulation and climate (Beek et al., 2004). Calcium carbonate dissolution or precipitation is controlled mainly by the bottom water (or pore water) saturation state, sediment pH, and metabolic release of carbon dioxide. Moreover, salinity also affects the chemical reactions that occur in sediment, like precipitation/dissolution of CaCO3 . Inorganic carbon (IC) concentrations in sediment vary little, or increase with depth due to the dissolution near the water-sediment interface. The direct tracer of CaCO3 dissolution is the increase in the calcium concentration of the pore water below the sediment-water interface. Most studies focused on the total inorganic carbon in sediment, few on IC forms. In fact, there are many kinds of carbonate mineral in sediment, such as aragonite, siderite, calamine, cerusite, phosgenite, magensite, and dialogite (Preda and Cox, 2004) with different dissolvabilities under different pH solutions; for example, calamine can be dissolved in NH3 ·H2 O cerusite, phosgenite can be dissolved in NaOH, and calcite and aragonite can be dissolved in acid. To understand the burial and diagenesis of inorganic carbon in marine sediments, it is necessary to identify, separate, and quantify the various solid-phase reservoirs of deposited carbon, but it is very difficult because of the fine-grained nature of most marine sediments. So it is necessary to find an indirect means to determine the identity and size of sedimentary IC reservoirs. One approach is physical separation of different sedimentary fractions by grain size, and measurement of total inorganic carbon (TIC) in the different fraction after each has been separately dissolved (Yang et al., 2002). However, this method can easily lead to ambiguous or incorrect identification of the C-bearing phase. Complete physical separation of different phases from fine-grained sediment rarely can be achieved and surface coating of various sorts, potentially important in IC removal to sediments, can remain undetected and unidentified in such treatment. The most promising methods for separating and quantifying the various IC reservoirs in marine sediment are by sequential extraction. So according to IC characters, IC in sediment is divided into five forms: NaCl form, NH3 ·H2 O form, NaOH form, NH3 OH·HCl form, and HCl form (Fig. 2.10, Li et al., 2004b). 2.1.3.1 Inorganic Carbon Forms in the Liaodong Bay Sediments Liaodong Bay (Fig. 2.11) is located in the northeastern part of the Bohai Sea (longitude 119◦ 50 00 ∼121◦ 08 01 E, latitude 38◦ 20 00 ∼39◦ 59 05 N). More than 10 rivers enter the bay, and a mass of sand inputs the bay every year from these rivers. The sediments in the eastern bay mainly consist of sand and muddy sand. But in the central and southern bay, sediments are sandy mud

2.1 Change Processes of Carbon in the Bohai Sea Step I-A I-B

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Fig. 2.10. Scheme of the sequential extraction method for different forms of IC in marine sediment (Li et al., 2004b) N 41

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Fig. 2.11. Location of the sampling stations

and sandy clay, part of which comes from the Yellow River. Three sediment cores in Liaodong Bay were collected with a gravity corer in December, 2001 (Fig. 2.11). The cores were successively cut into 3.0 cm thick slices from surface to bottom for DIC measurement. The contents of different IC forms and their vertical distributions in core sediments of Liaodong Bay were shown in Fig. 2.12. The contents of the NaCl form were similar in 3 core samples with a range of 0.16∼0.23 mg/g, and its vertical distribution maintained stability from surface to sea bottom. The contents of the NH3 ·H2 O form ranged from 0.16 to 1.96 mg/g, and the

2 Biogeochemical Processes of the Bohai Sea Content (mg/g) 3 1 2

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GS5

Fig. 2.12. Vertical profiles of different inorganic carbon forms in Liaodong Bay sediments. NaCl form, ; NH3 ·H2 O form, ; NaOH form, ; NH2 OH·HCl form, ×; HCl form, *; TIC, ◦

vertical distribution was basically consistent in 3 stations with prominent fluctuation in upper layers (above 66 cm at station GS1, 99 cm at station GS3, 36 cm at station GS5), but stable in the under layers. The content of the NaOH form was slightly higher than that of the NaCl form with a range of 0.19∼0.43 mg/g. Its vertical distribution was simple with a slight decrease from surface to sea bottom. The contents of the NH2 OH·HCl form, and HCl form were similar with a range of 0.36∼1.70 mg/g. Their vertical distributions were similar too, but the fluctuation of the HCl form was bigger than that of the NH2 OH·HCl form. Compared to regions of the Yangtze River Estuary and Jiaozhou Bay (Table 2.2), the content of TIC in Liaodong Bay was lower because its carbonate was diluted by a large amount of matter from the Liaohe River and other surrounding rivers. So the contents of every IC form were lower accordingly. However, the percentage of every IC form was similar to that in the Yangtze River Estuary and Jiaozhou Bay except for the NH3 ·H2 O form. The percentage of the NaCl form and NaOH form was similar, which accounted for a minority of the total inorganic carbon. The percentage of the Table 2.2. The characteristics of different IC forms in Liaodong Bay, Jiaozhou Bay and Yangtze River Estuary sediments Liaodong Bay Jiaozhou Bay∗ Yangtze River Estuary∗ Content Ratio Content Ratio Content Ratio (mg/g) (%)†† (mg/g) (%)†† (mg/g) (%)†† NaCl form 0.16∼0.23 4.49∼10.58 0.29∼0.54 1.6∼14.1 0.064∼0.518 0.86∼4.86 0.16∼1.96 10.47∼46.68 0.17∼0.56 1.25∼15.6 0.17∼0.79 2.33∼7.38 NH3 ·H2 O form NaOH form 0.19∼0.43 5.68∼19.41 0.29∼0.68 1.8∼17.8 0.20∼1.39 2.41∼14.19 NH2 OH·HCl form 0.36∼1.35 17.21∼39.74 1.03∼11.1 9.9∼82.4 1.30∼5.86 20.65∼49.59 HCl form 0.46∼1.70 18.67∼42.39 0.59∼21.1 6.0∼86.6 2.93∼7.58 35.80∼69.86 TIC 1.62∼4.24 3.03∼24.7 4.99∼13.72 ∗ The data of Jiaozhou Bay and Yangtze River Estuary are from Li et al. (2004a); †† The ratio of one form of IC to TIC in every sample IC form


151

NH3 ·H2 O form, NH2 OH·HCl form, and HCl form was similar; they accounted for most of the total inorganic carbon. 2.1.3.2 Relationship between Organic Carbon and Inorganic Carbon Organic matter can be oxidized to CO2 by O2 and NO− 3 in upper layers of sediment, and results in a decrease in pH, and this then promotes CaCO3 dissolution. However, in an environment where O2 is lacking, the organic matter will be oxidized by Fe, Mn oxide, and sulfate, and then will increase in alkalinity and result in CaCO3 precipitation. Therefore, the effect of organic matter diagenesis on inorganic carbon is more complicated and presents different characteristics in different regions. The study of Li et al. (2000) showed that lower CaCO3 concentrations corresponded exactly to the higher contents of total organic carbon (TOC), which suggested that the higher TOC contents may have played an important role in CaCO3 dissolution. The higher proportions of organic matter in sediments increased the accumulation of higher concentrations of CO2 in interstitial waters through bacterial degeneration, thus enhancing the dissolution of carbonate (Yuan et al., 2004). The molar ratio of organic and calcium carbonate in particulate material that reaches the seafloor can affect carbonate preservation. A higher content of carbonate in sediment will promote carbonate preservation, while higher content of organic matter and the release of CO2 due to organic carbon oxidation will lead to enhanced carbonate dissolution. When the ratio of the molar Corg /CaCO3 in sediment exceeded a maximum value of approximately 1.25, the organic matter oxidation may promote 100% CaCO3 dissolution in theory (Pfeifer et al., 2002). The content of organic carbon (OC) in Liaodong Bay sediments is shown in Table 2.3, which indicated that the discrepancy is small in different stations. The ratios of OC and different IC forms in Liaodong Bay sediments are very small (Table 2.4), which is predicted to promote 100% CaCO3 dissolution in theory. The ratios of the same IC form were similar at different stations, especially at station GS1 and station GS3, which showed that the influence of OC on carbonate dissolution is similar over the whole region. Table 2.3. The content of organic carbon in Liaodong Bay sediments (%) Station GS1 GS3 GS5

Min 0.57 0.56 0.39

Max 0.86 0.73 0.77

Average 0.69 0.64 0.59

Although much of the literature reported that organic carbon oxidation will promote carbonate dissolution, we do not know which part of carbonate will be dissolved because we cannot separate carbonate individually from their

152


Table 2.4. The ratios of organic carbon (OC) and every IC form in Liaodong Bay sediments Sample

Depth (cm)

NaCl/OC

NH3 ·H2 O/OC

NaOH/OC

NH2 OH·HCl/OC

HCl/OC

GS5

0 6 12 24 36 66 126 186 0 6 12 24 42 69 129 189 0 6 12 24 36 66 126

0.023 0.033 0.027 0.028 0.031 0.044 0.031 0.038 0.032 0.035 0.030 0.028 0.034 0.030 0.031 0.032 0.029 0.035 0.027 0.027 0.033 0.020 0.024

0.10 0.14 0.11 0.27 0.20 0.29 0.18 0.22 0.13 0.15 0.14 0.12 0.18 0.14 0.05 0.05 0.12 0.13 0.22 0.13 0.09 0.02 0.02

0.02 0.07 0.06 0.05 0.07 0.08 0.04 0.04 0.05 0.07 0.06 0.05 0.05 0.04 0.05 0.06 0.06 0.06 0.05 0.06 0.06 0.03 0.02

0.10 0.18 0.16 0.11 0.20 0.35 0.10 0.12 0.14 0.13 0.15 0.13 0.12 0.10 0.14 0.08 0.09 0.11 0.09 0.06 0.09 0.04 0.14

0.13 0.31 0.16 0.14 0.17 0.18 0.12 0.14 0.21 0.10 0.12 0.13 0.11 0.11 0.18 0.09 0.18 0.16 0.09 0.06 0.11 0.08 0.14

GS3

GS1

minerals on acount of their fine grain. However, in Liaodong Bay sediments, only the correlation between organic carbon and the NH2 OH·HCl form of IC is significant at the 0.05 level and the correlation is negative (Table 2.5), which showed that higher organic carbon will promote the NH2 OH·HCl form of IC dissolution first. Table 2.5. Correlation coefficients between iron, OC, and every IC form in Liaodong Bay sediments (n=23) Fe2+ Fe3+ OC

NaCl form 0.190 0.288 0.207

NH3 ·H2 O form 0.113 0.251 −0.095

NaOH form 0.430∗ 0.203 0.053

* Correlation is significant at the 0.05 level (2-tailed)

NH2 OH·HCl form 0.041 −0.256 −0.383∗

HCl form 0.484∗ −0.324 −0.052


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2.1.3.3 Relationship Between Fe and Inorganic Carbon Iron in sediments has been documented in much literature and has become recognized as an important constituent of the global carbon cycle (Cooper et al., 2005). The role that iron plays in biogeochemical cycles depends greatly on its redox species of iron (II) and iron (III). Their redox transformations at the oxic-anoxic boundary affect the distribution and cycle of carbon, sulfur, phosphorus, and other elements in sediments. Under well oxygenated condition Fe(III) is the stable oxidation state, and at neutral pH it forms highly insoluble oxides and hydroxides. Ferrous Fe is stable in an anoxic condition, and in the presence of high carbonate, sulfide, and orthophosphate concentrations it forms insoluble salts. In Liaodong Bay sediments, because the main redox environment characteristic is a relatively weak oxidation (Li et al., 2004b), the ratios of Fe3+ :Fe2+ were bigger than 1. Recent studies have shown that Fe(III) reduction can also occur in oxygenated, high pH, although this usually results in much lower steady state Fe(II) concentrations (Shaked et al., 2002). These redox reactions should have an impact on organic carbon reduction and carbonate dissolution or precipitation. In particular, the labile IC form may be affected strongly. Fe influence on inorganic carbon is embodied two ways. On the one hand, Fe3+ can oxidate organic matter into CO2 , and then cause carbonate dissolution or precipitation. Model application revealed that 3% of organic matter mineralization was due to Fe(OH)3 (Pfeifer et al., 2002). On the other hand, Fe3+ and Fe2+ can combine CO2− 3 ions and form carbonate. According to correlation between iron and IC (Table 2.5), different coefficients with different IC forms indicated that iron has different influences on different forms of IC in Liaodong Bay sediments. If iron has an effect on IC, the effect will drive the same IC form change as a steady mode. So the ratio of IC to iron will change regularly. The ratios of different IC forms to iron in Liaodong Bay sediments were shown in Fig. 2.13 and Fig. 2.14 (Niu et al., 2006). Their vertical distributions had obvious regularities. In general, the ratios of NaCl form, NH3 ·H2 O form, and NaOH form to Fe3+ had a clear trend of increasing with depth, especially in upper sediment layers. But the trends of the NH2 OH·HCl form and HCl form were not clear. So we thought the effects of Fe3+ on the NaCl form, NH3 ·H2 O form, and NaOH form were stronger than those on the NH2 OH·HCl form and HCl form, and the effects in upper sediment layers were greater than in lower sediment layers. The vertical distributions of ratios of different IC forms to Fe2+ were more complicated than those of Fe3+ . The distribution of the ratios of the NaCl form to Fe2+ was similar to that of the NaOH form, with more prominent characteristics than other IC forms. They decreased with depth in upper sediment layers (above 40 cm at stations GS3 and GS5, 12 cm at station GS1), then increased slightly below the depth of sediment at stations GS3 and GS5 or increased between the depths of 12∼36 cm and decreased below the depth of 36 cm at station GS1. The vertical distribution of the ratios of the NH3 ·H2 O form at station GS1 was similar to that at station GS3 but was the reverse of that at station

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GS5. The vertical distributions of the ratios of the NH2 OH·HCl form and HCl form were different at the three stations. So we reached a similar conclusion to Fe3+ that the effect of Fe2+ on the NaCl form, NH3 ·H2 O form, and NaOH form was stronger than that on the NH2 OH·HCl form and HCl form. Based on Li et al. (2005)’s studies, the NaCl form, NH3 ·H2 O form, NaOH form, and NH2 OH·HCl form were relatively labile IC, and their activities decreased from the NaCl form to NH2 OH·HCl form. The HCl form of IC was the steadiest. So we can conclude that the effect of iron on the labile IC form was stronger than on the stable IC form.

1.000

40 80 120 160

0

0.200

Ratios 0.600

1.000

0

40

Depth (cm)

Ratios 0.600

Depth (cm)

Depth (cm)

0

0.200

80 120 160

GS1

Ratios 0.500 1.000

1.500

40 80 120 160

GS3

200

GS5

200

Fig. 2.13. The ratios of every IC form and Fe in Liaodong Bay sediments. NaCl form, ; NH3 ·H2 O form, ; NaOH form, ; NH2 OH·HCl form, ×; HCl form, * (Niu et al., 2006) 3+

80 120 160

40 80 120

200

Ratios 2.000

3.000

80

120

160 GS1

1.000

40

Depth (cm)

40

Depth (cm)

Depth (cm)

Ratios Ratios 0.000 0.200 0.400 0.600 0.800 0.000 0.500 1.000 1.500 2.000 0.000 0 0 0

160 GS3

200

GS5

Fig. 2.14. The ratios of every IC form and Fe2+ in Liaodong Bay sediments. NaCl form, ; NH3 ·H2 O form, ; NaOH form, ; NH2 OH·HCl form, ×; HCl form, * (Niu et al., 2006)

2.1.3.4 Assessment of Influencing Factors The concentration of inorganic carbon in marine sediments is mainly controlled by the following factors: (1) The supply speed of autogenetic CaCO3 by


155

the ocean itself. (2) The supply speed of terrestrial matter. Terrestrial matter controls the content of CaCO3 in sediments. Under the condition of a steady supply of carbonate, the inputs of terrestrial matter will dilute the content of carbonate. The more the inputs of terrestrial matter, the lower the concention of carbonate. (3) Carbonate dissolution or precipitation in the early diagenetic process (Wu et al., 2001). In Liaodong Bay, the primary production was high and a mass of biogenic CaCO3 was produced (Wang and Gao, 2002), but it accepted a large amount of terrestrial matter from the Liaohe River and other surrounding rivers. So the carbonate concentration in Liaodong Bay sediments was lower than in other regions in the Bohai Sea (Yang et al., 1989). Except for the influence of the above-mentioned factors, all forms of IC in sediments were influenced by the sedimentary environment, such as pH, Eh, Es, water content, organic carbon, Fe3+ /Fe2+ , and so on. However, their influence was inconsistent and represented different characteristics in different environments (Li et al., 2005). Besides the effect of organic carbon and iron discussed above, pH, Eh, Es, and water content may influence the change of IC in sediments, but it is difficult to estimate the relative strength of their effect on IC. Cluster analysis encompasses a number of different algorithms and methods for grouping objects of similar kinds into respective categories. In other words, cluster analysis is an exploratory data analysis tool aimed at sorting different objects into groups in a way that the degree of association between two objects is maximal if they belong to the same group, and minimal otherwise. So cluster analysis can be used to discover structures in data without providing an explanation/interpretation, and may reveal deeper associations in data which, though not previously evident, nevertheless are sensible and useful once found. In this part, the aim of using cluster analysis is to sort all influencing factors into groups, and reveal the association between influencing factors and different IC forms. Fig. 2.15 showed the results of cluster analysis for all forms of IC and their influencing factors, which could indicate which had close relationships with which IC form. On the whole, all the IC forms and their influencing factors could be divided into two groups: pH group and iron group. The iron group could be subdivided into Fe3+ group and Fe2+ group. The pH group included the NH2 OH·HCl form, HCl form, and pH, which indicated that the NH2 OH·HCl form and HCl form of IC were influenced directly by pH and influenced indirectly by other factors. The Fe3+ group included Fe3+ , Es, and OC, which indicated that the deoxidization of Fe3+ has close relationships with the oxidation of S2− and organic carbon. They had similar effects on all IC forms. The Fe2+ group included the NaCl form, NaOH form, and NH3 ·H2 O form of IC, influencing factors of Fe2+ , Eh, and water content, which indicated that Fe2+ , Eh, and water content had close relationships with the NaCl form, NaOH form, and NH3 ·H2 O form of IC. Their effect on the NH3 ·H2 O form was stronger than on the NaCl form and NaOH form, and the Fe2+ relationship with the NH3 ·H2 O form of IC is weaker than that with Eh and water content. Comparing the influence of Fe3+ and Fe2+ on different IC forms, Fe3+ and Fe2+ had a stronger effect on the NaCl form, NaOH form, and

156

2 Biogeochemical Processes of the Bohai Sea 0

Rescaled distance cluster combination 5 10 15 20 25

NH3 H2O form Eh Water content Fe2+ NaCl form NaOH form Fe3+ Es NH2OH HCl form OC HCl form pH

Fig. 2.15. The result of cluster analysis for IC influencing factors in Liaodong Bay sediments (Dendrogram using average linkage between groups, Pearson correlation)

NH3 ·H2 O form than on the NH2 OH·HCl form and HCl form, and Fe2+ had a stronger effect on the NaCl form, NaOH form, and NH3 ·H2 O form than Fe3+ . The inorganic carbon in Liaodong Bay sediments was divided into 5 forms: NaCl form, NH3 ·H2 O form, NaOH form, NH2 OH·HCl form, and HCl form. The contents of the NaCl form and NaOH form were similar and occupied the minority of TIC. However, the NH3 ·H2 O form, NH2 OH·HCl form, and HCl form were the principal forms of TIC and accounted for more than 80% of TIC. In particular, the percentage of the NH3 ·H2 O form was much higher than that in the Changjiang River Estuary and Jiaozhou Bay sediments. The concentration of Fe3+ was higher than Fe2+ and Fe3+ /Fe2+ ratios showed that the main redox environmental characteristic in this region was relatively weak oxidation. Iron had little effect on the NH2 OH·HCl form and HCl form of IC which was influenced mainly by pH. However, iron had a stronger influence on the NaCl form, NaOH form, and NH3 ·H2 O form of IC; the influence of Fe2+ was higher than Fe3+ and its effect on the NH3 ·H2 O form was stronger than on the NaCl form and NaOH form. In a word, the redox action of iron affected mainly the labile IC form. 2.1.4 Biogeochemical Process of Organic Carbon in Sediments Organic matter enters the world’s oceans from two primary sources: marine primary productivity and terrestrial river runoff. Organic material is produced in marine surface waters by the photosynthetic processes of phytoplankton. This primary production is grazed upon by herbivorous zooplankton which releases biochemical compounds and excreta to the water column. As the detritus sinks through the water it is subject to breakdown, or remineralization, by bacterial activity releasing dissolved nutrients into the water. The original organic material produced in the surface waters is repeatedly recycled and sinks in particulate form to the sea bottom. The amount of organic material


157

that reaches the sediment is proportional to the depth of the water column and the amount of primary production in surface waters. The percentage of organic matter that makes it to the sediments is generally 15 μmol N/L. As already shown by Table 2.8, DON fractions generally accounted for more than 50% of TN. Exceptions were observed in September 1998 at stations B1 and C1 (west and north of the mouth of the Huanghe River, Fig. 2.23), where only 28%∼43% was reached and where DIN became the main fraction, reaching about 56% and 43%, respectively. Here also, the TN concentrations were between 25 and 30 μmol/L, and thus almost twice the concentration was found at the other stations. Further autumn maxima of DIN of >7 μmol/L were found in the bottom near layers at the eastern stations A2 through A4 (Laotieshan Channel), reaching >40% of TN. As for dissolved inorganic nitrogen, higher concentrations are found in the Huanghe River Estuary, Bohai Bay and the northern part of the central Bohai Sea (Fig. 2.24, Zhao et al., 2002). The highest concentration of DIN appears in the Huanghe River Estuary. It indicates that the Huanghe River was the most important source of DIN in the Bohai Sea in October 1998. During May 1999, dominating high DIN contributions to TN of >50% were found in the northwest, at the outer Bohai Bay (stations E1 and E2). Here, concentrations

2.2 Distributions and Transformations of Nitrogen in the Bohai Sea

165

Table 2.8. Mean concentrations (μmol N/L) of DIN, DON, PN, and TN in the Bohai Sea (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

Surface DIN DIN/TN (%) DON DON/TN (%) PN PN/TN (%) TN Bottom DIN DIN/TN (%) DON DON/TN (%) PN PN/TN (%) TN

September 1998, grid 1

September 1998, grid 2

May 1999, grid 1

May 1999, grid 2

2.81±3.29 15.24±11.26 10.18±2.19 62.49±12.03 3.51±0.85 22.27±7.69 16.50±4.33

2.54±3.16 13.72±12.01 10.97±1.52 68.90±10.28 2.79±0.76 17.38±4.52 16.30±3.42

5.3±4.73 24.00±15.95 10.87±2.94 57.30±16.06 3.86±3.77 18.70±8.45 20.08±8.18

4.73±4.10 21.20±14.12 11.84±2.28 62.80±14.47 3.26±2.02 16.00±5.69 19.84±6.26

3.75±3.55 19.78±13.95 10.99±3.89 61.12±13.67 3.28±0.98 19.10±4.99 17.91±4.76

3.39±3.70 18.48±15.90 11.03±2.82 65.45±13.28 2.66±0.59 16.07±3.84 17.08±4.77

5.42±4.37 22.96±13.59 11.28±4.53 55.00±17.19 5.05±4.94 22.04±9.81 21.75±10.27

4.87±4.03 21.15±13.25 11.22±2.12 59.04±14.55 4.20±2.72 19.81±6.17 20.27±7.07

N 41

40

39

38

37 118

119

120

121

122

E

Fig. 2.23. Composition of total N (μmol N/L) in surface and bottom water layers of the Bohai Sea (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

166


of TN were at least twice as high as in the central Bohai Sea. DIN contributions of >30% were found at stations G3 through G5 and at F4/F5 west of L¨ ushun. In the eastern part of the investigated area, at stations A1 through A4 and C4/D5 near the Bohai Strait and the Laotieshan Channel, the contribution of DIN to TN was in most cases 2 μmol/L higher on the bottom than at the surface and thus accounted for most of the local TN increase between the surface and the bottom (Fig. 2.23). Around stations F1, E1, and E2, this difference between the bottom and the surface partly exceeded 15% of the total PN contribution to TN. Also, at the inner part of the Laotieshan Channel, at stations E5, F5, and G5, the contributions of PN to TN were >10% higher in the bottom waters than at the surface.

Fig. 2.25. Dissolved inorganic nitrogen (μmol/L) of the Bohai Sea in spring (BH99) and autumn (BH9) cruises (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

2.2.1.2 Vertical Variation of Dissolved Inorganic Nitrogen In a section across the central Bohai Sea (section L) of the BH98 cruise, nutrient species showed a somewhat vertically well-mixed profile (Fig. 2.26,

168


1.0 0

Depth (cm)

BH99 Nitrate

0.0

0.0

0.60 0.80 1.00

0.40

A4

1.00

0.60

0.35 0.35 0 .40 0.45 0.50

0.40

40 50

0.40

0.80

30

0.35

0.35

1.40

0.60

Depth (cm)

0 0.6

0.40

A3

0 1.2

0.40

30

20

A1 A2 0 1.0 10 0 00 20 1. 1.20 0 1.4 30 40 50 60 BH98 Ammonium 0 10 20 1.0 1.0 30 2.0 3.0 40 4.0 50 BH98 Nitrate 60 0.80

0.40

20

G4

F4

0.60

10

10

Depth (cm) Depth (cm)

0.45

A4

0.60

40 BH99 Ammonium 50 0

Depth (cm)

0.30

7.00

0.30 0.4 5

4.00 5.0 0 6.0 0

(a) A3

A2

E4 0.40

A1

0.60

0.4 5

C1 D3 0 10 20 30 BH98 Ammonium 0 10 20 BH98 Nitrate 30 0.80

0 0.60.75

G4

F4

1.50 3.00 4.50 6.00 7.50 8.00

0

Depth (cm)

E4

0 7.0

C1 D3 0 10 20 BH99 Ammonium 30 0 3.00 3.00 10 20 BH99 Nitrate 30

0.30

Depth (cm) Depth (cm)

Zhang et al., 2004). Concentrations of nitrate, nitrite, and ammonia fell from stations C1 to G4. In the section across the Bohai Strait (section A) from south to north, nitrate was almost vertically homogenous in the upper waters of ca. 20 m, corresponding to the depth of pycnocline. Concentrations of nitrate increased then with depth and reached 6.0∼7.0 μmol/L, in near-bottom waters, higher in the south than in the northern part of the Bohai Strait. Ammonia and nitrite showed somehow a vertically mixed picture with a higher concentration at station A1 than at station A4.

1.0 2.0 5.0

3.0 4.0

6.0

(b)

Fig. 2.26. Vertical variation of nitrogen (μmol/L) at sections L (a) and A (b) of the Bohai Sea in spring (BH99) and autumn (BH98). L: central Bohai Sea; A: Bohai Strait (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

At section L of the BH99 cruise, the nitrate averaged 7.5 μmol/L at station G4, then reduced southwestwards to C1 (Fig. 2.26). Ammonia and nitrite showed well-mixed vertical profiles in coastal waters when taking into account the nature of stratification in the central Bohai Sea, with 1.0 μmol/L for ammonia in near-bottom waters and 0.12 μmol/L for nitrite at the surface. At sections across the Bohai Strait (i.e., A1 to A4), stratification is identified for nitrogen. Concentrations of ammonia and nitrite increase considerably from


169

surface to near-bottom waters. Nitrate shows a value of 0.40∼0.60 μmol/L in this region, similar between surface and near-bottom waters. 2.2.1.3 Seasonal Differences of Nitrogen In spring 1999, the DIN concentrations from Bohai Bay through the central Bohai Sea to the stations west of L¨ ushun were higher than in autumn and— through enhanced phytoplankton activity—caused generally higher values of total N itself. Fig. 2.27b (Raabe et al., 2004) shows distinct maxima of primary production in the Bohai Sea with the highest values of >700 mg C/(m2 ·d) at the Laotieshan Channel and with further maxima at stations G2 (southern Liaodong Bay) and C1 (north of the Huanghe River mouth). Simultaneously, there was an accumulation of organic matter with high concentrations of particulate N (Fig. 2.23) in the western Bohai Sea, especially near Bohai Bay. The production maxima in Fig. 2.27b seemed to have been moved by the main inflow current, triggered by the Yellow Sea Warm Current YSWC. From grid 1 to grid 2, the maximum located in the Laotieshan Channel around station D5 (Fig. 2.27b) moved in a western direction to station G4 west of L¨ ushun (Fig. 2.27d), while the maxima at station G2 (southwestern Liaodong Bay) and at station C1 (mouth of the Huanghe River) followed the anticlockwise south current in southern and eastern directions, respectively. It was considered that this is a typical spring-time situation with the developing phytoplankton originating from the Yellow Sea, fuelled by nutrients from deeper water layers and driven northwest by the YSWC (Fig. 2.27d). Following the anticlockwise circulation, the production maxima reached then the very shallow Bohai Bay and built up a high biomass, fed by the nutrient input of the Huanghe River. Apart from the Huanghe River supply and the stations west of L¨ ushun city fuelled by the YSWC (Fig. 2.23), the generally low DIN concentrations in 1998 in the central part of the Bohai Sea reflected the autumn season, when most of the phytoplankton blooms had ended and nutrient concentrations were exhausted. In particular, nitrate limited the primary production, for example at station B2 (Laizhou Bay): Although there was no actual maximum in primary production (Fig. 2.27a), the nitrate minimum of 4 mg/L (Wei et al., 2003) as well as by higher concentrations of particulate N (Fig. 2.23). Only in region influenced by the Huanghe River (from the eastern Bohai Bay to the northern Laizhou Bay) and in the area west of L¨ ushun city, did higher DIN concentrations persist—again fuelled by the YSWC and by the Huanghe River—and caused moderate increases in primary production in these areas between grids 1 and 2. At the same time, the production maximum observed in the Bohai Strait (stations C4/D5 in Fig. 2.27a) moved in an eastern direction (Fig. 2.27c). From this it can be assumed that with the seasonal decline in phytoplankton blooms and due to the overall nutrient exhaustion in the Bohai Sea, the phytoplankton and part of the particulate organic material left the Bohai Sea along the south coast through the Bohai

170


41

Fig. 2.27. Primary production (mg C/(m2 ·d)) at the surface in the Bohai Sea in autumn 1998 (a) and in spring 1999 (b), and differences between two grids in autumn 1998 (c) and in spring 1999 (d) (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)


171

Strait. The origin of PN was mostly coupled with phytoplankton standing stocks, because PN and chlorophyll correlated with the highest significance (P >99.99%) during both seasons (Figs. 2.28a and 2.29a, open squares). The correlation was strongest in spring 1999. There was also a correlation of PN with SPM of highest significance (P >99.99%, Fig. 2.29b, open squares) during spring, while the significance in autumn was much lower (P >95%, Fig. 2.28). This means that most of the SPM in autumn did not consist of fresh organic material, but originated from resuspended sediments and detritus.

Particulate N (mmol/L)

10

R2=0.2933, n=391

(a)

8 6 4 2 0

0

1

8

5

6

R2=0.1087, n=326

(b)


2 3 4 Chlorophyll a (mg/L)

6 4 2 0

0

2

4 6 8 10 12 14 Total suspended matter (mg/L)

16

18

Fig. 2.28. Correlation of particulate N with chlorophyll a (a) and with total SPM (b) of the Bohai Sea in autumn 1998 (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

In autumn 1998, no regional differences for the correlations were found (Figs. 2.28a and b), indicating that the biogeochemical processes were more or less in the same range throughout the investigated area. Support for this assumption came also from the PC and DOC fractions that did not show strong lateral gradients in the Bohai Sea (Fig. 2.23). In spring 1999, two main groups of data points could be identified (Fig. 2.29a). The samples from the central Bohai Sea inclusive of the southern and eastern regions (open squares) showed highly significant correlations of PN with chlorophyll. In Bohai Bay (filled squares), a highly significant correlation was also found, but with a much steeper slope; i.e., the samples had a much higher content of PN

172

2 Biogeochemical Processes of the Bohai Sea Particulate N (mmol/L)

30 25

Central, southern and eastern Bohai Sea Bohai Bay

20

R =0.4754, n=107 2

15 10 5 0

0

20


(a)

R =0.7303, n=349 2

1

2

5 3 4 6 Chlorophyll a (mg/L)

7

8

Central, southern and eastern Bohai Sea Bohai Bay

9

(b)

15 10 R2=0.7747, n=434

5 0

0

20

40 60 80 100 Total suspended matter (mg/L)

120

140

Fig. 2.29. Correlation of particulate N with chlorophyll a (a) and with total SPM (b) of the Bohai Sea in spring 1999 (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

compared to chlorophyll. These Bohai Bay samples were also characterised by high SPM values having a very close relationship with PN (Fig. 2.29b, filled squares). This was an indication of the production of fresh biomass with high N content in Bohai Bay during spring 1999, partly originating from in situ plankton production, partly from resuspension of the sediments or from imports via the residual currents from the coastal regions of Liaodong Bay into Bohai Bay. The biogeochemical processes were also strongly enhanced by interactions between sediments and the water column in this very shallow area and resulted in the described strong accumulation of particulate organic material during spring (Figs. 2.23 and 2.30). 2.2.1.4 Distinguishing between Transport and Conversion Processes of Nitrogen In order to identify net import and export processes into/from the Bohai Sea, it was necessary to examine whether changes in biogeochemical parameters were caused by transport processes or by phase-transfer and conversion processes. For this, analysis of changes in SPM data was a supportive tool, in particular when SPM concentrations showed a close relationship to particulate nitrogen (Figs. 2.28b and 2.29b). During both seasons, suspended matter was generally higher in the bottom layers than at the surface and, consequently, equilibrium between sedimentation and erosion was assumed (Jiang et al., 2004). In the Laotieshan Channel, currents and higher water density


173

N 41

40

39

38

37 118

119

120

121

122

E

Fig. 2.30. Composition of total C in surface and bottom water layers of the Bohai Sea on grid 1 in autumn 1998 and in spring 1999 (Raabe et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

of the YSWC caused sediment erosion and an increase in SPM in the bottom water. Then, via residual currents, the SPM was transported to the northeast of the Bohai Sea, west and north of L¨ ushun (Liaodong Bay), causing SPM maxima of 10∼20 mg/L throughout the water column (Jiang et al., 2004). In general, major parts of the suspension would settle here (Huang et al., 1999). Further maxima of total suspended matter found in the coastal zones from Bohai Bay to Laizhou Bay were caused by discharges of silt by the Huanghe River and Haihe River, partly indicated by lower salinity values as well as by stronger vertical salinity gradients, as reported by Zhang et al. (1998b) and Jiang et al. (2004). Apart from these local SPM maxima originated by riverine input and by sediment erosion by the YSWC, wind-induced local resuspension had played a major role in increasing SPM content in the shallow areas of the Bohai Sea: Jiang et al. (2004) ascribed the different SPM concentrations in spring and autumn to different meteorological forcing, namely the wind speed. Consequently, apart from the import of nitrogen by the YSWC and by the Huanghe River, resuspension processes in Bohai Bay released nitrogen into the water column. In spring and autumn, decreases in PN were observed in the coastal area from Bohai Bay to Laizhou Bay near the mouth of the Huanghe River during the two successive grids. These decreases were higher during spring, and they were probably caused by continuing decomposition

174


as well as by sedimentation, indicating highly dynamic processes, especially in the shallow areas of the Bohai Sea. Significant increases in PN (>0.15 μmol N/(L·d)) observed during spring at stations G3/G4 and F4 were well correlated with increases in primary production (Fig. 2.27d) as well as with decreases in DIN (>0.28 μmol N/(L·d)) and thus were caused by an enhanced biogeochemical conversion of nitrogen. In contrast to this, increases of >0.33 μmol PN/(L·d) throughout the water column at station B1 (northern Laizhou Bay) were accompanied by strong decreases in primary production (>100 mg C/(m2 ·d), Fig. 2.27d). Here, PN concentrations were apparently affected by the mentioned strong resuspension of surface sediments (Jiang et al., 2004). Other strong evidence for the resuspension of organic material near Bohai Bay in spring came from the very high particulate carbon content, especially at station D1 (Fig. 2.30). Furthermore, DOC concentrations were much higher in this region, indicating strong remineralisation processes followed by the beginning of decomposition and oxidation. As a consequence, oxygen values were lower (10 mg/L). This was confirmed by low oxygen values generally observed in Bohai Bay during early summer (Cui et al., 1994). Since the DON was generally >60% of TN in autumn and spring in the investigated area (Fig. 2.23), it can be assumed that the pelagic ecosystem was in a steady state of ongoing production and decomposition, including sorption processes. That means the situation during both seasons was comparable with the postspring situation in temperate waters described by Butler et al. (1979). High loads of DON observed in areas of high primary production (stations D5 and C4) can be related to DON release during exponential phytoplankton growth (Bronk et al., 1994). Short-time changes between the successive grid samplings indicated that besides changes induced by advection and resuspension a variable turnover of nitrogen species occurred. However, as already indicated by SPM data, major parts of the observed changes must be referred to as advection of gradients near the bottom in the deeper part of the Bohai Sea. This assumption was strongly supported by the high primary production of partly >700 mg C/(m2 ·d) at the Laotieshan Channel (Figs. 2.27a and b), requiring a net turnover of nitrogen of >9 μmol/(L·d). Due to the low DIN concentrations of 6 μmol N/L in autumn and to 2 μmol N/L in spring and thus feeding primary production by nitrogen import. During spring 1999, the high primary production of 5 μmol C/(L·d) near the Huanghe River did not fit in with the observed DIN decreases of 0.1 μmol N/(L·d) from grid 1 to grid 2 at stations C1/D1. Here also, a net import of nitrogen by the Huanghe River had to be postulated (Raabe et al., 2004).


175

2.2.2 Evolution of Nutrients and Primary Production The Bohai Sea is the main spawning and feeding site for many fish and shrimps. Fishery resources were once very abundant, but they declined significantly over the past 30 years. There are many factors influencing the variation of fishery resources. Phytoplankton, as the foundation of the whole ecosystem, are very important for fishery resources. The primary productivity (PP) in the Bohai Sea was high in the 1980s, but it became low from the 1990s onwards. On the other hand, the phytoplankton composition changed gradually from diatoms with absolute predominance to diatoms coexisting with dinoflagellates since the 1980s (Sun et al., 2002). Diatoms support carnivorous fishes which have a long food chain, while dinoflagellates are the main food for zooplankton, such as jellyfish that support a short food chain. Therefore, such a change in PP and phytoplankton community structure in the Bohai Sea can cause a change in the fish yield and a variation in the fishery composition, and can also affect the stability and function of the ecosystem. Nutrients, such as nitrogen and phosphorus, are fundamental for phytoplankton growth. They changed notably in the past 30 years in the Bohai Sea. Many studies were conducted to study the nutrients uptake of phytoplankton and the limitation of nutrients on phytoplankton by means of modeling in the laboratory or by in situ enclosure experiment. However, few researches were conducted to analyze the relationship between PP and nutrients over a timescale of several decades, and systematic analysis of the effect of nitrogen and phosphorus on PP in the past 30 years in the Bohai Sea has not been reported yet. Many comprehensive investigations of the Bohai Sea in the past 30 years provided lots of basic data about concentrations of nitrogen and phosphorus, and data of PP and phytoplankton composition. Mainly based on these basic data, this section tries to investigate the role of nitrogen and phosphorus in the variation of PP and phytoplankton community structure, in order to provide a scientific foundation for marine environmental protection. The Bohai Sea is the main receiving water body for contaminants from the Bohai Sea Economic Circle region which is composed of the surrounding provinces and cities, such as Shandong, Liaoning and Hebei provinces, and the city of Tianjin. It has about 21% of the national population and contributes 22% of the national GDP in 1999 (Zhang Z et al., 2006). The region is the most developed in northern China. With the rapid development of industrial society, the Bohai Sea Economic Circle region discharged many contaminants into the Bohai Sea. These pollutants have broken the balance of the Bohai Sea ecosystem. 2.2.2.1 Evolution of DIN and DIP in Bohai Sea Waters Evolution of DIN and DIP in Bohai Sea waters exhibited three stages in the past 30 years: in the early and middle 1980s; from the late 1980s to the middle 1990s; from the middle 1990s onwards. In the first stage, DIN was low and stable, while DIP decreased gradually; in the second stage, both DIN and

176


DIP increased; in the third stage, both DIN and DIP decreased in variational tendency; however, DIN was still above its level of the early 1980s, while DIP was already below its level of the early 1980s and even reached the lowest level in the past 30 years (as shown in Figs. 2.31 and 2.32).

DIN (mmol/L)

20 15 10 5 0 1978

1988

1998

2008

Year

Fig. 2.31. Evolution of DIN in Bohai Sea waters in the past 30 years

DIP (mmol/L)

1.5 1.0 0.5 0 1978

1988

1998

2008

Year

Fig. 2.32. Evolution of DIP in Bohai Sea waters in the past 30 years

The quantity of chemical fertilizer used can affect the DIN in the Bohai Sea greatly. Based on the series of China Statistical Yearbooks, the quantity of chemical fertilizer used in farmland increased by an average ratio of 6.7% every year from 1980 to 2000. There was a close relationship between DIN and the quantity of chemical fertilizer used in farmland from the early 1980s to the middle 1990s (r =0.907, n=14, sig.M3-7≈M10-1. Affected by the Huanghe River input enriched with organic matter, as for M9-12, the stable nitrogen formed in early diagenesis can be reactivated in a deeper layer sediment, and became a transferable form, which is the main reason why the transferable nitrogen in the deep layer is almost the same as in the surface layer. M10-1, mainly affected by the Haihe River inputs, shows the same characteristic. Song (1997) has found that “active ferrous” concentration in deeper layer sediment is higher than in surface layer sediment in this area, proving that such activation exists. Nitrogen in sediment may have a similar activation to ferrous concentration. In the core sediments with natural grain size of the southern Bohai Sea, OSF-N, and IEF-N are preponderant forms in transferable nitrogen. The core sediments show different vertical distributions: IEF-N decreases


193

with depth and IMOF-N has an abrupt transition with depth, due to redox enviroment change, and OSF-N is mainly affected by sediment resources and the mineralization process. Decomposition rate constants of ON and OC in the sediment are 15.51×10−3 and 4.79×10−3 yr−1 respectively and constants of C, N, P, Si have the sequence N>P>C>Si. Because there is plenty of nitrogen combined in the sediments in early diagenesis, the C/N ratio is much lower than ON/ON. A large amount of inactive components contained in organic nitrogen and ON adsorption to clay minerals result in ON enrichment in sediment, which is the main reason for OC/ON reduction with depth. In the surface sediment, there is about 34.25% of nitrogen that can be involved in recycling. As to core sediments, the proportion of transferable nitrogen to TN in the surface layer is higher than in the deeper layer, but some special sedimentation environments with organic matter can induce “stable” nitrogen to be active and, as a result, the two proportions become almost the same (Song et al., 2002b). 2.2.4 Sediment-Water Exchange of Inorganic Nitrogen Inorganic nitrogen regeneration in sediment plays an important role in the budget and in the dynamics of nutrients in a water column. The Bohai Sea is affected greatly by continental detritus input by river, which is a very important pathway of nitrogen to be the marine recipient. Nitrogen regeneration from sediments may also contribute significantly to marine primary production in the Bohai Sea. It shows the processes affecting the exchange fluxes at the sediment-water interface. The nitrogen release from and adsorption onto surface sediments were also examined via simulation. Based on the incubation experiments, the exchange flux of nutrients at the sediment-water interface is established in Table 2.14. In the BH98 cruise, data of incubation with airflow show an average of 0.34 mmol/(m2 ·d), indicating a release of NO− 3 from sediment to overlying seawater at stations E1, E3, and A2, with the exception of station G2 (Table 2.14, Zhang et al., 2004). In the case of incubation with N2 -flow, the NO− 3 exchange fluxes have a geometric average of −0.88 mmol/(m2 ·d), underlying a removal from seawater at stations G2, E1, and A2 except for E3 (Table 2.14). 2 The exchange flux of NO− 2 shows an average of −0.041 mmol/(m ·d), which indicates a removal from seawater in the incubations with air sup2 ply. The NO− 2 fluxes average 0.10 mmol/(m ·d) in N2 case, indicating a net − + increase of NO2 into seawater. The NH4 fluxes vary from −0.37 to 0.065 mmol/(m2 ·d) in case of air supply, suggesting a removal from seawaters. In 2 the N2 case, the NH+ 4 shows an exchange flux of 0.24∼0.52 mmol/(m ·d), + suggesting that NH4 turns out to be an important nitrogen species at the sediment-water interface in anaerobic conditions (Table 2.14). In the BH99 cruise, the observed sediment-water exchange fluxes at station A2 are 0.007, − + 0.009, and 0.034 mmol/(m2 ·d) for NO− 3 , NO2 , and NH4 , respectively. At stations E3 and A4, the exchange fluxes of N species in aerobic incubations

194


Table 2.14. The exchange flux (mmol/(m2 ·d)) of nitrogen at sediment-water interface in the Bohai Sea (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center) Station BH98 cruise A2 E1 E3 G2 BH99 cruise A2 A4 E3

Series

NH+ 4

NO− 2

NO− 3

DIN

Air N2 Air N2 Air N2 Air N2

−0.370 0.520 0.065 0.260 −0.160 0.240 −0.070 0.520

0.290 −3.48 0.145 −0.655 1.05 1.08 −2.33 −3.55

−0.120 0.410 0.009 0.019 −0.033 −0.009 −0.020 0.050

−0.190 −2.55 0.235 −0.380 0.860 1.31 −2.41 −2.98

Air Air N2 Air N2

0.034 −0.016 −0.065 −0.082 0.012

0.007 −0.004 −0.034 −0.048 −0.255

0.009 −0.007 −0.005 0.003 0.034

0.050 −0.026 −0.104 −0.129 −0.210

Negative values indicate the removal of nutrients from near-bottom waters, while positive values show the release of nutrients from sediment to near-bottom waters

indicate a net removal from seawaters to sediments, with a net exchange flux of − + −0.04, 0.003, and −0.06 mmol/(m2 ·d) for NO− 3 , NO2 , and NH4 , respectively (Table 2.14). With respect to the anaerobic incubation (i.e., N2 case), the aver− + 2 age fluxes of NO− 3 , NO2 , and NH4 are −0.2, 0.024, and −0.01 mmol/(m ·d), − respectively. The NO3 dominates in DIN and shows a removal from seawa+ ters. Data of NO− 2 and NH4 indicate an apparent release from sediment to seawater at station E3 but a net removal from seawater at station A4 (Table 2.14). In the Bohai Sea, the benthic fluxes of NO− 3 in N2 incubation can be 5∼10 fold those in aerobic conditions, suggesting an increase in nitrogen removal from seawater in anaerobic conditions. Since the NH+ 4 flux in anaerobic incubation is twice as high as that in the aerobic condition, the coupling of + NO− 3 and NH4 exchange fluxes indicates that denitrification happens at the sediment-water interface in N2 incubations (Rysgaard et al., 1993). The concentrations of dissolved oxygen (DO) in anaerobic incubation can be as low as ca. 0.91 mg/L, corresponding to a saturation of ca. 15%, and the DO concentration in aerobic incubations is 7.51 mg/L, with a saturation of 120%. It seems, however, that a simple relationship cannot be established between + NO− 3 and NH4 with the data in Table 2.14. In the BH98 cruise, the DO concentration of near-bottom seawaters is 5.60∼7.00 mg/L with a saturation of 80%∼99% at our incubation stations. In the BH99 cruise, the near-bottom water DO concentration increases to 9.63∼10.5 mg/L, having a saturation of 105%∼112%. Some of the ammonia will be transformed to nitrate via nitrification reaction.

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea

195

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea Despite the role of phosphorus as one vital nutrient, the distributions and dynamics of marine phosphorus pools are less well characterized in comparison with those of the bioelements carbon and nitrogen (Suzumura and Ingall, 2004). Phosphorus is the key element for phytoplankton growth and one of the factors in coastal estuarine and seawater eutrophication. Recently, phosphorus has been observed to limit phytoplankton production in both coastal waters and the open sea. 2.3.1 Distribution of Phosphorus and Silicate in Seawaters In coastal ecosystems, the anthropogenic inputs of phosphorus in coastal seas induce serious problems, such as reducing web diversity, altering phytoplankton compositions, and increasing the intensity and frequency of red tides (Nixon, 1993). Survey data in the past 40 years have shown that the content of inorganic nitrogen has greatly increased and the contents of inorganic phosphorus and silicon have decreased to a low level. The average phosphorus content was 1.06 μmol/L in 1982∼1983. It fell to 0.33 μmol/L in 1992∼1993 and was 0.50 μmol/L in 2000∼2001, half of that in the early 1980s, although it has slightly increased compared with its value in the early 1990s. Silicate concentration also shows a substantial decrease. The present silicate concentration (8.12 μmol/L) is only 35% that of the early 1980s (Li et al., 2003). Some researchers pointed out that these changes have caused an unbalanced proportion of nutrients, such as the ratio of N/P changing from 1.7 in the beginning of the 1980s to 23.7 in the late 1990s, Si/N from 13.7 to 2.4, and Si/P from 23.7 to 57 (Yu et al., 2000). The nutrient regime in the Bohai Sea ecosystem has changed from N-limitation to P-limitation (Tang et al., 2003). 2.3.1.1 Horizontal Distribution of Phosphate and Silicate The horizontal distributions of phosphate and silicate are shown in Fig. 2.40. In the autumn cruise, relatively high levels of reactive phosphate (i.e., PO3− 4 ) were found in Liaodong Bay with a mean value of ca. 0.35 μmol/L both at the surface and in near-bottom waters, followed by Bohai Bay (0.20 μmol/L) and Laizhou Bay (0.10 μmol/L). Concentrations of phosphate were relatively low in the central part of the Bohai Sea, where PO3− 4 averages 0.1∼0.2 μmol/L. The concentration of silicate was higher in the coastal waters than in the central part of the Bohai Sea, and the influence from riverine discharges could be identified from the horizontal profiles across the shelf. For example, concentrations of silicate decreased from 8.0∼10.0 μmol/L off the Yellow River Estuary to ca. 4.0 μmol/L in the central Bohai Sea. High levels of silica were also found in the area off Liaodong Bay (Fig. 2.40, Zhang et al., 2004).

196


Fig. 2.40. Horizontal distribution (μmol/L) of phosphate and silicate of the Bohai Sea in spring (BH99) and autumn (BH98) (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

In the spring cruise, phosphate in Laizhou Bay was 0.10∼0.20 μmol/L for both surface and near-bottom waters. The phosphate increased toward the north part of the Bohai Sea and reached 0.50∼0.60 μmol/L in Liaodong Bay. Compared to the autumn cruise, the concentration of silica was lower in the spring 1999 cruise. The influence of Bohai Bay could be seen from a relatively high level of 8.0∼9.0 μmol/L for silica, followed by Liaodong Bay. Concentration of silica was low at Laizhou Bay, with a mean level of ca. 2.0 μmol/L; this silica deficit water mass apparently dominated the southern part of the Bohai Sea (Fig. 2.40). 2.3.1.2 Vertical Distribution of Phosphate and Silicon On the autumn cruise, in the section across the Bohai Strait (section A) from south to north (i.e., A1 to A4), phosphate was almost vertically homogenous in the upper waters of ca. 20 m, corresponding to the depth of pycnocline. The concentrations of phosphate increased then with depth and reached 0.65 μmol/L, in near-bottom waters (Fig. 2.41), higher in the south than in the northern part of the Bohai Strait. With respect to silica, the concentration reduced from south to north across the strait; a high level of up to 12.0 μmol/L could be distinguished from near-bottom waters at station A2. At section L in the spring cruise, phosphate increased regularly from 0.10 μmol/L in the southwest (C1) to 0.60 μmol/L in the northeast (G4). Silica averaged 10.0 μmol/L at station G4, and then reduced southwestwards to C1. At sections


197

across the Bohai Strait (i.e., A1 to A4), stratification was identified for nutrients. Concentrations of phosphate increased considerably from surface to near-bottom waters. For silica, a vertical profile across the strait provided evidence of a silica-poor water mass moving up to the surface in the central part of the Bohai Strait, with high levels (2.5∼3.0 μmol/L) of this species found in both the northern and southern parts (Fig. 2.41). C1

D3

E4

G4

F4

0.27

0.24

3 0.3

5.50

4.50

10

6.50

BH98 Phosphate

7.50

30 0

4.50

7.50

Depth (m)

0.21

20

0.30

10 0.1 8

Depth (m)

0

20 BH98 Silicate 30

2.0 0

0 1.8 0 1.6

BH99 Silicate

A3 A1 A2 0 10 20 0.2 0.2 30 0.3 0.3 0.4 40 0.5 50 BH98 Phosphate 60 0 8.0 9.0 6.0 10 10.0 .0 20 11 9.0 8 40 .0 11. 30 0 50 BH98 Silicate 60

A4

5.0

7.0

0 2.2

0 1.6

0 1.8

2.2 2.60 02 .00 2.4 0

Depth (m)

0.28 0.28 0.24

2.0 0

0.200.22 0.24

Depth (m)

0 0.2

BH99 Phosphate 0 2.2

Depth (m)

0.18 0.18

A4

0.1

0.18

0 1.8 0 1.8

20 30 40 50

A3

A2

0.22

Depth (m)

A1 0 10 20 30 40 50 0 10

Fig. 2.41. Vertical distribution (μmol/L) of phosphate and silicate of the Bohai Sea in spring (BH99) and autumn (BH98) (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

2.3.2 Forms of Phosphorus and Silicon in Surface Sediments The geochemical phases of phosphorus are the important parameters that reflect the bioavailability and ecological effects of phosphorus. As is well known, aqueous phosphate shares with nitrate the characteristic of being a major growth-limiting nutrient in the global biosphere, and apatite and iron oxide weathering are the major aquatic supplies.

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2.3.2.1 Forms of Phosphorus and Silicon in the Surface Sediments Phosphorus and silicon are both major biogenic elements in the marine biogeochemical process and hence occupy an important place in oceanographic research (Song et al., 2000b; Song, 2001). At present, research into the cycling of phosphorus and silicon near the sediment-seawater interface tends to focus on the conversion and burial of phosphorus and silicon near the interface in combination with the effects of biological activities (chiefly of benthos and microorganisms) near the interface, such as the effects of the phytoplankton bloom period on the interface fluxes of phosphorus, silicon, and other biogenic elements (Conley and Johnstone, 1995), the relationship of bacterial production and biomass at the sea bottom to interface exchange rates of phosphorus and other nutrients (van Duyl et al., 1993). Tarapchak and Moll (1990) considered bacteria as the major user of dissolved organic phosphorus (DOP), but research conducted by Conter and Waizel (1992) demonstrated that the capability of bacteria to absorb DOP is poorer than that of phytoplankton. Functions of bacteria in the biogeochemical cycling of phosphorus in the marine environment remain to be further studied. Disturbance by benthos is another important factor affecting the interface cycles of phosphorus and silicon. Aller and Yingst (1985) investigated the influence of disturbances by such marine benthonic animals as Heteromastus filiformis, Macoma baltica and Tellina texana on the interface exchange and exchange rate of phosphorus. On the whole, biological activities near the sediment-water interface will affect to a great extent the exchange between the sediment and its overlying water (Gehlen et al., 1995). In fact, the forms of phosphorus and silicon must be studied closely to determine the functions of phosphorus and silicon in sediments (Song et al., 2003). The distributions of various forms of phosphorus and silicon in the surface sediments of the southern Bohai Sea are shown in Table 2.15. Their major geochemical features were as follows. (1) Ion-exchangeable form (IEF) The content of IEF-P in the surface sediments ranged from 0.033 to 0.137 μmol/g (mean of 0.077 μmol/g), and that of IEF-Si ranged from 0.54 to 2.30 μmol/g (mean of 1.17 μmol/g), accounting for 0.9% and 0.01% of their respective total amounts, being the lowest percentages of all the forms. Compared with other forms of inorganic phosphorus and silicon, IEF was more closely related to the aqueous environment. Therefore, under the same physicochemical conditions, IEF would be utilized by organisms more easily than other forms of inorganic phosphorus and silicon. This was also demonstrated by the fact that the distribution areas with higher contents of IEF-P and IEF-Si appeared in such high production areas as Bohai Bay, Laizhou Bay and its neighbouring waters.

IEF

IMOF

Transferable P and Si OSF TIF BF T Si P Si P Si P Si BP BSi P Si1) 2.35/0.02 0.12/2.1 7.84/0.06 0.50/8.9 2.15/0.02 0.44/7.8 11.43/0.09 1.18/20.9 5.64 5.91 3.43/0.03 0.23/3.6 6.91/0.06 0.82/12.9 1.81/0.01 0.64/10.1 11.07/0.1 3.21/50.6 15.18/0 .11 6.34 5.72 2.57/0.02 0.16/2.5 0.56/0.004 0.37/5.8 0.82/0.006 0.59/9.3 4.08/0.03 2.99/47.2 16.60/0 .12 6.34 5.86 6.55/0.06 0.43/4.7 4.77/0.05 0.90/9.8 3.80/0.04 0.82/9.0 11.86/0.12 2.05/22.4 4.20/0 .04 9.16 4.82 7.87/0.08 0.44/4.5 2.29/0.02 0.84/8.5 2.53/0.03 1.01/10.2 11.33/0.11 1.25/23.8 20.63/0 .20 9.86 4.42 7.04/0.07 0.40/4.3 0.11/0.001 0.81/8.8 2.76/0.03 1.01/1.1 8.77/0.09 4.35/47.5 19.58/0 .21 9.16 4.93 5.64/0.06 0.25/3.6 0.35/0.003 0.87/12.4 3.26/0.03 0.71/10.1 7.29/0.10 11.33/0 .12 7.04 5.32 3.55/0.03 0.16/2.3 2.35/0.02 0.91/12.8 2.31/0.02 0.54/7.6 6.95/0.06 3.90/55.4 6.65/0 .06 7.04 5.54 8.13/0.08 0.45/4.0 7.28/0.08 0.76/6.7 2.20/0.02 0.86/7.6 16.65/0.17 6.12/54.3 40.84/0 .41 11.27 4.41 3.38/0.03 0.26/4.1 2.60/0.02 0.70/11.0 2.10/0.02 0.67/10.8 6.83/0.06 3.71/58.5 22.02/0 .21 6.34 5.56 7.86/0.08 0.52/4.9 4.95/0.05 0.90/8.5 2.48/0.03 1.10/10.4 13.52/0.14 5.71/54.2 31.31/0 .33 10.57 4.26 7.24/0.07 0.45/4.3 2.29/0.02 0.99/9.4 3.14/0.03 1.01/9.6 10.89/0.11 4.56/43.1 41.77/0 .43 10.57 4.48 3.67/0.03 0.22/2.4 4.55/0.04 1.18/12.9 4.29/0.04 0.59/6.4 9.51/0.08 4.16/45.4 31.51/0 .34 9.16 5.34 2.37/0.02 0.22/2.1 0.03/0.00 1.50/14.2 4.01/0.03 0.55/5.2 7.74/0.06 3.37/31.9 42.87/0 .42 10.57 5.48 8.94/0.09 0.64/5.7 1.09/0.01 0.93/8.3 2.99/0.03 1.22/10.8 11.59/0.12 2.15/19.1 33.21/0 .41 11.27 4.29 7.60/0.08 0.48/4.3 1.36/0.01 1.03/9.1 3.54/0.04 1.04/9.2 10.56/0.11 4.82/42.6 42.72/0 .51 11.27 4.37 6.51/0.07 0.56/4.7 0.56/0.006 1.13/9.4 3.70/0.04 1.16/9.6 8.29/0.09 5.89/49.2 43.78/0 .51 11.98 4.10 7.41/0.08 0.52/4.3 1.78/0.02 0.90/7.5 2.52/0.03 1.03/9.6 9.89/0.11 6.74/56.3 17.92/0 .21 11.98 4.05 – 0.51/4.5 – 1.03/9.1 – 1.05/9.3 – 2.59/23.0 – 11.27 – – 0.37/3.7 – 1.16/11.8 – 0.84/8.5 – 4.17/42.3 – 9.86 – 4.37/0.04 0.34/3.2 2.61/0.03 1.30/11.8 2.75/0.03 0.81/7.7 9.09/0.09 3.58/33.9 22.50/0 .21 10.57 4.90 1.55/0.01 0.18/2.0 1.11/0.01 1.08/11.8 3.58/0.03 0.31/3.4 2.44/0.03 6.10/66.6 12.20/0 .11 9.16 5.53 8.89/0.09 0.57/4.8 4.03/0.04 1.10/9.2 2.64/0.03 1.05/8.7 14.08/0.14 7.42/61.9 26.40/0 .32 11.98 4.07 7.26/0.07 0.50/4.2 1.78/0.02 1.14/9.5 2.99/0.03 1.05/8.7 10.2/0.11 5.07/42.3 55.76/0 .62 11.98 4.16 1.84/0.02 0.12/1.2 1.32/0.01 1.57/15.9 3.92/0.03 0.37/3.7 4.15/0.04 1.96/19.9 43.09/0 .42 9.86 5.36 5.36/0.05 0.40/3.8 2.27/0.02 1.26/11.9 2.48/0.03 0.89/8.4 8.58/0.08 3.96/37.5 30.87/0 .32 10.57 4.41 2.47/0.02 0.08/1.6 2.44/0.02 1.53/16.7 4.63/0.04 0.41/4.5 5.65/0.05 3.86/42.1 3.45/0 .03 9.16 5.25 8.62/0.09 0.59/6.0 3.28/0.03 0.93/9.4 2.65/0.03 1.07/10.9 12.82/0.13 5.94/60.2 40.27/0 .42 9.86 4.20 3.12/0.03 0.26/3.1 1.71/0.02 1.25/14.8 2.87/0.03 0.61/7.2 5.94/0.06 4.06/48.0 9.25/0 .09 8.45 4.90 5.56/0.05 0.36/3.7 2.81/0.03 1.01/10.7 2.91/0.03 4.14/42.8 28.33/0 .33 P or Si, and B is the ratio percentage of the content of each form of P or Si to total P or Si in sediment. 1) unit:

CF

P Si P M3-5 0.069/1.2 1.24/0.01 0.25/4.4 M3-7 0.093/1.5 0.73/0.006 0.32/5.1 M3-9 0.092/1.5 0.95/0.008 0.34/5.4 M5-1 0.044/0.5 0.54/0.005 0.35/3.8 M5-3 0.087/0.9 1.17/0.01 0.48/4.9 M5-5 0.140/1.5 1.62/0.02 0.47/5.1 M5-7 0.074/1.1 1.30/0.01 0.39/5.5 M5-9 0.078/1.1 1.05/0.01 0.30/4.3 M6-5 0.051/0.5 1.24/0.01 0.36/3.2 M7-2 0.086/1.4 0.85/0.008 0.34/5.4 M7-4 0.078/0.7 0.71/0.007 0.50/4.7 M7-6 0.083/0.8 1.36/0.01 0.48/4.5 M7-8 0.096/1.0 1.29/0.01 0.27/2.9 M7-10 0.070/0.7 1.31/0.01 0.26/2.5 M8-2 0.077/0.7 1.56/0.02 0.50/4.4 M9-1 0.067/0.6 1.60/0.02 0.49/4.3 M9-3 0.085/0.7 1.22/0.008 0.51/4.3 M9-5 0.052/0.4 0.70/0.02 0.46/3.8 M9-7 0.096/0.9 – 0.44/3.9 M9-9 0.079/0.8 – 0.39/4.0 M9-11 0.100/0.9 2.11/0.008 0.37/3.5 M9-12 0.033/0.4 0.79/0.01 0.10/1.1 M10-1 0.066/0.6 1.17/0.01 0.41/3.4 M10-3 0.088/0.7 1.17/0.01 0.46/3.8 M10-7 0.066/0.7 0.99/0.01 0.18/1.8 M11-2 0.082/0.8 0.95/0.01 0.41/3.9 M11-4 0.053/0.6 0.74/0.007 0.21/2.3 M11-6 0.081/0.8 0.92/0.01 0.40/4.1 M11-8 0.060/0.7 1.11/0.01 0.29/3.4 Avg2) 0.077/0.9 1.17/0.01 0.37/3.9 ∗ A/B, A is the content of each form of mmol/L. 2) Avg is short for Average

Site

Table 2.15. Contents of different forms of phosphorus and silicon in the surface sediments (μmol/g)∗

2.3 Biogeochemical Processes of Phosphorus and Silicon in the Bohai Sea 199

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(2) Carbonate bound form (CF) CF-Si was the dominant form of transferable silicon. This was probably due to the transport of large quantities of silt with a high concentration of carbonates from the Huanghe River each year. The content of CF-Si measured at station M11-6 was the highest. CF-P was also the major existence form of inorganic phosphorus. In earlier researches on the bioavailability of various forms of phosphorus in sediments, CF-P was considered as an inapplicable form. However, the fact is that when environmental changes are large (for example, the pH value drops down), considerable amounts of carbonates are soluble and can enter into waters to participate in the biogeochemical cycling. In this research, we found that the high-value area of CF-P appeared in Bohai Bay and the waters off the Luanhe River mouth, thus providing some supporting evidence for the above conclusion. (3) Iron-manganese oxide form (IMOF) The content of IMOF-P was relatively high in Bohai Bay, and the waters off the Huanghe River Estuary and the Luanhe River mouth. This was related to two factors: the sediments’ redox environment and the constant transport of a high content of iron in large quantities of silt in the coastal runoffs. It is generally accepted that IMOF-P cannot be easily utilized by organisms in a non-reductive environment, whereas in a reductive environment Fe(III) is easily reduced to Fe(II) and causes the release of P to the overlying water. This phenomenon was reported to occur in the estuary of the Hooghly River (Vaithiyanathan et al., 1993). Due to the influence of human activities in the coastal region (mainly the discharge of pollutants and nearshore aquaculture) more quantities of organic matter settle down and bacterial activity is more active in Bohai Bay and Laizhou Bay, thus making the environment reductive in part of the bottom sediments in these areas, hence the lower distribution of IMOF-P here. However, in most parts of Bohai Bay and the western part of Laizhou Bay, the very prominent reductive environment led to a high value of IMOF-P there (Song, 1997). Obviously, the transfer of IMOF-P was influenced not only by the redox environment but also by other factors, which in some cases are not negligible. (4) Organic matter+sulphide form (OSF) The OSF-P was the main form of transferable phosphorus, and was influenced to a considerable degree by the redox properties of the sedimentary environment. (5) Biogenic form (BF) The contents of BSi and BP were much higher than those of silicon and phosphorus in transferable form, indicating that through a series of early diagenesis after the precipitation of organisms or their metabolic products, most of the phosphorus and silicon contained in them had transformed into relatively stable forms and no longer participated in the biogeochemical cycling.


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Of the forms of transferable phosphorus in surface sediments, OSF-P was the predominant form, accounting for 10.7% of the total phosphorus; CF-Si was the predominant form of transferable silicon, accounting for 0.05% of the total silicon. There was a certain similarity in the diagenesis of various forms of phosphorus. The content of the transferable form of silicon accounted for less than 0.12%, indicating that previous research on the biogeochemical process of silicon by means of the determination of the total silicon content cannot give us any valuable information (Song et al., 2003). 2.3.2.2 Biogeochemical Processes of Phosphorus and Silicon of Surface Sediments in the southern Bohai Sea Earlier research indicated that many factors affect the contents and distributions of P and Si in sediments as well as P and Si transfer across the sediment seawater interface. Marine sediments are an important source and sink of P and Si in sediments and have more important functions than scientists considered before. Hence, it is necessary to ascertain which factor has the largest impact on the cycling (release and burial) of P and Si in the southern Bohai Sea. In other words, the major factor controlling the interface cycle of P and Si must be determined. (1) Correlation analysis of different forms of phosphorus in the surface sediment The forms of phosphorus and silicon in the southern Bohai Sea surface sediments are described clearly above. The questions concerning the factors that control their forms and what are the functions of these P and Si forms are answered in the following sections. In general, the coefficients of the correlation r between various forms of P or Si contained in sediments and the sedimentary parameters may reveal some possible correlations among other constituents (Tables 2.16 and 2.17) and help provide some answers to the above questions. Of all forms of phosphorus, IMOF-P exhibits a significant positive correlation with CF-P, BP, TIP, and TP, indicating that IMOF-P plays an important role in the biogeochemical process of phosphorus, that the transfer of phosphorus near the interface is chiefly related to the oxidative-reductive process, and that the diagenesis processes of various forms of phosphorus are similar, whereas TSi exhibits a negative correlation with silicon in other phases (CFSi, BSi) or exhibits no significant correlation, indicating that very little of silicon in sediments can participate in the cycling and that no valuable information can be derived from previous studies on the biogeochemical process of silicon based on determination of the total silicon. BSi, as a main source of dissolved silicon, shows a good positive correlation with CF-Si and IEF-Si and a significant negative correlation with TSi. The possible dissolution effect on the transfer of silicon and the differences between the transfer mechanisms of phosphorus and silicon make them produce totally different results in the geochemical process. The relations of various forms of phosphorus and silicon

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Table 2.16. Correlation coefficients for different forms of phosphorus in surface sediment with respect to sedimentary parameters (n=29, P 0.217 (D4)>0.213 mg/g (D7). Moreover, both D4 and D7 had similar distribution patterns and close concentrations. Generally speaking, the distribution of TN is controlled by the source and particle size of sediments. There was no significant correlation observed between TN and OC, considering all the stations selected.

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea 0.01

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Fig. 3.52. Vertical profiles of nitrogen in Jiaozhou Bay sediments (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center)

358

3 Biogeochemical Processes of the Yellow Sea

(2) IEF-N The ion exchangeable form of nitrogen (IEF-N) is the most active part of nitrogen in sediments, which plays an essential role in the nitrogen cycle. And the NO3 -N was the dominant state in IEF-N in Jiaozhou Bay sediments. Factors influencing the distribution and concentrations of IEF-N in sediments include mainly temperature, salinity, pH, OC (organic carbon) and the characteristics of clay minerals, etc. The correlative coefficients of IEF-N and other sedimentary environmental parameters were calculated and tabulated in Table 3.29. NH4 -N is the first product of organic matter mineralization in sediments, which can be nitrified or assimilated by microorganism, or be released to overlying water, or absorbed by clay minerals. When there is a relatively high exchange capacity, the NH4 -N concentration produced by mineralization is very low when entering the overlying water. Moreover, especially in coastal sediments, where it is in a comparatively anoxic condition, the NH+ 4 absorbed onto the sediments surface can reach up to 2/3 of the total NH4 -N and only a little can be released into the water body. Accordingly, it will exert great influence on the transformation flux in the sediment-water interface, the supply of nutrients, and the primary production. For Jiaozhou Bay sediments, the average IEF-N concentrations of the selected three core sediments were 0.019 (D7)>0.016 (B3)>0.010 mg/g (D4), which accounted for 9.22% (D7), 5.38% (B3), and 4.42% (D4) of the TN average, respectively. The highest concentrations of IEF-N were 0.027 (D7), 0.018 (B3), and 0.015 mg/g (D4) and were 22, 78, and 18 cm below the surface sediments, respectively. And the lowest ones were 0.012 (D7), 0.013 (B3), and 0.006 mg/g (D4), which were 30, 22, and 68 cm below the surface sediments, respectively. The vertical distributions of IEF-N of the three core sediments were different but almost had the same distribution. However, the patterns, which were the IEF-N concentrations, decreased with depth. However, the pattern was not striking for B3 core sediments and varied greatly for D7. Moreover, below 40 cm, the IEF-N concentrations varied vertically by less than 0.001 mg/g, which showed there is little variation in the supply of IEF-N and, on the other hand, it also indicated that the mineralization of organic matter in sediments took place generally at the oxygenous surface and NO3 -N was the dominant form of IEF-N. As Table 3.29 shows, IEF-N had good positive correlation with φ (r=0.652, n=19, P B3>D7. Overall, the sediments of D4 and B3 are land-derived input and D7 is mainly autochthonous. Generally speaking, the Si/DIN and Si/P ratios can be used to identify the nutrient limiting factors for the growth of phytoplankton. With Si/PFe-P>Oc-P>Ex-P>Al-P. And the authigenic apatite-bound phosphorus and detrital phosphorus were the dominant components of inorganic phosphorus. (1) Exchangeable phosphorus (Ex-P) Loosely sorbed P or exchangeable phosphorus (Ex-P) is often omitted because the concentration of Ex-P is very low in normal sediments. The Ex-P is mainly the phosphorus that is absorbed into the oxide and oxyhydrate and onto the surface of clay mineral particles in sediments, which is very labile in oxic conditions. The physical-chemical parameters such as temperature, pH, hydrodynamics and biological turbations have great influence on the release and adsorption of phosphorus. The concentrations of Ex-P are 0.006∼0.016 mg/g (D4), 0.024∼0.049 mg/g (D7), and 0.007∼0.014 mg/g (B3) with a percentage of 2.20%∼6.62% (D4), 6.85%∼18.50% (D7), and 2.94%∼6.53% (B3) of TP, respectively. The Ex-P concentrations vary vertically and are comparatively complicated and drastic (as Fig. 3.55 shows), which indicates that there may have been larger variations in the supply of Ex-P. Take B3 as an example, the highest concentration of Ex-P is in the surface sediments but the lowest is at 78 cm below surface sediments. However, for D4 and D7, the trends are not very clear, the highest concentrations both appeared in the middle of sediments and the lowest appeared in the deep sediments. Generally, the Ex-P concentrations decrease from the inner bay and mouth of the bay to the outside bay. (2) Aluminum-bound P (Al-P) Aluminum-bound P (Al-P) is the lowest among the six inorganic phosphorus types. The concentrations of Al-P are 0.0018∼0.0036 mg/g (D4), 0.0020∼0.0030 mg/g (D7), and 0.006∼0.009 mg/g (B3), accounting for 0.61%∼ 1.32% (D4), 0.51%∼1.10% (D7), and 2.01%∼4.07% (B3) of TP, respectively. The highest Al-P concentrations mostly appear in the surface and subsurface sediments. Except for D7, the lowest Al-P concentrations of D4 and B3 were at the bottom of sediments. On the whole, the Al-P concentrations decrease from the inner bay to the outside bay; that is, they follow the order B3>D4>D7. Except for B3, the Al-P concentrations of D4 and D7 vary vertically by less than 0.001 mg/g, which showed there are little variations in the supply of Al-P. (3) Iron-bound phosphorus (Fe-P) Phosphorus also occurs in minerals containing iron in different forms, such as vivianite [Fe3 (PO4 )2 ·8H2 O] and strengite [FePO4 ·2H2 O]. The Fe oxides have a high affinity for phosphorus and Fe oxides present in the oxic layer of the sediments may act as a trap for upward diffusing phosphate. This fraction is usually considered as a potentially mobile pool of P and is algal available. The Fe-P concentrations in Jiaozhou Bay sediments are 0.018∼0.030 mg/g (D4), 0.012∼0.067 mg/g (D7), and 0.015∼0.042 mg/g (B3) with a percentage

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of 6.92%∼12.25% (D4), 4.70%∼18.66% (D7), and 7.44%∼20.93% (B3) of the total inorganic phosphorus, respectively. The Fe-P concentrations in the surface sediments are not the highest but are still relatively high and decrease with depth. The vertical profiles of Fe-P in Jiaozhou Bay sediments are similar to those of Al-P with the highest concentrations in surface or subsurface sediments and the lowest ones are at the bottom except for D7. The average Fe-P concentrations follow the order B3 (0.032 mg/g)>D7 (0.031 mg/g)>D4 (0.024 mg/g). The Fe-P concentrations are greatly affected by the sources, characteristics of sediments, mineral compositions, particle size of sediments, human activities, and other environmental factors. The redox cycle Fe greatly affects P geochemistry after burial (Cha et al., 2005) and the oxidation-reduction cycle is important in controlling the fate of iron in most aquatic systems (El-Azim and El-Moselhy, 2004). Generally speaking, redox-sensitive phosphorus forms include phosphorus absorbed on Fe-oxides or in humic Fe(III)complexes, Fe(III)-phosphates and phosphorus occluded in Fe oxides. Under reducing conditions, the oxidized form of iron, defined as Fe3+ , is transformed to the reduced form, what is called Fe2+ . The former is a hardly soluble compound with phosphate ions and the latter a soluble one. Therefore, the Fe-P fraction is ready to be deposited in sediments under oxidized conditions and released to overlying water under reduced conditions. The pH is also an important factor influencing the vertical distributions of Fe-P concentrations. When the pH increases in sediments, it creates intense competition between hydroxyl and phosphorus ions at the same time, thereby weakening the combination of phosphorus and Fe3+ , and leading to the release of phosphates to the overlying water. (4) Occluded phosphorus (Oc-P) Occluded phosphorus (Oc-P) is the phosphorus that is tightly sealed by the oxyhydroxides of Fe and Al and it is difficult to release and utilize. The Oc-P concentrations in Jiaozhou Bay sediments are 0.015∼0.025 mg/g (D4), 0.011∼0.019 mg/g (D7), and 0.014∼0.031 mg/g (B3) accounting for 5.29%∼11.17% (D4), 4.19%∼7.26% (D7), and 6.53%∼13.86% (B3) of TP, respectively. The high concentrations of Oc-P in Jiaozhou Bay account for the high concentration of oxides in the zone. The vertical distributions of Oc-P concentrations are more variable with no marked features but, by and large, they decrease with depth and decrease from the inner bay to the outside bay. Moreover, the surface sediments have the highest concentrations in the whole column. 3.5.3.3 Authigenic Apatite-Bound Phosphorus and Detrital Phosphorus Apatite group minerals are by far the most prevalent phosphate in sediments. General marine apatites can be hydroxylapatite [Ca10 (PO4 )6 (OH)2 ], fluoroa-

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea

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patite [Ca10 (PO4 )6 (FOH)2 ], and chlorapatite [Ca10 (PO4 )6 Cl]. However, fluoroapatite (CFA) is considered to be the primary diagenetic sink in tropical marine carbonate sediments. The calcium-bound phosphorus in the sediments is generally made up of two parts, detrital carbonate-bound P and authigenic apatite-bound P. Calcium-bound P is a relatively stable fraction of phosphorus and contributes to the permanent burial of phosphorus in sediments. In all phases, the calciumbound phosphorus is the most dominant form in the sediments of the D4 and D7 columns. In general, neutral or alkaline sediments contain calcium phosphate as the dominant phosphorus phase as water soluble phosphates react to form calcium phosphate precipitates. The authigenic apatite-bound phosphorus (ACa-P), consisting of the active phosphorus fractions, is mainly the parts of biogenic phosphorus, CaCO3 -bound phosphorus. And the fluor apatite presents itself only below 2 cm, so the marine plankton is the main source of the surface sediments. The ACa-P concentrations ranged from 0.025 to 0.058 mg/g (D4), from 0.039 to 0.079 mg/g (D7), and from 023 to 0.102 mg/g (B3) with a percentage distribution range of 9.09%∼24.00%, 11.00%∼27.03%, and 9.07%∼41.05% of the TP, respectively. The detrital phosphorus (Det-P) concentrations of D4, D7, and B3 stations vary from 0.042 to 0.109 mg/g, from 0.051 to 0.116 mg/g, and from 0.042 to 0.063 mg/g accounting for 20.39%∼42.14%, 14.52%∼40.28%, and 14.87%∼30.96% of TP, respectively. The vertical distributions of the two forms of Ca-P are very obvious, which increase with depth and it shows that the Ca-P tends to be buried. The CaP concentrations in the sediments of Jiaozhou Bay are very high, which can illuminate the same sources and they are calcium orthophosphates from sea facies. The Ca-P is the inert component in the sediments. For a high solubility product (Ksp : 1.66×1044 ), it has a minor influence on the overlying water. The Ca-P concentrations, including detrital carbonate-bound P and authigenic apatite-bound P, generally increase with depth, though there are some fluctuations, which may be due to the bioturbation. Generally speaking, the Ca-P is ready to be released to the overlying water when the pH of sediments decreases, but the process is difficult to realize. The ACa-P and the Det-P concentrations in Jiaozhou Bay sediments also have obvious turning points at 38 cm below the surface, shown by a sudden increase in the two Ca-P concentrations and this can account for the bloom of the plankton and further emphasizes the changes in the sedimentary environment so that the pH may increase abruptly. On the other hand, the sediment of D7 changed from clayey silt to fine sand below 38 cm, so the grain size is important for the phosphorus concentrations and distributions. From the discussions given above, we may infer that the forms and distributions of phosphorus vary with different sedimentary environments. Generally speaking, calcium and iron have a different reactivity in the pore water of sediments. The reactivity of iron becomes weaker during transformation from the freshwater sediments to the sea facies sediments, whereas calcium shows the reverse pattern, whose reactivity becomes stronger along with the

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change from freshwater sediments to sea facies sediments. Therefore, in the freshwater sediments, Fe-P is the main component and Ca-P is the primary part in the sea facies sediments. In the sediments of two cores in Jiaozhou Bay, the Ca-P accounts for the dominant part of TP and the Fe-P is the relatively subdominant portion, but still an important part of TP. Thus, it can be concluded that Jiaozhou Bay is influenced by human activity and the open sea, the YS, and it is a typical transition zone between land and ocean but, on the whole, the greater influence is what imposed on it by the whole marine system. 3.5.3.4 Burial Fluxes of Phosphorus To be buried with the sediments is not only the end but also the start of marine recycling of phosphorus. That is to say, sediments also act as both source and sink of phosphorus. We should know the phosphorus release from sediments and phosphorus buried by the sediments because the phosphorus is very important for the primary production in seawater. In some sense, the burial flux can be an indicator of environmental change at a definite time. Based on equations (3.15) and (3.16), the burial fluxes of phosphorus were descriptive in Fig. 3.56. As it showed, the burial fluxes of all nitrogen forms began to increase in the 1980s with a peak at the end of the last century. For example, the burial flux of inorganic phosphorus (IP) of B3 was 1.37 μmol/(cm2 ·yr) at the beginning of the 1980s, but with a rapid augmentation of 3.48 μmol/(cm2 ·yr) in the middle of the 1990s. And the burial flux of organic phosphorus (OP) at the beginning of the 1980s was 0.46 μmol/(cm2 ·yr) with a rapid increase of 1.57 μmol/(cm2 ·yr) at the end of the last century. Correspondingly, for B3, the burial flux of total phosphorus (TP) increased from 1.83 μmol/(cm2 ·yr) to 5.04 μmol/(cm2 ·yr) during the same period. Fortunately, at the beginning of this new century, with measurements adopted to manage pollution and eutrophication, the environmental quality has been improved greatly with the IP, OP, and TP burial fluxes dropping to 1.67, 0.15, and 1.82 μmol/(cm2 ·yr), respectively, even lower than those in the 1980s when the rapid development of Qingdao occurred. Therefore, the decrease in the extent of the OP burial flux is the greatest and the level may therefore even be the lowest in the past hundred years. It suggests that the burial flux of nutrient is a more effective proxy indicating the eutrophication history of Jiaozhou Bay compared with the abundance of nutrients. We can divide the eutrophication in Jiaozhou Bay in the past 100 years into three periods, according to the burial flux changes in nitrogen. In detail, the quantity and burial fluxes of phosphorus have similar trends: (1) increasing steadily from 1900 to 1980, (2) increasing rapidly after 1980, (3) accelerating from 1900 to a peak in 2000, and (4) decreasing from 2000 to the present. Fertilizers applied in the catchments areas are a dominant source of eutrophication in coastal areas. The amount of fertilizers applied annually in the catchment area of Jiaozhou Bay increased by three times from

373

2 Burial fluxes ( m mol/(cm yr))


.

Fig. 3.56. The burial fluxes of phosphorus at different periods in Jiaozhou Bay sediments (Li et al., 2006) (With permission from Springer)

1980 to 1998 indicating that a strong increase in fertilizer consumption that started in the early 1980s was followed by an increase in nutrient concentrations. The burial fluxes of phosphorus decreased from 2000 to the present. This trend is attributed to source reduction associated with increased regulation of wastewater discharge, industrial production of phosphorus fertilizers, increased industrial waste recycling and cleaner industrial processes. Much regulatory attention has also been given to sewage effluent and its detrimental environmental effects. Sewage treatment practices have been upgraded and monitored to reduce the concentrations of nutrients being introduced to the sediment record. The improvement in the Jiaozhou Bay sedimentary envi-

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ronment can be attributed to the measurements presented by the municipal agencies of Qingdao, which have also appropriated nearly 1.7 millions RMB in 2005. Anthropogenic activities have accelerated cycling and increased the nitrogen delivery to Jiaozhou Bay. It is therefore of great importance to understand how human activities change the nitrogen inputs. To interpret recent humaninduced impacts on phosphorus burial in Jiaozhou Bay, we consider the mean burial flux before the 1980s as the natural background and the mean between the 1980s and 2000 after subtracting the mean before the 1980s as the humaninduced input. We define the ratios as the anthropogenic factor (AF) based on equation (3.28). The AFs of phosphorus in Jiaozhou Bay sediments are tabulated in Table 3.33. Table 3.33. The anthropogenic factors (AF) of phosphorus inputs in Jiaozhou Bay (Dai et al., 2007a) (With permission from Elsevier’s Copyright Clearance Center) AF B3 D4 D7 Average

Ex-P 1.17 1.62 0.83 1.21

Al-P 0.97 1.94 1.78 1.56

Fe-P 1.21 2.52 2.76 2.16

Oc-P 0.73 3.38 0.96 1.69

ACa-P 1.18 2.14 0.85 1.39

Det-P 1.22 2.84 1.41 1.82

IP 1.11 2.59 1.27 1.66

OP 0.21 0.57 2.94 1.24

TP 0.74 1.84 1.70 1.43

As it shows, the relative anthropogenic inputs of phosphorus to the bay have been 1.21∼2.16 times greater than the values of natural inputs before the 1980s, which may indicate there are considerable changes in phosphorus burial fluxes between the 1980s and 2000 and perhaps it is the result of phosphorus fertilizers and agricultural discharge and other sources derived from Qingdao City. In particular, Det-P and Fe-P are sensitive to human impact in their inputs to the bay and the AF values were 1.82 and 2.16, respectively. As it shows, the AFs of inorganic phosphorus are larger than those of organic phosphorus. Moreover, the AFs of TP in different stations follow the order: D4>D7>B3 (Dai et al., 2007b). 3.5.4 Biogenic Silica in the Sediments Silicate, or silicic acid (H4 SiO4 ), is a very important nutrient in the ocean. Unlike other major nutrients such as phosphate and nitrate or ammonium, which are needed by almost all marine plankton, silicate is an essential chemical requirement only for certain biota such as diatoms, radiolarian, silicoflagellates, and siliceous sponges. But siliceous phytoplankton contributes significantly to the primary production in the world’s oceans. More than 40% of the entire primary production is contributed by diatoms, which reveals a close coupling of silica and carbon in the ocean. Therefore, silicate cycling has received sig-


375

nificant scientific attention in recent years and many scientists have studied silicate behavior in the marine environment. Biogenic silica is one kind of amorphous silica or is termed biogenic opal, or opal in brief. Biogenic silica in sediments mainly consists of fecal pellets and reliquiae of siliceous plankton such as diatoms, radiolarian, silicoflagellates, and siliceous sponges. It is estimated that about 240×1012 mol silica was fixed by marine silica biota every year. Nevertheless, most of the biogenic silica produced in the euphotic zone dissolves during settling. Nelson et al. (1995) estimated that on a global scale at least 50% of the silica produced by diatoms in the euphotic zone dissolves in the upper 100 m of a water column. Diatom frustules that escape dissolution during settling reach the sea floor, where a major fraction of the silica dissolves at the sediment-water interface or in surface sediments, and only 3% of the silica produced in the euphotic zone is finally buried in the sediment. Calculation shows that the silica supplied to the world’s oceans from biogenic silica dissolution in sediment is 4 times that from rivers, and the regeneration of biogenic silica in sediment is the main source of silica in seawater. There are a number of factors that influence the dissolution of biogenic silica in sediment. Besides the source of silica, the dissolution is related to the water depth, water temperature, the degree of the saturation of silicate in pore water and overlying water too. Silica accumulation in sediment will strongly influence the silica concentration in seawater. Indeed, silica may limit the growth of diatoms under some particular conditions. (1) High content of biogenic silica in Jiaozhou Bay sediment The biogenic silica contents in three cores sediment from Jiaozhou Bay are shown in Table 3.34 and Fig. 3.57, by which an overall low discrimination of biogenic silica content is indicated. From the contrast between the biogenic silica content in surface sediment (0∼10 cm) and subsurface sediment (10∼20 cm), supposing the sedimentation rate is constant, the content of biogenic silica in recent years is obviously higher than that in earlier years in the inner bay, indicating a faster accumulation of biogenic silica in recent years. On the whole, the average of biogenic silica in locations B3, D4, and D7 is 1.54%, 1.48%, and 1.39%, respectively, showing that the biogenic silica content decreases from the inner bay to the bay mouth and outer bay, which is the same trend as that of phytoplankton distribution (Wu et al., 2004). Therefore, the biogenic silica content in Jiaozhou Bay sediment is positively correlated to the biomass of phytoplankton. The vertical changes are small at the bay mouth and outer bay. But the change is bigger at the inner bay, and several high value intervals at 4∼6 cm, 22∼24 cm, 68∼70 cm, and 92∼94 cm are denoted, which is due to slow water exchange with the YS in the inner bay, and massive N, P nutrient inputs from surrounding rivers, which would bring about eutrophication and result in algae bloom if conditions are favorable. Consequently, biogenic silica that accumulated in sediment could fluctuate with the bloom, because the

376


Table 3.34. The content (%) of biogenic silica in Jiaozhou Bay sediments (Li et al., 2006) (With permission from Springer) Location 0∼10 cm 10∼20 cm Total

B3 (inner bay) 1.58 1.29 1.54

D4 (bay mouth) 1.44 1.49 1.48

D7 (outer bay) 1.48 1.40 1.39

Biogenic silica content ( % ( 0.5 0

1.5

2.5

3.5

Depth (cm)

20 40 60 80 100

D4 (bay mouth) B3 (inner bay) D7 (outer bay)

120

Fig. 3.57. The vertical distribution of biogenic silica in Jiaozhou Bay sediment (Li et al., 2006) (With permission from Springer)

frequency and scale of algae bloom in the inner bay are often higher than those in the bay mouth and outer bay. Therefore, the biogenic silica content in the inner bay is more variable. In addition, water exchange continuously increases from the inner bay to outer bay and the concentration of nutrient can decrease. As a result, the frequency and scale of the bloom will decrease from the inner bay to outer bay. Therefore, the content of biogenic silica in sediment remains more stable in the outer bay than in the inner bay. According to the sedimentation rate we calculated for Jiaozhou Bay, the high content of biogenic silica at 4∼6 cm in location B3 may probably be related to the event of a large red tide in 1998.


377

(2) The ratios of Si:N and Si:P The ratios of Si:N and Si:P can be used to estimate limitation nutrient factors for phytoplankton growth. In general, when Si:POC>Si, which is consistent with the phenomenon that the

3.5 Biogeochemical Processes of Jiaozhou Bay, South Yellow Sea 3

379

n=15, r=0.65

2.5 2 1.5 1

Biogenic silica (%)

0.5 0 0 2

B3 0.2

0.4

0.6

0.8

0.4

0.6

0.8

n=15, r=0.79

1.5 1 0.5 D4 0 0

0.2

2

n=13, r=0.54

1.5 1 0.5 D7 0 0

0.1

0.2 0.3 0.4 Organic carbon (%)

0.5

Fig. 3.58. Relationship between biogenic silica and organic carbon in Jiaozhou Bay sediments (Li et al., 2006) (With permission from Springer)

decomposition rates of C, N, and P are faster than that of biogenic silica indicated by the ratios of OC:BSi, BSi:TN, and BSi:TP in sediment of Jiaozhou Bay, which improved the BSi that accumulated in the Jiaozhou Bay sediment. (4) Flux of biogenic silica sedimentation in Jiaozhou Bay sediment The sedimentation flux can reflect quantitatively how much sediment may accumulate. Being compared with primary production, the amount of matter that decomposed and came back to the water during settling can be estimated. By comparing it with the flux across the interface of sediment-water, we can calculate how much matter is buried. Therefore, how much biogenic silica can be buried and how much biogenic silica can be taken back to the seawater in Jiaozhou Bay can be determined based on equations (3.14) and (3.15), with the results tabulated in Table 3.36.

380


Table 3.36. The fluxes of biogenic silica sedimentation in Jiaozhou Bay (Li et al., 2006) (With permission from Springer) Region Inner bay Bay mouth Outer bay

Water content (%) 65.6 65.4 80.3

Sedimentation rate (cm/yr) 0.85 1.63 0.45

Sedimentation flux (mmol/(m2 ·d)) 5.34 9.87 2.08

According to the primary production of (41.9±15.3) mmol/(m2 ·d) (Wang, 2003), the biogenic silica production from diatom, calculated in the Redfield ratio is (6.32±2.35) mmol/(m2 ·d). Using the sedimentation flux at B3 (inner bay) as the average flux of biogenic silica sedimentation in the bay, we found that only about 15.5% of diatom-produced biogenic silica can be decomposed during sinking down to the bottom, about 84.5% of biogenic silica can reach the bottom and be deposited. Because the water depth of Jiaozhou Bay is small (7 m on average), most primary production may be deposited before decomposition takes place. The flux of biogenic silica from the sedimentwater interface to water, which can represent the silicate release rate, is 3.3 mmol/(m2 ·d) (Jiang et al., 2002), less than the inner bay sedimentation flux. Consequently, silica continuously transported from sea water to sediment results in low silica concentration in seawater for phytoplankton growth. Above all, the accumulation of biogenic silica in sediment is a major mechanism for Si limitation in phytoplankton growth in Jiaozhou Bay. Based on information about biogenic silica revealed from three sediment cores from Jiaozhou Bay, the reasons for silica limitation in local phytoplankton growth were discussed. The main conclusions can be summarized as follows: •

The biogenic silicate content in Jiaozhou Bay sediment is obviously much higher than that in the YS and the Bohai Sea. The sediment of the bay is characterized by a high content of biogenic silicate, being 1.58%, 1.44%, and 1.48% in locations B3, D4, and D7, respectively. In the inner bay, the content at the sediment surface is higher than it is underneath, indicating that the accumulation rate in recent years is greater than that in earlier years. • In Jiaozhou Bay sediment, OC, N, and P decompose much faster than BSi does in similar conditions, which means that most biogenic silicate will be buried and separated from silicate recycling. • Only 15.5% of biogenic silicate is hydrolyzed during the journey from the surface to the bottom in seawater, thus ca. 84.5% can be deposited on the bottom. The silicate release rate at the sediment-seawater interface is quite a bit lower than its accumulation rate from water to sediment. They are the main reasons for a constantly low silicate concentration in sea water and Si-limitation in phytoplankton primary production. In one


381

word, silicate accumulation in sediment is the key factor to explain silicate limitation in phytoplankton growth in Jiaozhou Bay (Li XG et al., 2006). 3.5.5 Nutrients (N, P, Si) in the Seawaters We all know that marine sediment acts as source or sink for nitrogen and phosphorus in seawater. The sediment as the source will release nitrogen and phosphorus into the water for the phytoplankton use. However, when the nitrogen and phosphorus in seawater are oversupplied by river input mainly from human activities, the phytoplankton will grow fast to bloom, and produce more faecal pellets and reliquiae into sediments, the sediment becomes the sink of the nitrogen and phosphorus in seawater (Li et al., 2004). Therefore, nitrogen and phosphorus in sediment play a key role in a marine ecosystem, on which subject many studies have been published. Nitrogen and phosphorus in sediment can be divided into an organic form and an inorganic form. Both can be utilized by phytoplankton as well as zooplankton and microorganisms. The distributions of nitrogen and phosphorus in the two forms in sediment are obviously different. In general, the contents of IN and IP increase with depth in surface sediments, and then gradually stabilize in the subsurface. However, the vertical distributions of ON and OP are the reverse, and the corresponding distributions are clearer with an increase in depth. The reason is that most microorganisms live in surface sediment, causing rich organic nitrogen and phosphorus, which decrease with depth in surface sediment, or remain roughly stable in deeper sediment. The content and distribution of the nutrients bear rich environmental and biogeochemical information about the nitrogen and phosphorus cycle, which is presented in this paper taking Jiaozhou Bay as an example (Fig. 3.59).

Fig. 3.59. Investigation sites in Jiaozhou Bay (Yang et al., 2003) (With permission from Yang DF)

382


N and P inputted into Jiaozhou Bay by river and by sewage effluents of cities have made the bay become more and more eutrophic day by day. It was thought that phytoplankton growth was limited by the change from nitrogen to phosphorous, and that the silicate concentration in Jiaozhou Bay is important for limiting diatom growth. Inorganic phosphorous and total inorganic nitrogen were thought to be abundant enough to meet the requirements of phytoplankton growth and reproduction; i.e., N and P of Jiaozhou Bay were not limiting nutrients for phytoplankton growth. Zhang and Shen (1997) indicated that the probability of dissolved inorganic element nitrogen and phosphorous as the limiting factors in phytoplankton growth was very small or close to zero at the surface layer of Jiaozhou Bay. (1) Seasonal variation and horizontal distribution of nutrients N, P, and Si DIN (the sum of nitrate, nitrite, and ammonium) and phosphate concentrations did not have an obvious annual, less than annual, or even seasonal cyclical variation. Throughout Jiaozhou Bay in one year, the DIN values were >2.36 μmol/L and the phosphate values were >0.16 μmol/L. Silicate seasonal variation showed a repeated annual cyclic pattern. Every year there was only one highest value of silicate concentration in summer. Throughout Jiaozhou Bay, silicate values were 2 μmol/L. For example, at stations 1 and 4 in Jiaozhou Bay, the seasonal variations of DIN concentration, phosphate concentration and silicate concentration were as shown in Figs. 3.60, 3.61, and 3.62.

DIN1

DIN4

Fig. 3.60. Seasonal variations of DIN concentration at stations 1 and 4 (Yang et al., 2003) (With permission from Yang DF)

(2) Seasonal variation and horizontal distribution of the nutrient Si:DIN ratio From December to May, Si:DIN ratios were 0.5 generally from June to November (Fig. 3.63), with such a distribution pattern being repeated each year. The variation of nutrients N and P in Jiaozhou Bay is mostly due to human activity. In the last 30 years the population around Jiaozhou Bay has increased

0.6

P1

383

P4

0.5 0.4 0.3 0.2

1994-2

1993-11

1993-8

1993-5

1993-2

1992-11

1992-8

1992-5

1992-2

1991-11

0

1991-8

0.1

1991-5

Phosphate concentration (mmol/L)


Date (Year-Month)

8

Si1

Si4

6 4

1994-2

1993-11

1993-8

1993-5

1993-2

1992-11

1992-8

1992-5

1992-2

1991-11

0

1991-8

2

1991-5

Silicate concentration (mmol/L)

Fig. 3.61. Seasonal variations of phosphate concentration at stations 1 and 4 (Yang et al., 2003) (With permission from Yang DF)

Date (Year-Month)

Si:DIN and Si:16P ratios

Si:DIN and Si:16P ratios

Fig. 3.62. Seasonal variations of silicate concentration at stations 1 and 4 (Yang et al., 2003) (With permission from Yang DF) Si:DIN2

Si:16P2

Si:DIN5

Si:16P5

Fig. 3.63. Seasonal variations of Si:DIN and Si:16P ratios at stations 2 (a) and 5 (b) (Yang et al., 2003) (With permission from Yang DF)

384


rapidly, and the amount of industrial waste and daily living sewage inputted into Jiaozhou Bay has increased rapidly. Among the rivers flowing into the bay in 1980, the Haibo River carried 140×103 t/yr of sewage into Jiaozhou Bay. Also, in recent years, the long time application of nitrogenous fertilizers to the bay’s beach lands and coastal waters favorable for agriculture resulted in an input of much N and P into bay waters. The direct and indirect intervention of human activity has broken down the ecological cycle of nutrients N and P so that they now show an increasing trend (Yang et al., 2003).

3.6 Biogeochemical Characteristics of Heavy Metals in Yellow Sea Sediments Since the 1960s, China and Korea have experienced rapid industrial developments, resulting in the degradation of the marine environmental quality mainly due to the increase in population, production of pollutants and deterioration of natural habitats. The rapid marine environmental change with respect to its oceanographic characteristics has drawn increasing international attention over the last three decades (Song, 2004). A certain level of heavy metals in the ocean may result in changes in the ecological equilibrium. With the call for sustainable development and marine environmental conservation, it was critical to understand the characteristics of pollution and the ecological risk of heavy metals. 3.6.1 Distributions of Heavy Metals The distributions of Cu and Zn were similar (Fig. 3.64), which showed a bimodality trend that went from low to high, was then low, and finally a high trend from west to east. There was a descending trend from north to south. The YS trough was the dividing line separating the south YS from the western and eastern parts. In the west, distributions resembled grain size distribution, and in the east they were parallel to the Korean coastline. A high content area existed in 35◦ ∼36◦ N, 122◦ ∼123.5◦ E district. B7 and C5 stations were high centers of Cu while A6, B6, and B7 were Zn’s high centers, with a highest figure of 186.2 mg/kg at the B7 station. In those high content stations, clay (dII region>III region; i.e., the concentrations of the four heavy metals of Cu, Hg, Pb, and Zn increased as the clay proportions increased and the sediment grain size decreased, and As and Cd vary irregularly without any consistent pattern with the sediment characteristics. Moreover, the relationships between heavy metal concentrations in the TOC contents, as well as sand, silt, and clay proportions, did not display consistent trends in the three regions either. For example, in I region, all the heavy metal concentrations positively related to the clay concentration except for Pb, with the r value ranging from 0.39 to 0.90, while in III region, a negative relationship occurred, with the r value ranging –0.67∼–0.34. The status of the sand proportion was even reversed. Inside, Cd and Cu showed well their positive linear relationships with clay in I region, and it was also true of TOC contents because they were subject to chelate with the organic matter. Cd and Cu can be transformed with the complex compounds, which lead to an increase in Cd and Cu concentrations in sediments. Analyzing from the general to the specific, not all the heavy metals related well with the sediments characteristics. The factors controlling metal distributions in the sediments were mainly classified according to natural conditions and anthropogenic sources, wherein the anthropogenic sources include various contamination inputs caused by human dispersal into the environment, including dry and wet deposition, industrial and municipal sewage, marine transportation pollutant and the exploitation of underground mining. The natural conditions include the proportion of sediment types caused by hydrodynamic conditions, the salinity, pH and Eh of seawater and sediments, and so on. According to the above discussion, it is clear that the source and the sediments type did play the dominant role. Therefore, further studies about the other factors will be quite necessary, and emphasis should be placed on the salinity, pH and Eh of sediments and the hydrodynamics conditions of the SYS. Analyzing the linear relationship between a pair of heavy metals, the coefficients matrix of the linear relationship was shown in Table 3.40. As and Cd,

3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments

405

Table 3.40. Correlation coefficients between heavy metal concentrations and the sediment types and TOC concentration of the SYS surface sediments (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center) Heavy

Range

metal

(mg/kg)

TOC vs. heavy (mg/kg) metal P r Average

Sand vs. heavy metal P r

I region (6 stations) As 5.20∼8.28 6.85 0.3 0.5 0.03 Cd 0.081∼0.239 0.188 0.6 0.25 0.4 Cu 19.1∼33.0 25.6 0.5 0.34 0.02 Hg 0.046∼0.066 0.057 0.6 0.29 1 Pb 13.74∼26.35 20.35 0.9 0.07 0.06 Zn 74.3∼141.0 105.2 0.03 0.84 0.5 II region (12 stations) As 3.80∼8.35 5.88 0.8 0.1 0.4 Cd 0.042∼0.227 0.0969 0.3 0.33 0.1 Cu 8.2∼25.6 15.88 < 0.01 0.81 0.3 Hg 0.011∼0.058 0.037 0.04 0.59 0.6 Pb 4.91∼35.59 16.29 0.01 0.86 0.5 Zn 32.7∼108.0 64.35 0.1 0.47 0.3 III region (9 stations) As 4.34∼33.0 8.85 0.2 −0.55 0.3 Cd 0.061∼0.416 0.135 0.1 −0.58 0.1 Cu 6.8∼20.0 13.4 0.06 −0.69 0.09 Hg 0.015∼0.047 0.03 0.9 −0.07 0.1 Pb 7.9∼29.11 14.78 0.5 −0.26 0.2 Zn 44.0∼76.6 61.2 0.9 −0.06 0.1

Silt vs. heavy metal P r

Clay vs. heavy metal P r

−0.86 −0.43 −0.87 −0.01 0.79 −0.34

0.1 0.69 0.4 0.47 0.5 −0.37 0.02 0.90 0.4 0.44 0.1 0.71 0.5 −0.31 0.6 0.30 0.1 −0.7 0.5 −0.38 0.9 0.06 0.4 0.39

−0.29 0.5 0.31 0.19 0.22 0.33

0.2 0.08 0.1 0.3 0.2 0.3

0.37 −0.52 −0.47 −0.33 −0.44 −0.35

0.7 −0.14 0.7 −0.12 0.6 0.18 0.5 0.19 0.3 0.31 0.8 −0.067

0.38 0.54 0.60 0.54 0.45 0.57

0.5 0.3 0.09 0.07 0.3 0.07

−0.24 −0.42 −0.59 −0.64 −0.37 −0.64

0.1 0.05 0.2 0.4 0.1 0.3

−0.54 −0.67 −0.52 −0.34 −0.53 −0.41

Cu and Cd, Cu and Hg, Cu and Zn, and Zn and Hg presented significant linear relationships. Their correlation indices range from r =0.60 to r =0.76, which indicated that such elements had the same sources and a similar occurrence in the same samples. Correlation analysis results presented by Fig. 3.72 showed that the six heavy metals, except for As, positively related to PCBs concentrations, and there was no significant relationship between As concentration and PCBs concentrations. The positive relationships suggested that they had the same source, and the factors controlling PCBs distribution also worked on the heavy metals of As, Cd, Cu, Hg, Pb, and Zn. In addition, their function medium, mode, and outcomes displayed similarities. (4) Phytoplankton and zooplankton biomass Organic matter in sea surface sediments was mainly terrigenous and biogenous. So the phytoplankton biomass impacting on organic matter in the water column and on the PCBs distribution in the surface sediments of the study area was discussed in detail as follows. Chlorophyll a concentrations and phytoplankton cell abundance in seawater were commonly used as indicators of

3 Biogeochemical Processes of the Yellow Sea PCBs concentration (pg/g, dw)

35

5000 4000 3000 2000 1000 0

0

5

10 15 20 25 30 As concentration (mg/kg)

PCBs concentration (pg/g, dw)

r=0.36 P=0.07 6000

6000

5000 4000 3000 2000 1000 0

5

10 15 20 25 30 Cu concentration (mg/g) r=0.16 P=0.41

5000 4000 3000 2000 1000 0

0

5

10 15 20 25 30 35 40 Pb concentration (mg/g)


35


r= 0.13 P=0.51

6000



406

r=0.18 P=0.38

6000 5000 4000 3000 2000 1000 0

0

0.10 0.20 0.30 0.40 Cd concentration (mg/kg) r=0.14

P=0.48

6000 5000 4000 3000 2000 1000 0

0.01 0.02 0.03 0.04 0.05 0.06 0.07 Hg concentration (mg/g) r=0.32 P=0.10

6000 5000 4000 3000 2000 1000 0 20

40

60 80 100 120 140 Zn concentration (mg/g)

Fig. 3.72. Relationships between PCBs concentration and the concentrations of As, Cd, Cu, Hg, Pb, and Zn in the surface sediments of the SYS (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

phytoplankton biomass and regarded as important parameters to describe ocean ecosystems and environmental characteristics. The horizontal distributions of chlorophyll a concentration, phytoplankton cell abundances, and zooplankton wet weights in the SYS were shown in Fig. 3.73. As indicated in Fig. 3.73, there were general downtrends from northwest to southeast with the higher values of chlorophyll a concentrations and phytoplankton cell abundance observed in inshore surface seawater, especially around stations A1 and D9. The horizontal distribution of zooplankton wet weight was similar to the distributions of chlorophyll a concentrations and phytoplankton cell abundance. It showed a good correlation with PO4 -P and

3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments N 38

N 38

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Chl a-0m Cells density 33 33 120 121 122 123 124 125 126 127 128 E 120 121 122 123 124 125 126 127 128 N 38

407

E

37 36 35 34 Wet weight 33 120 121 122 123 124 125 126 127 128

E

Fig. 3.73. Horizontal distribution of the chlorophyll a (μg/L), plankton cell density (×103 cells/L), and zooplankton biomass based on wet weight (mg/m3 ) in the surface seawater of the SYS (Zhang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

a negative correlation with TOC. There were so many environmental factors imposing their effects on the phytoplankton biomass and primary productivity, such as nutrients, light availability, temperature, and so on (Falkowski and Woodhead, 1992). It was reported that in respect of controlling the photosynthetic capacity of the phytoplankton community, nutrients occupied the driving seat rather than temperature. Since the 1980s, the YS ecosystem has been suffering the P limit (N:P=25>16). Apparently, A1 and D9 came under the strong influence of the Changjiang River’s effluent waters and the surface runoff in the SYS western coast, where the P concentrations were very high. High nutrients promoted the growth of phytoplankton, and thus formed high chlorophyll a concentration regions. However, in the middle area away from the seaboards, nutrients could not be supplied from terrestrial input. Therefore, nutrient limitation restrained the growth of phytoplankton, thus forming a plume and a closed center with a significantly low chlorophyll a concentration. Studies showed that the nutrients level in seawater had a great impact on the phytoplankton biomass, sedimentation rate, suspended particle concen-

408


tration, particle organic carbon, and dissolved organic carbon. Phytoplankton were the major organisms that fix CO2 , in either particulate or dissolved forms via the photosynthetic process. They constituted the base of the marine food chain and thus might affect the dynamics of higher trophic levels due to the variability in their biomass and productivity. The dead phytoplankton, the byproduct of zooplankton metabolic activity through fecal pellet production and the vertical migration of zooplankton through feces production, represented fluxes of carbon at depth. The deep sea was the ultimate sediment trap. In the productive sea area, the vertical flux of organic matter was relatively high. Besides, O2 was almost exhausted by plankton and the remineralization of organisms in sediments was weak. So, the relatively high concentration of the particulate matter reaching the sea floor was maintained and TOC in surface sediments was high. In the middle of the SYS where there was a nutrient deficit, TOC in sediments was relatively low. So the PCBs concentrations were low, correspondingly. However, we saw a contrary trend in PCBs contribution. The discrepancy between biomass and TOC distribution indicated that the phytoplankton biomass was not a key factor to affect PCBs distribution, and the factors controlling suspended particles distribution played a more important role in the TOC concentration. (5) Impact of water hydrodynamics on PCBs in sediments Many previous reports proposed that the PCBs abundance fell precipitously as the distance increased from the contaminant source in many estuaries and coastal areas, while the PCBs horizontal distribution tendency in the SYS was very special (Mora et al., 2005; Chen et al., 2003; Colombo et al., 2005). From Fig. 3.31, it was easy to find that the middle sediments area presented a special characteristic, where the fine particle sediments developed and, through the above discussion, it could be indicated that the factors working mainly on PCBs distribution had direct and indirect contact with the sediment characteristics to some extent, such as sediment components and proportions, TOC contents, plankton biomass, and so on. According to earlier publications, hydrodynamics was the dominant driving power to affect the sediment types and distribution (Li et al., 2003; Dai et al., 2007). In the YS, the function of the warm current, coastal current, and circumfluence presented a piece of valid evidence that hydrodynamic conditions controlled the sediment deposition process. In general, the intensity of the current corresponded to the sediment grain size (Tang et al., 2000). It was thought widely that the sediments of the SYS middle and east areas were affected by Kuroshio and the Cheju Channel (Lan et al., 2005). In summer, a stable southerly coastal current in the west SYS existed; at the meeting of the YS and the East Sea, the flash water mainly originated from the Changjiang River and obviously expanded to Cheju Island, while in the east of the SYS, a northerly coastal current developed. Consequently, as a whole, the surface water approximately formed a marine basin scale anticlockwise circumfluence. In the bottom, a cold water mass developed and occupied the whole SYS. By way of CTD ob-

3.7 Biogeochemistry of PAHs and PCBs in the Yellow Sea Sediments

409

servation and the excursion of the man-made acaleph, the bottom current of the SYS approximately took on an anticlockwise cyclonic movement, too. In winter, a warm current with high salinity flowed from south to north in the SYS middle area, the so-called traditional YS Warm Current, on both sides of which there were two southerly coastal currents, forming a little circumfluence on the west coast. Furthermore, in the east, the YS Warm Current flowed on the outside continental shelf from south to north. A nearly circular weak current area came into being centered on the point of 36◦ N, 123◦ E, with the velocity of flow less than 0.4 m/s (Tang et al., 2000). Because of the effect of the cold mass and marine circumfluence, a large clay sediment region developed. In conclusion, the PCBs distribution was affected by the characteristics of sediments, such as grain size, clay proportion and TOC content, but the hydrodynamic condition was the most important factor affecting the sediment mode and sediment transportation tendency. In the middle sediment region, whose dominant component was clay, the surface area and sorption capacity increased with a decrease in the grain size. Meanwhile, due to the high TOC contents in fine particle sediments, the sorption capacity of the organic pollutant increased further. Thus the conclusion could be drawn safely that the characteristics of the sediments were the dominant factor controlling the PCBs distribution, and the driving power of the hydrodynamic conditions was the reason that led to this result (Zhang et al., 2007). 3.7.3 Contamination History of Polycyclic Aromatic Hydrocarbons and Polychlorinated Biphenyls in the 20th Century The natural sediment cores used to measure PAHs and PCBs were reported in many previous studies. In China, the studies on dating sediment trap experiments designed to reconstruct the historical usage of PAHs and PCBs were not carried out frequently for various reasons, and the sampling areas where they were studied mainly focused on the Pearl River Estuary, Changjiang River Estuary and Bohai Bay. Although many investigations have been presented on the distribution and sources of PAHs and PCBs in the surface sediment of the YS, the sedimentation rate and flux of core sediment samples, no available investigation was documented on the vertical distribution of PAHs and PCBs in the core sediment and this aspect of historical usage had not been included in the targets. D6 station (33.855◦ N, 123.033◦ E) was sampled in II region controlled by weak hydrodynamic conditions and covered with middle grain size sediment particles (Fig. 3.31). 3.7.3.1 Sediment Aging Quantitative reconstruction of contaminant spatial and temporal trends requires precise chronology. A common approach of 210 Pb has been in use. The

410


vertical profiles of total and excess 210 Pb activities have shown that the logarithm of excess 210 Pb activities is significantly and linearly related to depth (r =0.99, P PCFA (5.8%). Basically, the inorganic P was the major form, accounting for 72%∼93% of the total P pool, and the contribution of POrganic was relatively small at the study stations. The concentration of PCDB in the surface sediments seemed to increase eastward. The minimum contour of PCDB appeared near 123.5◦ E and 30◦ N, which is outside of Hangzhou Bay. In contrast, the maximum contour of PCFA was located outside of Hangzhou Bay, and the concentration of PCFA decreased eastward. The distribution of PDetrital differed from those of PCDB and PCFA and there were two higher N 33 32 31 30 29 28

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121 122

123 N 33

124

125

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Fig. 4.41. The distributions of different forms of phosphorus in the surface sediments in the middle shelf of the ECS (μmol/g). (a) PCDB ; (b) PDFA ; (c) PPetrital ; (d) POrganic ; (e) Total P (Fang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center)

490

4 Biogeochemical Processes of the East China Sea

values of PDetrital in the ECS. One was close to 123.5◦ E and 32◦ N, approximately 200 km east of the mouth of the Changjiang River Estuary, and the other was located 125◦ E and 29◦ N, in the south-western part of Hangzhou Bay. The distribution pattern of total P was similar to that of PDetrital . The maximum contour of POrganic also appeared about 200 km east of the mouth of the Changjiang River Estuary. Inorganic phosphorus (IP), organic phosphorus (OP), and total phosphorus (TP) in the Changjiang River Estuary sediment were also studied separately. Similar to that in the middle shelf of ECS, IP occupied the majority of TP (about 77%) while OP occupied 23% in the Changjiang River Estuary. The content of IP, OP, and TP was about 0.25∼0.5, 0.02∼0.25, and 0.29∼0.61 mg/g, respectively, with a mean value of 0.36, 0.11, and 0.49 mg/g, respectively. Fig. 4.42 (Li et al., 2006b) shows the distribution of P in sections A and B in the Changjiang River Estuary sediment, and that the change in P was smaller in all regions except the area of stations 1 to 5 in section A.

Fig. 4.42. P distributions in the Changjiang River Estuary sediments (Li et al., 2006b) (With permission from Springer)

4.4.3 Phosphorus Burial Fluxes Mass accumulation rate (MAR) in the ECS varied from >2 to 0.05 g/(cm2 ·yr). The maximum MAR appeared in the mouth of the Changjiang River, and the value generally decreased southward along the inner shelf and eastward offshore. Based on this valuable published data, Fang et al. (2007) calculated the phosphorus burial flux in the ECS. To facilitate the calculation, the calculated area is divided into five boxes: estuary (box I), inner shelf (box II), middle shelf (boxes III and IV), and outer shelf (box V) (Fig. 4.43), according to the value of MAR in each box observed by Huh and Su (1999) and to the phosphorus content in surface sediments found by Fang et al. (2007). The total P concentration in the sediments in the ECS area remained within a narrow range, 14∼23 μmol/g. Thus, the error in calculation caused by the variation in the total P concentration in sediments in different boxes was probably less than 20%. Table 4.14 (Fang et al., 2007) shows the values of the parameters used in the calculations and the calculated results. The

4.4 Phosphorus Biogeochemistry in the East China Sea

491

Fig. 4.43. The area within the ECS was divided into five boxes for burial flux calculations. The five boxes representing different MAR area and phosphorus content are: (I) estuary; (II) inner shelf; (III) middle shelf; (IV) middle shelf; (V) outer shelf (Fang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center) Table 4.14. The area, mass accumulation rate, P total concentration, P accumulation rate, and P burial flux for each box of the ECS (Fang et al., 2007) (With permission from Elsevier’s Copyright Clearance Center) Box I II III IV V Total

Mass accum. Area (km2 ) rate (g/(cm2 ·yr)) 42,400 0.6∼1.0 (0.8) 38,200 0.2∼0.4 (0.3) 57,400 0.2∼0.5 (0.35) 69,700 0.1∼0.4 (0.25) 197,000 0.05∼0.1 (0.075) 404,700

P total conc. (μmol/g) 15.8∼17.2 (16.1) 14.5∼19.8 (16.5) 13.5∼22.2 (17.5) 17.5±3.6 17.5±3.6

P accum. rate (μmol/(cm2 ·yr)) 9.48∼17.2 (12.88) 2.90∼7.92 (4.95) 2.72∼11.13 (6.13) 1.42∼8.41 (4.38) 0.71∼2.12 (1.31)

P burial flux (×109 mol/yr) 4.02∼7.29 (5.46) 1.11∼3.03 (1.89) 1.55∼6.37 (3.52) 0.98∼5.85 (3.05) 1.37∼4.13 (2.59) 9.03∼26.68 (16.50)

P burial flux was found to be in the range of 9.03×109 ∼26.68×109 mol/yr (average 16.5×109 mol/yr) for the calculated area. 4.4.4 Phosphorus Balance The water fluxes of the ECS are chiefly influenced by the Kuroshio surface water (KSW), the Kuroshio tropical water (KTW), and the Taiwan Strait water (TSW) entering into the ECS, and by the shelf surface water (SSW) leaving the ECS. The annual variation in concentration of DIP in these waters is small. If the geochemical cycle of phosphorus in the ECS is assumed to have reached a steady state, then the P budget of the ECS can be calculated using water and phosphorus mass-balance, using data obtained in previous studies.

492


The major water masses near the ECS continental shelf break were divided into four groups, namely, the KSW (0∼75 m), the KTW (76∼200 m), the KIW (201∼600 m), and the SSW. The annual flux of each water mass was as follows: the KSW, 22,248×109 m3 /yr; the KTW, 11,827×109 m3 /yr; the KIW, 3,942×109 m3 /yr; the SSW, 50,573×109 m3 /yr (Chen and Wang, 1999). Although the major Kuroshio currents are parallel to the isobaths, the SSW has net transport offshore because of fresh water discharged from rivers, mainly the Changjiang River, and net precipitation, while the KSW, KTW, and KIW have net onshore transport. In addition, there is input from the TSW. The annual water flux flowing through the Taiwan Strait is still uncertain and ranges from 0.2 to 2.31 Sv (1 Sv=1×106 m3 /s). The annual water flux of the Taiwan Strait, 0.2 Sv, calculated by Chen and Wang (1999) is relatively small. Thus, a reasonable value, approximately 1.0 Sv, was adopted in the present study. In addition, Liu et al. (2000) estimated that based on two field surveys conducted in August 1994 and in March 1997, the Kuroshio upwelling water intrusion into the southern ECS off northern Taiwan was about 0.6∼0.8 Sv. The value of 0.7 Sv was substituted here for the water flux of the KIW by Chen and Wang (1999). The water fluxes of SSW, KSW, KTW, and KIW are estimated to have an uncertainty of ±20%, mainly due to uncertainties involved in estimating the relative proportions of the Kuroshio water masses (Chen and Wang, 1999). For matching the water mass balance calculation, the uncertainty of the Changjiang River (R) and TSW fluxes is also considered as ±20% (Fang et al., 2007). Based on the above information, a simple box model (Fig. 4.44, Fang, 2004) was used to calculate the P budget of the ECS according to the water and P mass balance, as shown in the following equations: QR + QTSW + QKSW + QKTW + QKIW = QSSW QR (CR−DIP + CR−DOP + CR−PIP )

(4.1)

+QTSW (CTSW−DIP + CTSW−DOP + CTSW−PIP ) +QKSW (CKSW−DIP + CKSW−DOP + CKSW−PIP ) +QKTW (CKTW−DIP + CKTW−DOP + CKTW−PIP ) +QKIW (CKIW−DIP + CKIW−DOP + CKIW−PIP ) = QSSW (CSSW−DIP + CSSW−DOP + CSSW−PIP ) + SPP

(4.2)

where Q and C are the water flux and different fractions of the P concentration. Subscripts R, TSW, KSW, KTW, KIW, and SSW denote the river input from the Changjiang River, the Taiwan Strait water, the Kuroshio surface water, the Kuroshio tropical water, the Kuroshio intermediate water, and the shelf surface water, respectively. SPP represents the P flux deposited in the sediment. The concentrations of DIP, DOP, PIP, and SPM in each water mass were taken as the average concentrations in the samples taken from


493

each water mass. The error constraint of each parameter was calculated as one standard deviation of the concentration in all samples of each water mass. QR

QKSW

QSSW

QKTW

SPP

QTSW

QKIW

Sediment

Fig. 4.44. Schematic diagram of the box model used to calculate the phosphorus budget in the ECS (Fang, 2004) (With permission from Elsevier’s Copyright Clearance Center)

The import flux of DOP from the Changjiang River and the inputs of atmospheric P fluxes into the ECS were excluded from the calculation because measurements were not available. Table 4.15 (Fang, 2004) summarizes the parameters used to calculate the P budget of each water mass in the ECS and the calculated results. The high annual flux of these parameters was calculated from the high value of water flux and high concentration in each column, and low annual flux was calculated from the low value of the water flux and low concentration. The average value was calculated from the value without error constraint. The results show that the P input flux from the various water masses into the ESC was (78.8±48.8)×109 mol P/yr. The inputs of the TSW and the KIW were the major sources, and both contributed more than 65% of Table 4.15. Values of parameters used and phosphorus fluxes calculated using the box model for the ECS (Fang, 2004) (With permission from Fang TH) Annual flux (×109 mol/yr)

Concentration Water mass

River water (QR )

DOP PIP SPM Water fluxes DIP (μmol/L) (μmol/L) (μmol/g) (mg/l) (×109 3 m /yr)

0.53± 0.1

PIP

TP

11.2± 2.3

11.8± 2.3

0.57

No data

23.98

504

Kuroshio surface 22,248± water (QKSW ) 4,450

0.138± 0.072

0.212± 0.112

0.586± 0.179

1.23± 8.6± 0.50 5.7

0.02± 0.015

8.6± 5.7

Kuroshio 11,827± tropical 2,365 water (QKTW )

0.343± 0.31

0.145± 0.066

0.340± 0.113

1.77± 6.7± 0.69 5.6

0.009± 0.007

6.7± 5.6

Kuroshio 22,075± intermediate 4,415 water (QKIW )

0.854± 0.682

0.301± 0.094

0.519± 0.125

1.09± 28.9± 0.45 22.2

0.015± 0.011

28.9± 22.2

Taiwan Strait 313,536± water (QTSW ) 6,307

0.450± 0.133

0.181± 0.112

5.56± 3.78

4.41± 21.4± 2.87 11.7

1.3± 1.2

22.8± 13.0

88,614± Shelf surface water (QSSW ) 17,723

0.356± 0.145

0.099± 0.069

1.348± 1.006

2.01± 44.1± 0.63 27

0.35± 0.31

44.5± 27.3

Spp

928.2± 185

TDP

34.3± 21.5

494


the total fluxes. The influx from the KSW and KTW, and from the Changjiang River, accounted for 19% and 15%, respectively, of the total fluxes. The ECS is a phosphorus sink, and approximately (34.3±21.5)×109 mol P, 43% of the total P flux, is deposited annually in the sediments. The SSW exports about (44.5±27.3)×109 mol P/yr out of the ECS. The import and export of P in each water mass, except for the Changjiang River, are dominated by the dissolved phase. The fluxes of the particulate phase are very small and can be neglected. In contrast, the annual import of particulate P from the Changjiang River is about (11.2±2.3)×109 mol P, which accounts for over one third of the total P fluxes deposited in the sediment in the ECS, because the particulate P input from the Changjiang River was omitted from the calculation of (Chen and Wang, 1999). 4.4.5 Cycling of Phosphorus It is clear that elucidating the cycling of all nutrients in marine systems is extremely important if one is to understand current controls on primary production and particulate carbon export in the world’s oceans. The intensity of coastal P cycling, in particular, can have a direct impact on a wide range of processes, ranging from nutrient limitation to particulate matter exported from the euphotic zone to underlying sediments. Hence, long-term changes in P cycling will affect the residence time of P over geological time scales. 4.4.5.1 Phosphorus Residence Time The time scales for P turnover are very important in understanding the mechanisms that control plankton distributions, particulate export, and the biological cycling of nutrients in marine systems. Using an initial 33 P/32 P input ratio of 0.99 and the steady-state model, the residence time of TDP was calculated to range between 3.7 and 13.9 d (Fig. 4.45a, Zhang et al., 2004). The TDP residence time was similar to the PO4 -P residence time (1.5∼24 d) determined in the sea of Okhotsk using artificial radiolabeled 32 PO4 -P, but shorter than that (20∼52 d) determined previously using naturally produced 32 P and 33 P in the North Pacific Subtropical Gyre. Furthermore, the residence time is also much shorter than expected in the ECS based on the phosphate advection-diffusion model, which is in the order of 15∼80 d. In fact, the TDP in seawater is divided into two main pools: SRP and soluble non-reactive phosphorus (SNP). The SNP is referred to as the fraction that is not available for biological uptake, and generally ranges from 0% to 50% of the TDP pool in coastal marine environments to as great as 75% in the open ocean. If SRP instead of TDP was used for the application of P residence time estimation, then it would be predicted that SRP residence time was much shorter than that of TDP. The dissolved P turnover rates varied significantly in different marine environments, and the residence time of TDP increased from 3∼4 d in the coastal

495

P assimilation rates (d)


Fig. 4.45. The phosphorus residence time, the uptake rates of phytoplankton and zooplankton and POC export fluxes at three sampling stations in the ECS. (a) Residence time for TDP, suspended matter, and net-plankton; (b) The phosphorus assimilation rates and carbon assimilation fluxes of phytoplankton; (c) The phosphorus grazing rates and carbon grazing fluxes of zooplankton; (d) POC export fluxes from the upper 35 m (Zhang et al., 2004) (With permission from Elsevier’s Copyright Clearance Center)

regime and mid-shelf area, to about 14 d in the outer-shelf area. Differences in residence time between environments are most likely related to rates of biological assimilation and/or nutrient concentrations, since residence time is a function of the concentration and the flux of a substance in seawater. During the course of this study, surface TDP concentrations dropped from 0.99 μmol/L at the inshore station to 0.40 μmol/L at the offshore station, whereas phytoplankton biomass in the water column decreased from 4×106 to 4×104 cells/m3 . The relative fraction of phytoplankton biomass in the outer-shelf decreased by about a factor of two compared to the coastal water, whereas surface TDP concentrations decreased only by 60%. The decrease in the outershelf phytoplankton biomass, however, could not be accounted for by TDP concentrations alone. The present finding provides the in situ evidence that TDP has rapid turnover rates in the coastal water, which suggests that low TDP concentrations can support relatively high levels of biological production than previously thought. If correct, the present results also imply that the role of P in supporting primary production has been underestimated. This information is key for current ecological models of the food web in the upper ocean, especially since many of these models assume P limitation to be based only

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on inorganic P concentration relative to N. The rapid turnover and regional variability of P within the dissolved pools reflect the dynamic nature of P in marine ecosystems. The short residence time of TDP suggests that studies of dissolved nutrient concentrations alone are insufficient for determining either nutrient limitation or maximal primary production. It has been shown that the abundance and assemblage structure of the marine ecosystem can greatly affect the residence time of nutrients in the upper ocean. Turnover models provide a unique tool with which to investigate the in situ variability of P turnover in marine ecosystems. Assuming that the dominant P source of phytoplanktonic pools was from TDP, it is calculated that the 0.45∼77 μm size class had P residence time of 15∼30 d. In the coastal waters and mid-shelf area, this 0.45∼77 μm size class consisted predominantly of the phytoplankton: Chaetoceros affinis, Chaetoceros debilis, Chaetoceros compressus, Chaetoceros laciniosus, and Nizschia pungens, and had a short residence time of 15∼25 d. On the outer shelf, however, the 0.45∼77 μm size class contained mixed assemblages of organisms dominated by Pyrocystis fusiformis, Ceratium contortum, Ceratium gravidum, etc., as well as decomposing fecal matter. These mixtures of phytoplankton and decomposing fecal matter had a longer P residence time of about 30 d. Assuming all the P uptake of the >77 μm size class was derived from the phytoplanktonic pools (0.45∼77 μm), residence time in this size class was relatively short, ranging from 9.7 to 15.4 d. In the coastal waters, plankton (>77 μm) were dominated by mature copepods, such as Calannus finmarchicus, and some developmental stages of Euphausids and P residence time of about 10 d. In the mid- and outer-shelf area, residence time in this size class increased to about 15 d, and was dominated by zooplankton: Eucalanus crassus, Eucalanus subcrassus, Eucalanus subtenuis, Rhincalanus cornutus, Pareuchaeta russelli, etc. The present estimate of in situ zooplanktonic P residence time is comparable to 19 d found by laboratory studies in which copepods were fed with 32 P-labeled phytoplankton. Furthermore, the estimates were within the 12∼21 d range determined in the Gulf Stream using mass balance estimates. 4.4.5.2 Phosphorus Uptake Rates The rates in which the nutrient is fixed by phytoplankton and transferred upward in the food web to zooplankton can largely determine the productivity of fish in the marine system. Traditionally, studies of nutrient flux rates have been conducted using incubation experiments with artificial C or N isotope tracer and the transfer of activity over time into the various size classes of phytoplankton and zooplankton examined. The difficulty with such types of research is that they involve significant perturbations to the system of interest. For example, samples are first separated from the ecosystem before incubation. Bottle incubations will, at best, miss sporadic bloom events and provide rate estimates that are only valid for discrete depths and time. Recently, naturally produced 32 P and 33 P have been used to investigate directly the uptake rates


497

of phytoplankton and zooplankton. The advantage of using naturally produced 32 P and 33 P is that they not only are a nutrient directly utilized by organisms, but also enable an examination of the uptake rate of P without disturbing the regime of interest. Furthermore, results will integrate over all of the processes that have affected the distribution of 32 P and 33 P over the prior 20 to 35 d. The estimated radioactive P assimilation rate of phytoplankton was low and ranged from 0.014 d−1 offshore waters to 0.035∼0.043 d−1 in nearshore waters (Fig. 4.45b, Zhang et al., 2004). The high variability in radioactive P assimilation rates suggests that dynamic uptakes of TDP decreased with increasing distance from the coast. Furthermore, assuming the C/P assimilation ratio of phytoplankton follows the Redfield ratio (C/P=106/1), phytoplankton assimilation fluxes for carbon from the calculated assimilation rates and measured inventory of TDP can be estimated. The fluxes of carbon assimilated by phytoplankton were thus estimated to be in the range of 189∼814 mg C/(m2 ·d), with a mean of 593 mg C/(m2 ·d) (Fig. 4.45). These estimates of 32 P (or 33 P) derived carbon assimilation rates of phytoplankton were about 2 times lower than average 14 C derived primary production of 1,100 mg C/(m2 ·d) from the same general area and in the same seasons. Although the 32 P (or 33 P) method might have large uncertainties associated with a number of factors, such as compartmentalizing phytoplankton and zooplankton into discrete size classes inappropriately, it is believed that the reason for the discrepancy was most likely due to the different mechanisms of C and P uptake by phytoplankton. The 14 C method measures mostly photosynthesis; the C fixation rates are usually close to net primary production. By contrast with the 14 C method, 32 P (or 33 P) derived P assimilation rates seem to be independent of photosynthetically available radiation; several studies have shown little effect of light on the rate of P assimilation by phytoplankton. The measurements made in this study have allowed comparisons of carbon fixation and phosphate uptake rates by natural plankton assemblages, where each method gives a different measure of plankton dynamics. In a similar manner, if it is assumed that the 32 P and 33 P activities in net-plankton and suspended particulate matter were similar to those in zooand phytoplankton, zooplankton grazing rates for radioactive P can at this point be derived from the steady-state model. The estimated values ranged from 0.025 per day in mid-shelf water to as high as 0.103∼0.077 per day in coastal and outer-shelf waters (Fig. 4.45c). The differences in grazing rates may be a result of different assemblage structure and life periods in each area. Although the zooplankton grazing rate for radioactive P was the highest in the outer shelf, the net grazing flux for P was the smallest due to extremely low phytoplankton biomass in the outer-shelf. It has been found that P was ingested by zooplankton as efficiently as C; zooplankton grazing fluxes for carbon can thus be calculated from the measured inventory of particulate P (PP) and the Redfield ratio. The estimated value was the highest in coastal waters and ranged between 10 and 161 mg C/(m2 ·d), with a mean of 64 mg C/(m2 ·d) (Fig. 4.45c). Given the estimated phytoplankton assimilation rates

498


for carbon above, it is possible to estimate the ecological efficiency of this carbon transfer from the ratio of zooplankton carbon uptake to phytoplankton carbon uptake. The ecological transfer efficiency thus ranged from 3% to 20%, which means that grazing fluxes of carbon by zooplankton corresponded to 3%∼20% of primary production (Zhang et al., 2004).

4.5 Silicate and Biogenic Silica in the East China Sea In recent years, the transformation, retention, and transport of nutrients through the vast continuum of rivers, lakes, wetlands, and estuaries to coastal waters was the subject of many large research projects including the LOICZ-Program (Land-Ocean-Interactions-in-the-Coastal-Zone-Program), a core project of the International Geosphere-Biosphere Programme (IGBP). Compared to our knowledge concerning N and P processing, transport and cycling of dissolved silica in the aquatic continuum is significantly less known. In contrast to N and P, with large human inputs, anthropogenic input of Si to estuarine systems is negligible. The amount of dissolved silica that eventually reaches coastal waters through estuaries is, however, essential in influencing the occurrence of eutrophication problems in the coastal zone. High anthropogenic inputs of N and P can eventually induce dissolved silica limitation of diatoms and subsequent succession of a phytoplankton community dominated by diatoms to a nondiatom phytoplankton community. Biogenic silica in sediments is known to be an important parameter to understand the biogeochemical processes and paleoenviromental records in estuarine and coastal ecosystems. The content of biogenic silica in sediments is found to have a close link with biosiliceous productivity in overlying waters. Alterations to phytoplankton species composition are likely to have a large effect on levels of CO2 in the atmosphere, and have a strong potential as a powerful proxy for paleoproductivity reconstructions. Therefore, it is of great significance to investigate the accumulation and distribution of biogenic silica in estuarine and coastal sediments. However, data on biogenic silica contents in estuarine and coastal sediments are relatively scarce, and factors controlling the preservation of biogenic silica in such complex systems still remain unclear. The Changjiang River is regarded as an important source of materials in the North Pacific coastal seas, and the materials carried by the river have a significant influence on coastal environmental health, sustaining the productive fisheries in the adjacent seas. With the development of the economy and the increased demands for preventing floods and saving water resources in China, thousands of water conservancy projects have been constructed in the Changjiang River drainage basin during recent decades. Anthropogenic perturbations have caused considerable changes in riverine nutrient concentrations and fluxes to the sea. However, few comprehensive data are available on the variations in nutrient concentrations and fluxes and the effects on the coastal ecosystem, in particular in the riverine silicon variation. Despite the

4.5 Silicate and Biogenic Silica in the East China Sea

499

considerable efforts that have been devoted to the study of the Changjiang River and the ECS, our understanding of the dissolved silicate and biogenic silica in this region is still limited. The main objectives of the section are to introduce the spatio-temporal variations of the dissolved silicate and biogenic silica in the ECS and to explore the factors and mechanisms that affect the distributions of the dissolved silicate and biogenic silica. 4.5.1 Spatial Distribution of the Dissolved Silicate in Seawaters The surface distributions of dissolved silicate (DSi) in the ECS are shown in Fig. 4.46 (Wang et al., 2003). Concentrations of DSi decreased gradually from the estuary to the adjacent sea, indicating that they were influenced by Changjiang diluted water. DSi concentrations underwent strong seasonal cycles, especially in the Changjiang River Estuary. DSi was significantly higher in summer than that in autumn, winter, and spring. Previous research showed that DSi showed conservative behavior in winter and a slight depletion due to biological uptake in summer. Shen et al. (2001) pointed out that the DSi concentration was generally high during the transition period from the wet to dry season (November) while it was low during the transition period from the dry to wet season (April∼May). Since the river runoff and flood deliver terrestrial materials, the seasonal pattern of the DSi can be an indicator of terrestrial origin. In spring, the distributions of silicate were quite similar to those of DIN and phosphorus (Fig. 4.46a). The waters with elevated concentrations of silicate covered the whole coastal area from 35◦ N in the north to 26◦ N in the south, and to the southwest of Cheju Island at about 126◦ E in the east. In contrast, the concentrations of silicate in the outer continental shelf area of the ECS were very low. During the summer, the surface distribution of silicate was quite similar to that of DIN, but was different from that of phosphorus. The waters with elevated concentrations of silicate covered the whole coastal area from 35◦ N in the north to 27◦ N in the south (Fig. 4.46b). In autumn, the combination of the low discharge and the prevailing northeasterly wind confined the influence of the Changjiang River water to a narrow band southward along the coast. Waters with high concentrations of silicate (>10 μmol/L) were found in the coastal area of the ECS, but confined to a narrow band (Fig. 4.46c). In winter, the surface distribution of temperature was more representative of the water circulation pattern than that of salinity. The waters with high concentrations of silicate (>10 μmol/L) were found in the coastal area of the ECS (Fig. 4.46d). The distribution pattern of nutrients in winter is quite similar to that in autumn. It was still nutrient deficient in the offshore area of the ECS, but the area in winter was larger than that in the other three seasons. The spatial and diurnal variations of dissolved silicate in the Changjiang River Estuary were investigated from May 19 to 26, 2003. The results showed

500


N 38

N 38

36

36

34

34

32

32

30

30

28

28

(a)

26 N 38

120

122

124

126

128

E120 N 38

36

36

34

34

32

32

30

30

28

28

(c)

26 120

122

124

126

(b)

26 122

124

E 120

128

E

(d)

26 128

126

122

124

126

128

E

Fig. 4.46. Distribution of surface water silicate concentration in the ECS. (a) May, 1998; (b) Aug., 1998; (c) Nov., 1997; (d) Jan., 1999 (Wang et al., 2003) (With permission from Elsevier’s Copyright Clearance Center)


501

that SiO3 -Si distribution is different clearly from that of NO3 -N, NO2 -N, NH4 N, and PO4 -P along transect A, which decreased continually seawards (Fig. 4.47). SiO3 -Si concentration in the inner Changjiang River Estuary is about 101.97 μmol/L. However, the variation in SiO3 -Si concentration at station 15 and its eastern stations was small and SiO3 -Si concentration was less than 30 μmol/L commonly. The distribution of SiO3 -Si along transect B indicated that the difference in SiO3 -Si concentration between 0 m and 2 m above the bottom layers in southern stations is smaller than that in northern stations (Fig. 4.48), which was similar to that of NO3 -N, NO2 -N, and PO4 -P. The variation in SiO3 -Si concentrations vs. time for each layer at anchor station 13 indicated that SiO3 -Si concentrations on the 0 m layer was higher than that in the layer of 2 m above the bottom most of the time (Fig. 4.49). 0 5 10

Depth (m)

15 20 25 30 35 40 45 50

1 2 3 4 5 6 7 8 9 1011121314151617181920 Station

Fig. 4.47. The concentration isolines of SiO3 -Si along transect A (μmol/L) 120

SiO3-Si (mmol/L/)

105 90 75 60 45 0m 2 m above the bottom

30 15 21 22 23 24

25 26 Station

27

28 29

Fig. 4.48. The concentration isolines of SiO3 -Si along transect B (μmol/L)

502

4 Biogeochemical Processes of the East China Sea 110 0m 2 m above the bottom

SiO3-Si (mmol/L)

100 90 80 70 60

6:00

10:00

May 24 2:00

18:00

22:00

10:00

14:00

6:00

21:50

May 23 1:30

13:50

17:50

40

May 22 9:50

50

Sampling time

Fig. 4.49. The variation trends of SiO3 -Si concentrations vs. time for each layer at station 13

The variation trends of SiO3 -Si concentrations vs. time in the two layers were similar; viz., if SiO3 -Si concentrations in one layer decreased or increased, they might decrease or increase simultaneously in another layer. The variation in SiO3 -Si concentration might be significant during 4 h, especially in the layer of 2 m above the bottom where its variation might be more than 60 μmol/L. On May 22, an SiO3 -Si concentration of 107.50 μmol/L decreased to 47.14 μmol/L from 13:50 to 17:50. The variation in SiO3 -Si concentrations vs. time for each layer at anchor station 20 indicated that its variation is similar in the layers 0 m and 2 m above the bottom, especially in the first 24 h when the variation went all the way (Fig. 4.50). SiO3 -Si concentration was the highest in the layer at 5 m, and the lowest at 20 m. During the investigation time, the variation was the smallest in the layer at 10 m, but the biggest in the layer at 5 m. Different to the change in N and P nutrients, silicate flux in the Changjiang River has decreased remarkably over the last 50 years and dissolved silicate concentration at Datong Hydrographic station decreased by 53.3 μmol/L from 1959 to 1984. Especially after the Danjiangkou Dam was completed in 1968, silicate concentration decreased sharply. Up to now, there were 48,000 reservoirs in the Changjiang River basin. Li and Cheng (2001) attributed the decrease in silicate concentration to the dam constructions and the eutrophication caused by industrial and domestic sewage, fertilizers, etc. What is more important, the Three Gorges Project is being built in order to make use of the abundant resources of the Changjiang River. The Three Gorges Project is the largest hydroelectric project ever built in China, and in the world. China’s Three Gorges Dam began to fill as its sluice gates started closing in 2003, with

60 50 40 30 20 10 125 120 40 30 20 10 50 40 30 20 10 60 50 40 30 20 10 90 75 60 45 30 15 0

503

May 19 16:00 20:00 May 20 0:00 4:00 8:00 12:00 16:00 20:00 May 21 0:00 4:00 8:00 12:00 16:00

SiO3-Si (mmol/L)


Sampling time

Fig. 4.50. The variation trends of SiO3 -Si concentrations vs. time for each layer at station 20 (from top to bottom: 0, 5, 10, 20 m from the surface, and 2 m above the bottom)

the water level reaching 139 m. The export of Si to the sediments in reservoirs has resulted in declines in dissolved silicate in many river waters after dam closure and the decline in Si transport to the coastal seas can change the nutrient ratios available for the phytoplankton community. In order to represent the nutrient conditions in the euphotic zone, the average concentrations of nutrients in the upper 20 m in the coastal area of the ECS, and the upper 50 m in the outer continental shelf of the ECS were used for calculation, for the depth of the euphotic zone is about 20∼30 m in the coastal area of the ECS, and 50 m or so in the outer continental shelf area of the ECS. The Si/N ratios were calculated by using the average concentrations of silicate and total DIN (DIN=nitrite+nitrate+ammonia) in the upper layer at each station. The maximal Si/N ratios were found in the

504


Changjiang River mouth, and the maximal average Si/N ratios were found in spring and summer, which implies that the Si/N ratios in the ECS were mainly influenced by the runoff of the Changjiang River (the runoff of the Changjiang River during the rainy season (from May to October) is 70% of the total runoff in a year). The Si/N ratios were larger than 1 at most stations. The stations with Si/N ratios less than 1 were located mostly in the Changjiang River mouth, but the actual concentrations of silicate were higher than 5 μmol/L. Therefore, it seems that silicate was not the limiting nutrient for the growth of phytoplankton in the ECS. 4.5.2 Distribution of Biogenic Silica in Sediments The biogenic silica (BSi) record in marine sediments acts as a potential proxy for oceanic changes and has a close link to the carbon cycle. BSi is preferentially transported over organic carbon throughout the water column and preserved in sediments; it has been used to reconstruct past production of surface waters. However, lateral advection of water masses, sediment redistribution and spatial variations in the preservation of silica frustules affect the signature produced in the water column and its burial in marine sediments. Because of spatial variations in BSi preservation, and coupling/uncoupling between Si and C in marine biogeochemical cycles, calibration of the BSi proxy in the modern ocean is required, which requires a full understanding of the mechanisms that control the Si cycle. In the coastal environment, the BSi record in core sediments provides evidence of eutrophication. It has been shown that the ratio of plant nutrients and availability of silicate control both the silica productivity and the phytoplankton species composition. Silicon limitation in marine systems has been examined in a variety of marine environments. Knowledge of the origin and fate of BSi in the ECS is relevant to the understanding of the population dynamics and community function. It was reported that the riverine flux of dissolved silica has decreased, but those of DIN and phosphate have increased in the Changjiang River, leading to decreasing Si:N and Si:P ratios in the adjacent coastal waters, with potential silica limitation of diatom growth. The Si:N ratios in the Changjiang River may decrease in the near future, which will influence the ecosystem in the adjacent coastal environment after the construction of the Three Gorges Dam is completed. Benthic regeneration of dissolved silica is known to be an important source in supporting the production of diatoms. Silicate released from sediment can provide 55% of silicate required by phytoplankton in the ECS (Shi et al., 2004). So it is important to enhance our understanding of biogenic silicate in the ECS. 4.5.2.1 Biogenic Silica in the SPM BSi concentrations in 2002 surface waters varied between 0.21 and 2.76 μmol/L, with an average±SD of (1.3±0.98) μmol/L. The highest concentrations of BSi were observed in coastal areas (Fig. 4.51), which were affected by


505

abundant terrestrial sediment discharge from the Changjiang River (Liu et al., 2005). Biogenic opal produced by vascular plants, diatoms, and siliceous sponges has been found in soils and terrestrial sediments of continents, with opal ranging from 2% to more than 5%, except in Antarctica. This can partly explain the high content of BSi near the coast, where SPM was high. Elevated concentrations of BSi in the Changjiang River Estuary are also related to the strong vertical mixing that conveys diatom cells upward from the near-bottom and/or sediments back to euphotic surface waters. Silica production extending to depths greater than that where there is 1% of light radiation has been reported in most coastal environments. Abundant nutrient supply, shallow water depth, and strong vertical mixing are prerequisites for the success of large diatoms in estuarine environments. It has been reported that freshwater diatoms contain 10 times more silica per cell volume than marine diatoms. In addition, diatoms are abundant in the surface sediments of coastal areas, including freshwater species.

N 32

N 32

31

31

30

30

29

29

121

122

123

124

E 121

122

123

124

E

Fig. 4.51. Horizontal distributions of BSi in surface and near-bottom waters in 2002 (μmol/L) (Liu et al., 2005) (With permission from Marine Ecology Progress Series)

BSi increased with depth, varying from 0.26 to 5.10 μmol/L, with an average of (1.9±1.63) μmol/L, in the intermediate layer, and 1.47 to 9.19 μmol/L, with an average of (3.2±2.08) μmol/L, in the bottom waters (Fig. 4.51) (Liu et al., 2005). High concentrations of BSi in near-bottom waters are presumably due to high sedimentation rates and resuspension of bottom sediments. Planktonic diatoms are abundant in ECS, with Pseudo-nitzschia delicatissma, P. pungens, and Melosira sulcata being the dominant species (Gao et al., 2003). Phytoplankton abundance was low (100 mg/L), and the dominant species are

506


Paralia sulcata and Coscinodiscus jonesianus. The distribution of chlorophyll (Chl) a during the 2002 cruise was different from that of BSi, in that the content of Chl a was high near 123◦ E, but low in the surface layer of the coastal areas. This is probably caused by the fact that only a small amount of BSi was bound to identifiable diatom cells in the river plume, while a large fraction of the remaining BSi consisted of disintegrated diatom cells. The concentrations of silicate during the 2002 cruise showed higher values in the coastal areas than in offshore areas, especially in surface waters, owing to high freshwater discharge from the Changjiang River, and the silicate levels were higher at the near-bottom than in surface waters due to resuspension of sediments. In summary, the distribution of BSi is closely related to that of silicate, and the silica kinetics associated with SPM is a factor affecting the distribution of BSi in coastal areas. Compared with the BSi content of settling particulate matter at an open-shelf station further offshore in the ECS, the BSi content showed a significant gradient from the coast to the shelf edge and the Okinawa Trough. 4.5.2.2 Biogenic Silica in Sediments BSi varied from 0.2% to 0.82% in ECS core sediments. The variation of BSi at different depths for a given sediment core and the differences in BSi between stations were both significant at the 95% confidence level, based on the application of ANOVA. High concentrations of BSi were observed at stations DC10, D34, E4, and E5, while low BSi concentrations were found at stations E6 and DB6 (Fig. 4.52) (Liu et al., 2005). Field observations at stations E6 and DB6 showed the bottom sediments consisted of silt and sand, with considerable shell fragments (e.g., DB6). Accordingly, the pigment content (e.g., Chl a plus pheo-pigments) was higher at stations E4, E5, and DC10 than at stations E6 and DB6. The variations of BSi within a given core are probably related to changes in primary production and resuspension of bottom sediments. No obvious trend with depth was observed in core sediments, except at stations DC10, E5 and, to a lesser extent, D34. The BSi content in ECS sediments was less than 1%, which is similar to those from the Bohai and Yellow Seas, but lower than that in Jiaozhou Bay sediment (Li et al., 2006a). The low concentrations of BSi can be attributed to the high content of SPM and the shallow euphotic zone. It has been reported that iron may stimulate diatom growth and enhance nitrate uptake in coastal areas, but limited increase in silica uptake leads to rapid dissolution of BSi. 4.5.3 Silica Balance on the East China Sea Shelf The silica balances in the ocean are controlled by many processes, such as terrestrial input, silica dissolution in sediment, and biogenic silica production. In the ECS, the silica balance is related to the river inputs, atmospheric deposition, BSi deposition and dissolution in sediments, export toward the deep sea and the input from Kuroshio water, etc.


507

Fig. 4.52. BSi in core sediments at stations E4 [122.61◦ E, 30.95◦ N], E5 [122.48◦ E, 28.94◦ N], E6 [125.00◦ E, 29.58◦ N], DC10 [122.47◦ E, 30.99◦ N], D34 [122.77◦ E, 30.53◦ N], DB6 [122.47◦ E, 31.48◦ N] in October 2000, May 2001, and April∼May 2002 (Liu et al., 2005) (With permission from Marine Ecology Progress Series)

4.5.3.1 Inputs of Silicate The main land-source input to the ECS is the Changjiang River. Part of the discharge from the Changjiang River is carried into the Yellow Sea, especially in summer, when the Changjiang River plume reaches Cheju Island. The annual freshwater discharge into the ECS is 86% (i.e., 794.8×109 m3 ) of the Changjiang River runoff, with a silicate discharge of 81×109 mol/yr to the ECS (FR ) based on a previous investigation (Fig. 4.53) (Liu et al., 2005). Total deposition fluxes of silicate for dry (i.e., aerosol) and wet (i.e., rain) depositions were (1.01±0.61)×109 mol/yr (Fig. 4.53, Liu et al., 2005). In the exchange between the ECS and the Kuroshio, the Kuroshio surface water (KSW), Kuroshio subsurface water (KSSW), and Kuroshio intermediate water (KIW) have net onshore transport, while shelf mixed water (SMW) has net offshore transport, because net precipitation and freshwater discharge from rivers exceed evaporation. In addition, there is silicate input through the Taiwan Strait water (TSW). The silicate input was estimated to be 22.3×109 mol/yr from the KSW, 47.3×109 mol/yr from the KSSW, and 237×109 mol/yr from the KIW, with a total input flux of 306.6×109 mol/yr from Kuroshio water. The silicate input flux was 56×109 mol/yr from the TSW. In contrast, the silicate offshore transport was 100.4×109 mol/yr (Fig. 4.53) (Chen and Wang, 1999; Liu et al., 2005).

508


F E(export)

Fig. 4.53. Biogeochemical cycle of Si on the ECS shelf (×109 mol/yr). River inputs, FR ; atmospheric deposition, FA ; net deposition of BSi in sediments, FB ; BSi gross production, FP(gross) ; silicate flux recycled in the surface layer, FD(surface) ; BSi flux exported toward the deep layer, FE(export) ; silicate flux recycled in the deep layer, FD(deep) ; silicate flux transferred from the deep layer to the surface layer, Fup ; silicate flux at the sediment-water interface, FD(benthic) ; BSi flux that reaches the sedimentwater interface, FS(rain) ; silicate input through the Taiwan Strait water, FTSW , and Kuroshio water, FKW ; offshore transport of silicate, FSMW (Liu et al., 2005) (With permission from Marine Ecology Progress Series)

4.5.3.2 Production of Biogenic Silica Silicate is required by planktonic organisms to grow (e.g., diatoms), and a significant amount of BSi is remobilized by dissolution of silica cells. Production of BSi was estimated by a combination of factors: 14 C primary production, the relative contribution of diatoms, and Si/C ratios. However, use of measured Si/C ratios for natural particle assemblages to transform 14 C productivity data into BSi productions is subject to uncertainties. Average primary production in the ECS was (9.0±3.9), (9.3±4.8), (15.7±9.6), and (11.3±4.7) mol C/(m2 ·yr) in winter, spring, summer, and autumn, respectively, with an annual mean primary productivity of (12.1±5.7) mol C/(m2 ·yr) (Gong et al., 2003). Estimation of a representative BSi/POC ratio for diatoms in natural environments is difficult; diatoms growing in nutrientreplete conditions show a reasonably constant BSi/ POC atomic ratio of (0.13±0.05), but in nutrient-poor conditions large variations of the BSi/POC ratio can occur, in both oceanic and coastal areas. The BSi/POC atomic ratio in settling particles in this study was 0.22 to 0.45, and the BSi/POC atomic ratio in settling parti-

4.6 Dissolved Oxygen and O2 Flux across the Sea-Air Interface of the ECS

509

cles was 0.12 to 1.4 on the ECS shelf. The average BSi/POC atomic ratio was hence estimated to be 0.48. Therefore, the annual BSi production on the ECS shelf was (3.1±0.6)×1012 mol/yr, given a surface area of ca. 53×104 km2 over the shelf region of water depth 60% 40% to 60% C20:0

CPI16∼30 CU (%) CP (%) a+iC (%) 7.27 27.52

35.63 51.20

7.20 9.27

13.57 3.57

C18:1 Δ11 C18:1 Δ9

C16:0 C18:0

C16:1 C18:1

1.87 0.29

2.40 7.24

0.46 1.57

NS: Nansha Lagoon; ECS: East China Sea; CU , total unsaturated fatty acids; CP , total polyunsaturated fatty acids; a+iC, anteiso- and iso-fatty acids

596

5 Biogeochemical Processes of the South China Sea

a small amount of higher carbon number fatty acids (>C20:0 ) deriving from higher land plants could be transported by aeolian dusts (Simoneit, 1977). However, the absence of fatty acids (>C20:0 ) deriving from higher land plants implies an exclusive marine source of fatty acids in sinking particulate organic matter from the continental shelf of the East China Sea. Unsaturated fatty acids from C14 to C22 detected in the two studied regions exist in abundance. They comprise 35.63% of the total fatty acids for the Yongshu Reef Lagoon and 51.20% for the continental shelf of the East China Sea (Table 5.22). Polyunsaturated fatty acids are mainly C18:2 , C20:4 , C20:2 , and C22:4 ones which originate mainly from marine plankton, particularly from diatom. The relative content of these fatty acids in the Yongshu Reef Lagoon (7.20 % of the total fatty acids) is lower than that in the continental shelf of the East China Sea (9.27% of the total fatty acids), showing that the latter has more diatom-derived fatty acids. Monounsaturated fatty acids are mainly C16:1 Δ9 , C18:1 Δ9 , and C18:1 Δ11 ones. In particular, C18:1 Δ11 acids are considered to derive mainly from bacteria and the C18:1 Δ11 /C18:1 Δ9 ratio is used for estimating the input of bacterial organic matter. C18:1 Δ11 /C18:1 Δ9 ratio is 1.87 for the Yongshu Reef Lagoon and 0.29 for the continental shelf of the East China Sea, indicating that more fatty acids in the Yongshu Reef Lagoon sample derive from bacteria. The further evidence is that the relative content of iso- and anteiso-fatty acids in the Yongshu Reef Lagoon (13.57% of the total fatty acids) is higher than that in the continental shelf of the East China Sea (3.57% of the total fatty acids). The fatty acids are contributed to by bacteria. These results clearly show that microbial transformations and biogeochemical reactions of particulate organic matter in the study areas strongly occur during sinking. The degree of these processes in the Yongshu Reef Lagoon is greater than that in the continental shelf of the East China Sea (Duan et al., 1998). (5) Carbon isotopic studies of individual lipids in organisms from the Nansha Islands waters Sediments contain abundant lipid compounds in general, which are used as biomarker compounds to study organic matter sources and reconstruct the palaeo-environments. However, lipid compounds in sediments are generally a mixture of various genetic components so it is difficult to correctly decouple their biological sources only by the results of biochemical research. Carbon isotopic studies of individual sedimentary lipid compounds can discover their genetic information, which provides a new way to understand their origins. The reasonable interpretation of carbon isotopes of sedimentary lipid compounds relies heavily on the carbon isotopic studies of individual lipids in the modern organisms. Biological lipids are precursors of sedimentary lipids and their carbon isotopic compositions are controlled by the environmental conditions, lipid biosynthetic pathways, and so on (Thompson and Calvert, 1994; Popp et al., 1998; Laws et al., 1997). However, the carbon isotopic studies of individual lipids in extant organisms are limited (Monson and Hayes, 1982;

5.5 Biogeochemical Processes in the Nansha Islands Waters

597

Table 5.22. Percentage content of fatty acid (Duan et al., 1998) (With permission from Duan Y) Peak number

Fatty acid

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

C9:0 C10:0 C11:0 C12:0 iC13:0 aiC13:0 C13:0 iC14:0 C14:1 Δ7 C14:0 iC15:0 aiC15:0 C15:0 iC16:0 C16:1 Δ9 C16:0 iC17:0 aiC17:0 C17:1 C17:0 C18:2 Δ9,12 C18:1 Δ9 C18:1 Δ11 C18:0 iC19:0 C19:0 C20:4 Δ5,8,11,14 C20:4 C20:2 Δ11,13 C20:1 C20:0 C21:0 C22:4 Δ7,10,13,16 C22:1 C22:0 C23:0 C24:0 C26:0 C28:0

ND, not delected; NS, Nansha; ECS, East China Sea

Contents (%) NS 1.40 0.16 0.16 1.90 0.16 0.16 0.33 1.31 0.65 5.89 5.23 3.44 3.11 0.98 8.02 18.49 1.31 0.82 0.81 2.62 2.13 6.05 11.29 7.69 0.16 0.49 2.13 1.64 0.98 0.49 2.29 0.32 0.32 0.65 3.11 0.15 1.47 0.30 0.32

ECS 0.21 Trace Trace 0.84 ND ND Trace 0.21 0.42 15.37 0.63 2.52 1.47 0.21 24.84 22.95 ND ND 0.84 0.42 2.74 12.21 3.57 3.16 ND 0.21 0.42 6.11 ND ND 0.63 ND ND ND ND ND ND ND ND

598


Summons et al., 1994; Collister et al., 1994; Schouten et al., 1998; van Dongen et al., 2002). The Nansha sea area of China is a special marine ecological environment. This is because it is located in the low-latitude tropical zone and semi-enclosed drainage area of the SCS which is far away from the continent and has abundant tropical organisms, especially coral organisms. A carbon isotopic study of individual lipids in marine organisms from this environment can provide new data for research on the carbon isotope of lipid compounds in marine organisms. On the other hand, we have determined carbon isotopic compositions of individual sedimentary lipids in the Nansha sea area (Duan and Wang, 2002). However, their genetic interpretations are made only by extrapolation from more conventional studies on bulk carbon isotopic compositions of organisms grown under different conditions. If we can know the carbon isotopic compositions of lipid compounds in various organisms from this marine region and compare them with those of these sedimentary lipid compounds, we will establish the genetic relationship between them. With this object in mind, plankton, coral, and macro-algae in the Nansha sea area were collected, and their soluble organic matter was extracted and fractionated. δ 13 C values of individual lipid compounds were determined using a new technique of gas chromatography combined with isotopic ratio mass spectrometry (GC-IRMS) and the features of their carbon isotopic compositions and isotopic genetic relationship between biological and sedimentary lipid compounds were discussed (Duan et al., 1998). 5.5.4.3 Carbon Isotopic Compositions of Fatty Acids The carbon isotope analytical results of lipids in plankton, coral, and macroalgae in the low-latitude tropical region of the Nansha sea area are listed in Table 5.23. Carbon isotopic compositions of C10 ∼C22 saturated fatty acids in plankton range from −25.6‰ to −28.6‰ with a mean of −27.4‰. Compared with other fatty acids, C16:0 fatty acid is enriched in 13 C, while C10:0 and C20:0 fatty acids are enriched in 12 C. δ 13 C values of C10:0 ∼C22:0 fatty acids in Gorgonia are in the range of −26.0‰∼−29.7‰ averaging −28.2‰. Fatty acids in Gorgonia have similar carbon isotopic compositions with the exception of C14:0 fatty acid enriched in 13 C. Carbon isotopic compositions of C14:0 ∼C22:0 fatty acids in macro-algae Laurencia vary from −24.4‰ to −28.1‰ with an average of −26.4‰. It is also observed that C14:0 fatty acid is enriched in 13 C and C20:0 fatty acid is enriched in 12 C relative to C16:0 and C18:0 fatty acids which are quite similar isotopically. If δ 13 C value of C16:0 fatty acids in the studied organisms is used as a carbon isotopic reference point and compared with other fatty acids in each studied organism, their isotopic differences are mostly within ±2.0‰(Table 5.23), reflecting that these fatty acids are biosynthesized by carbon chain elongation in the same ecological environment (Schouten et al., 1998). The mean carbon isotopic composition is light for Gorgonia and heavy for Laurencia, and the difference in mean δ13 C


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values of saturated fatty acids between the studied organisms is 1.8‰ showing that carbon isotopic compositions of saturated fatty acids do not differ significantly in different organisms from the Nansha sea area (Schouten et al., 1998). Table 5.23. Carbon isotopic compositions of fatty acids and n-alkanes in organisms from the Nansha sea area (‰ PDB) (Duan et al., 2004) (With permission from Duan Y) Comp.

Organisms C10 C12 C14 C15 C16 C18 C20 Plankton −28.6 −27.5 −26.7 −27.7 −25.6 −26.6 −28.3 Fatty acid Gorgonia −27.3 −29.7 −26.0 −27.5 −29.0 −28.2 −29.1 Laurencia −24.4 −26.4 −26.7 −28.1 Means −28.0 −28.6 −25.7 −27.6 −27.0 −27.2 −28.5 Plankton −29.7 −28.2 −28.7 Gorgonia −29.7 −29.7 −27.5 n-Alkanes Laurencia −29.2 −29.0 −28.0 Means −29.5 −29.0 −28.1 δ 13 C values are the mean of 1∼3 measurements for each sample

C22 Means C= C= 16 18 −27.3 −27.2 −20.1 −24.7 −28.1 −28.2 −20.5 −24.4 −26.4 −20.1 −23.9 −27.7 −27.3 −20.2 −24.3 −28.6 −28.9 −28.9 −28.8

The carbon isotope analytical data of individual fatty acids in organisms are limited. van Dongen et al. (2002) have determined carbon isotopic compositions of individual fatty acids in Sphagnum cuspidatum from the Bargerveen peat bog, the Netherlands, which are from −33.1‰ to −35.0‰. We have measured δ 13 C value of individual fatty acids in herbaceous plants and tree leaves from the Gansu Marsh and obtained similar isotopic data, ranging from −33.0‰ to −35.4‰. If carbon isotope compositions of individual saturated fatty acids in marine organisms from the Nansha sea area are compared with those studied results above, we will find that the former have heavier carbon isotopic compositions than the latter, which is consistent with the studied results that marine organisms are enriched in 13 C (Deines, 1980). One finding is that C16 and C18 unsaturated fatty acids in the three studied organisms are evidently enriched in 13 C and have similar carbon isotopic compositions in various organisms, respectively. δ 13 C values range from −20.0‰ to −20.5‰ for C16 unsaturated fatty acid (C= 16 ) and from −23.9‰ to ). The data show that C16 unsat−27.4‰ for C18 unsaturated fatty acid (C= 18 urated fatty acid is isotopically heavier than C18 homologue. Compared with C16:0 fatty acid, C16 and C18 unsaturated fatty acids are enriched in 13 C by a range from 0.9‰ to 8.5‰ (Table 5.24). It shows that unsaturated fatty acids have lighter carbon isotope compositions as compared with saturated fatty acids, and the cause is interpreted from the biosynthetic pathway. However, the results here indicate that desaturation of fatty acids also causes enrichment in 13 C of fatty acids. The reasons for the enrichment are unclear, and thus further investigation is required.

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Table 5.24. Carbon isotopic compositions of fatty acids and n-alkanes relative to the C16 fatty acid and C16 n-alkane, respectively (‰ PDB) (Duan et al., 2004) (With permission from Duan Y) Compounds Organisms C10 C12 C14 C15 C18 C20 C22 C= C= 16 18 Plankton −3.0 −1.9 −1.1 −2.1 −1.0 −2.7 −1.7 +5.5 +0.9 Fatty acid Gorgonia +1.7 −0.7 +3.0 +1.5 +0.8 −0.1 −0.9 +8.5 +4.6 Laurencia +2.0 −0.3 −1.7 +6.3 +2.5 Plankton +1.5 +1.9 +1.0 n-Alkanes Gorgonia 0.0 +1.0 +2.2 Laurencia +0.2 −0.4 +1.2

(1) Carbon isotopic compositions of n-alkanes The carbon number distributions of n-alkanes range from C14 to C22 . δ 13 C values of abundant C16 ∼C22 even-carbon-number n-alkanes are from −27.8‰ to −29.7‰ for plankton, from −27.5‰ to −29.7‰ for gorgonia, and from −28.0‰ to −29.2‰ for Laurencia. Their average δ 13 C values, ranging from −28.6‰ to −28.9‰, are quite similar. The isotopic compositions of different carbon-number n-alkanes do not differ significantly, whose differences are within a range of ±2.2‰. Similar to fatty acid, if δ13 C value of C16 n-alkanes is compared with that of other homologues, the variance range of δ 13 C values in the three studied organisms is from −0.4‰ to 2.2‰. These data show that carbon isotope compositions between n-alkanes do not differ significantly in the three studied organisms from the ecological environment of the Nansha sea area, reflecting that carbon isotopic compositions of n-alkanes in organisms from the same ecological environment are similar. The mean δ13 C values of n-alkanes in C3 plants analyzed by Collister et al. (1994) are from −31.4‰ to −38.6‰ and from −30.1‰ to −38.4‰, respectively. Our measured n-alkanes in herbaceous plants and tree leaves from the Gansu Marsh have carbon isotopic compositions from −31.3‰ to −34.9‰. Compared with those mentioned above, n-alkanes analyzed here are enriched in 13 C, indicating the carbon isotopic characteristics of the marine aquatic organisms. On the other hand, the mean δ 13 C difference between n-alkanes and saturated fatty acids is only 1.5‰, reflecting that they are biosynthetically similar. (2) Carbon isotope genetic relationship of individual lipids in organisms and sediments Sedimentary lipids can have multiple origins, so that their carbon isotopic compositions are various. The previous recognition of carbon isotope genesis of sedimentary lipids relies mostly on the extrapolation from the sedimentary lipid-formed environments and the studied results of bulk carbon isotopic compositions of organisms grown under different conditions. The comparative study of the carbon isotope genetic relationship of individual lipids in organisms and sediments from the same region and same ecological environment can provide a scientific basis for correctly understanding the carbon isotopic


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origin of sedimentary lipids. We have determined carbon isotopic compositions of individual fatty acids, alcohols, and n-alkanes in the sediments from the Nansha sea area (Duan and Wang, 2002). The results indicate that mean δ 13 C values of C14 and C15 saturated fatty acids in sediments are −31.9‰ and −31.4‰, respectively, which is 6.2‰ and 3.8‰ lighter than those in algae and coral (−25.7‰ and −27.6‰), respectively. This demonstrates that C14 and C15 saturated fatty acids are derived mainly from bacteria, not from marine algae or animals. Carbon isotopic compositions of C16:0 and C18:0 fatty acids in sediments are −28.2‰ and −27.9‰ on average, respectively, which are similar to those in algae and coral (averaging −27.0‰ and −27.2‰, respectively), especially coral. No presence of long chain (>C22 ) fatty acids in algae and coral indicates that those compounds in sediment come mainly from higher-land plants. δ 13 C values of short chain alkanols (C16 ∼C18 ) in sediments range from −25.0‰ to −28.8‰ averaging at −27.7‰, which are close to those of fatty acids in plankton and coral (Schouten et al., 1998). From a biosynthetic stand point, alkanols and fatty acids in the same organisms should have similar carbon isotopic compositions. Therefore, the characteristics of carbon isotopic compositions of short chain alkanols in sediments reflect that they mainly originate from plankton and coral. δ 13 C values of short chain n-alkanes (C16 ∼C20 ) in the three sediments from the Nansha sea area range from −29.8‰ to −32.9‰, with an average of −32.0‰. This mean value is 3.2‰ lighter than that of n-alkanes in marine organisms in the Nansha sea area, reflecting that these n-alkanes are derived mainly from bacteria, since bacteria and algae contain abundant low-carbon-number n-alkanes. A predominance of C16 ∼C20 n-alkanes and lack of >C22 homologues in the studied organisms further demonstrates that the abundant long-chain n-alkanes in sediments from the Nansha sea area with lighter carbon isotopic compositions come from higher plants and bacteria (Duan and Wang, 2002). The Nansha sea area, which is located in a low-latitude tropical zone and is far away from the continent and has abundant tropical organisms, especially coral organisms, is a special marine ecological environment. It was determined with carbon isotopic compositions of individual fatty acids and n-alkanes in plankton, Gorgonia and Laurencia from the Nansha sea area. δ 13 C values of individual saturated fatty acids are from −25.6‰ to −28.6‰ for plankton, from −26.0‰ to −29.7‰ for Gorgonia, and from −24.4‰ to −28.1‰ for Laurencia, respectively. Their averages are −27.2‰, −28.2‰, and −26.4‰, respectively. These data show that Laurencia is relatively enriched in 13 C and Gorgonia is relatively enriched in 12 C. However, their mean difference is small, whose range is only 1.8‰. C16 and C18 unsaturated fatty acids have heavy carbon isotopic compositions compared with the same carbon-number fatty acids and their differences are 6.8‰ and 2.9‰, respectively. The reasons for the enrichment are unclear. δ 13 C values of n-alkanes in the three studied organisms range from −27.8‰ to −29.7‰, from −27.5‰ to −29.7‰, and from −28.0‰ to −29.2‰, respectively. Their mean values are quite similar, ranging from −28.6‰ to −28.9‰. Small differences in δ 13 C values between

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saturated fatty acids and n-alkanes in the same organism reflect that they are biosynthesized by carbon chain elongation. Heavy carbon isotopic compositions of lipids in marine organisms from the Nansha sea area, as compared with land C3 plants, reflect the carbon isotopic characteristics of marine aquatic organisms. The comparative studies of carbon isotopic compositions of individual lipids in organisms and sediments from the Nansha sea area have established the carbon isotopic genetic relationships between the biological and sedimentary lipids, which provides a scientific basis for carbon isotopic applied research of individual lipids (Duan et al., 2004). 5.5.4.4 Geochemical Significance of Compositional Features of Ketone, Aldehyde, and Alcohol Compounds (1) Acyclic isoprenoid ketones, aldehydes, and alcohols Abundant C18 isoprenoid ketone, Z/E pristenal, and Z/E phytenal as well as phytol biomarker compounds were detected in the sinking particulate materials from the Yongshu Reef Lagoon and the continental shelf of the East China Sea (Figs. 5.28 and 5.29). This appears to be the first report of these compounds in China marine environment. Detection of these compounds in the studied regions is of some importance to understand the evolutional processes of acyclic isoprenoid compounds in the seawater column and their formative pathways in marine sediments. Their identifications were based on chromatographic retention time, mass spectrum features (Figs. 5.28 and 5.29), and comparison with those reported previously. The mass spectrum of phytol trimethylsilyl ether exhibits a base peak at m/z 143 and a molecular ion at m/z 368. C18 isoprenoid ketone has a base peak at m/z 58, a molecular CH3 O Si CH3 143 CH3 Mass cleavage


100

Phytol

Sterols

n-Alkanols

50 nC16

nC22

nC12

0 20

30

40

50

60

t (min)

Fig. 5.28. Total ion chromatogram of the alcohol fraction in sinking particulate material from the Yongshu Reef Lagoon (Duan et al., 1997) (With permission from Duan Y)


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100

A

B

50 C

0

10

20

30

D E

40 t (min)

50

60

Fig. 5.29. Total ion chromatogram of ketone+aldehyde fractions in sinking particulate material from the Yongshu Reef Lagoon. A, C18 isoprenoid ketone; B, Z pritenal; C, E pristenal; D, Z phytenal; E, E phytenal (Duan et al., 1997) (With permission from Duan Y)

ion at m/z 268, and other characteristic fragment ions at m/z 165, 210, and 250. The base peaks of Z/E pristenals and Z/E phytenals are m/z 84 and their molecular ion peaks are m/z 280 and 294, respectively. Z and E isomers were determined according to the different intensity of m/z 97 in the mass spectrum. It is generally considered that in the water column phytol is formed mainly from chlorophyll a by biological and biochemical hydrolyses, while C18 isoprenoid ketone, Z/E pristenals, and Z/E phytenals are intermediates in the conversion of phytol to other isoprenoid compounds and two pathways of biochemical and photochemical degradations have been proposed. Therefore, the presence of these abundant isoprenoid compounds shows that sinking particulate organic matter is strongly transformed before reaching a depth of 5 m from the sea floor. The phytol/Σn-alkanol ratio is 0.48 for the Yongshu Reef Lagoon and 0.03 for the continental shelf of the East China Sea, indicating that the relative content of phytol in the Yongshu Reef Lagoon is higher than that in the continental shelf of the East China Sea. However, the content of chlorophyll a does not differ significantly. This difference in their phytol contents may be related to the extent of biological and biochemical processes. These imply that phytol and its degraded products are formed by biological and biochemical pathways. (2) n-Alkanols Sinking particulate materials in the Yongshu Reef Lagoon and the continental shelf of the East China Sea contain abundant n-alkanols. Their carbon numbers range from C12 to C28 with a strong even-over-odd predominance (with a CPI16∼28 value of 11.88 and 4.69, respectively), indicating the feature of modern organism genesis. n-Alkanols, with a maxmum at C16 and

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− a ΣnC+ 22 /ΣnC22 ratio of 6.10 and 6.70 respectively, are dominated by lower carbon number n-alkanols (Tables 5.25 and 5.26). This distribution shows that they mainly come from marine plankton. However, a higher abundance of C22 n-alkanol is also present in the Yongshu Reef Lagoon, indicating the contribution of submerged macrophytes to n-alkanol. Higher carbon number n-alkanols (>C22 ) are generally thought to be derived from the surface wax of higher plants, but carbon isotope data show that they can also come from bacteria. The relative content of higher carbon n-alkanols is 14.08% for the Yongshu Reef Lagoon and 12.65% for the continental shelf of the East China Sea, showing that the former has higher plant or bacterium-derived n-alkanols. This is identical with the studied results of fatty acids.

Table 5.25. Relative contents of n-alkanols and sterols (Duan et al., 1997) (With permission from Duan Y) Number

n-Alkanols

1 2 3 4 5 6 7 8 9 10 11 12 13

nC12 nC13 nC14 nC15 nC16 nC17 nC18 nC19 nC20 nC21 nC22 nC23 ∼nC28

Relative content (%) NS ECS 7.04 1.15 1.41 0.86 4.23 6.31 2.82 3.16 29.58 28.16 1.41 7.18 5.63 20.98 1.41 2.87 8.45 7.76 1.41 2.30 22.53 6.61 14.08 12.65

Sterols C27 Δ5.22 C27 Δ22 C27 Δ5 5α-C27 Δ0 C28 Δ5.22 C28 Δ22 C28 Δ5 5α-C28 Δ0 C29 Δ5.22 5α-C29 Δ22 C29 Δ5 5α-C29 Δ0 5α-C30 Δ22

Relative content (%) NS ECS 6.44 1.39 2.03 1.85 28.81 37.96 21.02 12.50 14.24 9.25 3.73 4.17 5.08 6.94 1.36 2.32 6.78 2.32 1.02 4.17 7.12 3.70 1.02 0.46 1.36 12.96

Δ5.22 , stenols; Δ0 , stanols; NS, Nansha Sea; ECS, East China Sea

Table 5.26. The parameters of n-alkanol and sterols (Duan et al., 1997) (With permission from Duan Y) Region NS ECS

Σ nC+ 22 /ΣnC− 22 * 6.10 6.70

CPI16∼28 11.88 4.69

Phytol/ Σn-alkanol 0.48 0.03

sterols (C27 Δ0 /C27 Δ5 ) 0.56 0.33

Relative content of sterols (%) C27 C28 C29 C30 58.30 24.41 15.94 1.36 53.70 22.68 10.65 12.96

− *ΣnC+ 22 /ΣnC22 , Σ(C12 −C22 )/Σ(C23 −C28 ) ratio of n-alkanols; NS, Nansha Sea; ECS, East China Sea

(3) Sterols Thirteen sterols have been detected in sinking particulate materials from the Yongshu Reef Lagoon and the continental shelf of the East China Sea. Their


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carbon numbers range from C27 to C30 . The most abundant sterols of the two regions are cholest-5-en-3β-ol (3) and 5-cholestan-3-ol (4), which account for about 50% of the total sterols (Table 5.25). They are generally thought to originate primarily from zooplankton. This suggests a similarity in the contribution of zooplankton to sinking particulate organic matter in the Yongshu Reef Lagoon and the continental shelf of the East China Sea. C28 sterols in the two regions represent 24.41% and 22.68% of the total sterols respectively (Table 5.26), and the most abundant C28 sterol is 24-methyl cholest-5,22-dien3β-ol (5) which is generally considered to be derived mainly from diatom and used as a biomarker for diatom lipids. C29 sterols are present in low abundances, and 24-ethylcholest-5,22-dien-3β-ol (9) and 24-ethylcholest-5-en-3β-ol (11) are major components of the sterols in higher plants. However, for the marine environment, the two sterols can also come from some phytoplankton. Combining the results with those of hydrocarbon compound and fatty acid distributions, it indicates that the two sterols in sinking particulate materials from the Yongshu Reef Lagoon and the continental shelf of the East China Sea partially originate from higher land plants and their relative contents in the Yongshu Reef Lagoon are higher than those in the continental shelf of the East China Sea (Table 5.25). The input of the higher land plant component in sinking particulate material of the Yongshu Reef Lagoon is most probably in the form of aeolian dusts or polluted materials formed by human activity on the Yongshu Reef island, as the Yongshu Reef is separated from land by deep sea. C30 4α, 23,24-triethyl-5α-cholest-22-en-3β-ol (13) derived from dinoflagellates in Pyrrophyta comprise 1.36% and 12.96% of the total sterols in the Yongshu Reef Lagoon and the continental shelf of the East China Sea respectively, showing that sinking particulate organic matter in the continental shelf of the East China Sea has a more dinoflagellate-derived component. Sterols can come from both organisms and biochemical conversion of stenols to stanols. A C27 Δ0 /C27 Δ5 ratio of 0.1∼0.2 is thought to represent the constitution of stenols and stanols in plankton. The C27 Δ0 /C27 Δ5 ratio is 0.56 for the Yongshu Reef Lagoon and 0.33 for the continental shelf of the East China Sea, showing that the biochemical transformation process of sterols is greater in the Yongshu Reef Lagoon than in the continental shelf of the East China Sea. This is consistent with the result for isoprenoid compounds (Duan et al., 1997). 5.5.5 Vertical Transferring Process of Major and Rare Elements in the Nansha Coral Rreef Lagoons In geology, rare elements (REs) are defined as the elements whose contents in the earth are low, or have difficulty in forming signal mineral in nature although their total amount in the earth is very huge (Zhao and Yan, 1994). The major elements in the various environment matrixes are one of the most important subjects in the environment sciences.

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5.5.5.1 Rare Elements Most REs were discovered in the 19th century. More than one hundred years has passed since their discovery. However, some geochemical issues are still paradoxical (Song and Li, 1997). The contents of REs in the earth are so small that they seem more sensitive to environmental changes than common elements do. For example, common elements usually change in content from an order of magnitude of 10−3 to 10−2 when the circumstances suddenly change; however, the contents of REs will change from tens to hundreds of times (Song and Li, 1998). Studying the changes in REs in environments would infer accurately what had happened to the environmental changes in the past. So, studying the geochemistry of REs becomes more and more significant in the context of global climate change. Previous research into REs in the oceans mostly concentrated on sediments and have made great progress in understanding the origin of sediments and the contributing factors of authigenic mineral forming. However, RE research in seawater is far from satisfactory; what is more, in some sea areas the research is still rare. Over a long period, RE research in seawater was limited due to their extraordinarily low content (10−10 μg/L) and difficulty in determining this. But, in any case, the study certainly plays an important part in the chemical oceanography of REs. By weathering, dripping and other processes, part of the REs in the earth’s crust finally enter oceans from rivers, and then take part in the ocean’s biogeochemical cycle. REs in seawater are transferred from ions into particulates. The transformation of REs in seawater leads to their redistribution in different phases. It is obvious that REs take part in almost all biogeochemical processes in the ocean. The vertical transformation of REs in sinking particulates is one of the most important processes. The characteristics of the vertical transformation of REs would reflect the information about environmental changes in certain contents. (1) Characteristics and components of sinking particulates In the sinking particulates collected from coral reef lagoons in the Nansha Islands, SCS, during three cruises great differences were found to exist in the characteristics and the chemical components compared to those in open oceans and compared to sediments. More organic particulates, such as secretion, excreta, and dead bodies, were trapped. Chemical analysis showed that organic carbon content in sinking particulates reached 0.96%∼96%, organic nitrogen, 0.11%∼0.26%, and organic phosphorus, 209.17×10−10 ∼912.46×10−10 μg/L. Biogenic CaCO3 was the main component in mineral detritus and its content reached 90%. The vertical fluxes of sinking matter during three cruises (93-5, 94-4, 93-94) were 3.26, 4.60, and 22.93 g/(m2 ·d) respectively. (2) Vertical fluxes and contents of REs in sinking particulates The vertical fluxes and contents of REs in sinking particulates collected during three cruises are listed in Table 5.27. The contents of 18 elements in sinking particulates are greatly different; for instance, Ti is higher than 150 mg/g, As,


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Zn, and V are higher than 1 mg/g, respectively, and the rest are below 1 mg/g. The data indicate that the fluxes of most elements in the Yongshu Reef Lagoon are higher than those in the Zhubi Reef Lagoon except the fluxes of Zr, Co, Zn, and Sb. Results obtained from the same lagoon (the Zhubi Reef) during two cruises are similar. The nature of REs and environmental conditions in the coral reef determined the distribution of REs in different phases of each element. Geography, density of water exchange species, biomass of biocoene, etc., have a strong influence on the distributions of REs. The Yongshu coral reef is a nearly open lagoon, parts of the sinking particulates would come from the external sea by water exchange, so the mass flux of materials is higher than that in the Zhubi Reef, a typical closed lagoon. The water dynamic factor greatly influenced the flux. The sea current and tide observed at the same time indicated that current sea velocity and tide in the Yongshu Reef were obviously higher than those in the Zhubi Reef. Strong water dynamics would increase the collision of particles, so lots of sinking particulates would form. The average velocity of the current in the Zhubi Reef was 34 cm/s during the 94-4 cruise and 16 cm/s during the 93-5 cruise (5 m layer) (Yu and Xu, 1994). The vertical fluxes of the elements during the 94-4 cruise were evidently higher than those during the 93-5 cruise except for Au, Cr, Co, Zn, Sb, and Se, as shown in Table 5.27. The details will be discussed below. Table 5.27. Contents (μg/g) and vertical fluxes (μg/(m2 ·d)) of REs in sinking particulates (Song and Li, 1998) Element Zr Hf Ta V Ti Rb Cs Ag Au Cr Co Zn As Sb Sc Se U Th

93-5 Content Flux 1.00 3.28 0.002 0.007 0.0038 0.0125 2.47 8.10 200 656 0.006 0.020 0.001 0.003 0.004 0.013 0.0027 0.0089 1.16 3.81 0.019 0.062 17.3 56.7 1.0 3.3 0.33 1.09 0.0053 0.0174 0.024 0.079 2.66 8.72 0.0032 0.0105

94-4 Content Flux 0.76 3.5 0.006 0.025 0.0006 0.0028 5.84 26.86 200 920 0.060 0.276 0.003 0.014 0.004 0.018 0.0160 0.0736 0.33 1.51 0.017 0.078 5.0 22.9 2.4 10.9 0.15 0.69 0.0057 0.0262 0.009 0.041 2.66 12.24 0.0047 0.0216

93-94 Content 0.03 0.002 0.0034 5.09 180 0.054 0.001 0.004 0.0024 0.09 0.002 0.7 1.8 0.03 0.0032 0.081 2.35 0.0008

Flux 0.66 0.041 0.0790 116.81 4, 127 1.238 0.021 0.083 0.0557 2.13 0.041 16.3 41.3 0.70 0.0722 0.186 53.86 0.0186

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(3) Vertically transferring forms of REs Table 5.28 lists the fluxes of REs in different forms and their relative ratios in sinking particulates in the Yongshu Reef Lagoon. The data indicate that Ta, As, Th were transferred to the sea floor mainly in the ion-exchange form. The elements transferred in this form are likely to be combined with particulates in a weak chemical bond, such as adsorption. A living organism as “a swimmer” which enriched REs was trapped. This is possibly another method of transfer (Song and Li, 1996a). The elements transferred in the carbonate form are Zr, Hf, V, Ti, Rb, Cs, Au, Cr, Co, Zn, Sc, Se, and U. Their transformation could include two ways: one is in detritus wrapped REs, the other is in a compound of carbonate. Zr, Hf, Rb, Cs, and Sc transferred in the former way, while V, Ti, Table 5.28. Contents and fluxes of difference RE forms in sinking particulates of the Yongshu Reef Lagoon (Song and Li, 1998)

Zr Hf Ta V Ti Rb Cs Ag Au Cr Co Zn As Sb Sc Se U Th

C ND ND 0.0029 ND 1.4 ND ND ND ND ND ND 0.09 0.80 ND ND 0.0001 0.179 0.0061

IEF F ND ND 0.0663 ND 32.1 ND ND ND ND ND ND 1.97 18.34 ND ND 1.0014 4.140 0.0399

R ND ND 64.2 ND 0.6 ND ND ND ND ND ND 12.1 44.4 ND ND 0.7 8.6 75.3

C 0.614 0.0021 0.0004 4.86 166.0 0.0502 0.0035 0.0025 0.0024 0.046 0.0018 0.35 0.33 0.004 0.0013 0.0054 0.043 0.0003

CF F 14.074 0.0482 0.0087 113.86 3, 806.4 1.1511 0.0800 0.0573 0.0555 1.043 0.0413 7.93 7.61 0.083 0.0287 0.1247 46.846 0.00069

OSF C F R Zr 0.035 0.812 5.3 Hf ND ND 0 Ta ND ND 0 V 0.11 2.51 1.6 Ti 27.2 622.8 13.4 Rb 0.0006 0.0138 1.1 Cs 0.0001 0.0023 2.6 Ag 0.0080 0.0183 38.60 Au ND ND 0 Cr 0.020 0.459 19.1 Co 0.0009 0.0197 20.5 Zn 0.02 0.54 3.3 As 0.06 1.45 3.5 Sb 0.018 0.413 49.2 Sc 0.0012 0.0284 34.5 Se 0.026 0.0605 30.9 U 1.166 1.516 2.8 Th 0.0010 0.0220 12.3 ND, not detected; C, content; F, flux; R, relative ratio (%); iron-manganese oxides form; OSF, organic matter+sulphide

R 92.5 77.5 8.4 69.9 81.5 93.0 88.8 12.4 92.0 43.5 42.9 48.7 18.4 9.8 34.8 63.6 87.0 3.7

C ND 0.0004 ND 1.94 ND 0.0005 0.0001 0.0100 0.0001 0.032 0.0011 0.25 0.56 0.001 ND ND 0.026 0.0001

IMOF F ND 0.0089 ND 44.72 ND 0.0115 0.0023 0.2298 0.0022 0.743 0.0252 5.66 12.82 0.023 ND ND 0.596 0.0028

SIF C F 0.015 0.335 0.0002 0.0050 0.0012 0.0282 0.05 1.05 9.1 209.1 0.0027 0.0619 0.0002 0.0055 0.0002 0.0041 0.0001 0.0028 0.007 0.151 0.0004 0.0101 0.01 0.19 0.05 1.05 0.014 0.321 0.0011 0.0252 0.0004 0.0094 0.035 0.805 1.0006 0.0147 IEF, ion exchangeable form; form; SIF, silicate form

R 0 14.4 0 27.8 0 0.9 2.5 18.4 3.4 31.0 26.2 34.8 31.1 2.7 0 0 1.1 1.3 R 1.2 9.1 27.4 0.7 4.5 5.0 6.1 0.6 4.6 6.4 10.4 1.1 2.6 38.3 30.7 4.8 1.5 7.4 IMOF,


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Cr, Co, Zn, and U did so in the latter way. The trend in the transformation of Ag, V, Cr, Zn, As, and Hf was to the form of iron-manganese oxides, such as the co-precipitation of AgO or AgOH combining with iron-manganese oxides. The fine and close con-structure of the co-precipitation makes it easy to sink. Sb was transferred in a combination of organic matter and sulphide, where it was likely that the Sb-sulphide form was dominant. Another transferring forms of Sc, Ag, Se, and Th would be their organic compounds in sinking particulates. The main transferring forms of 18 elements in the Yongshu coral reef are listed in Table 5.29. Table 5.29. Main vertical transferring forms of REs in sinking particulates of the Yongshu Reef Lagoon (Song and Li, 1998) Element Zr Hf Ta V Ti Rb Cs Ag Au

Main forms CF CF, IMOF IEF, SIF CF, IMOF CF, OSF CF CF IMOF, OSF, CF CF

Elements Cr Co Zn As Sb Sc Se U Th

Main forms CF, IMOF, OSF CF, IMOF, OSF, SIF CF, IMOF, IEF IEF, IMOF, CR OSF, SIF CF, OSF, SIF CF, OSF CF IEF, OSF

(4) Comparison of RE contents in sinking matter and in sediment of the Nansha Islands Table 5.30 shows the ratios of RE contents in sinking matter (SM) to those in sediment (S). REs may be classified in five groups according to the ratios. Table 5.30. The classification of the REs based on the ratios of RESM /RES (%) (Song and Li, 1998) Clssified number 1 2 3 4 5

RESM /RES (%) >100 10∼100 1∼10 0.1∼1 V>As>Ti>U>Zn>Sb>Hf>Ag>Cs. Vertical flux changes of Rb, V, As, Ti, U, Zn can be used for forecasting SST variation. Table 5.32. The relationship between RE vertical flux (μg/(m2 ·d)) in lagoon and SST (◦ C) (Song and Li, 1998) Month 1 SST 27.6 Zn 17.48 Sb 0.68 V 1410 Ti 7355 As 359.6 Rb 321.3 Ag 0.03 U 45.89 Cs 0.02 Hf 0.04

2 3 4 5 6 7 8 9 10 27.5 28.2 29.6 30.6 30.2 29.7 29.5 29.5 29.4 17.31 19.03 32.27 70.03 49.43 34.35 30.43 40.43 28.80 0.68 0.69 0.72 1.12 0.86 0.74 0.72 0.72 0.71 1945 206.7 10.07 8.08 8.30 9.52 10.86 10.86 11.96 9166 2248 706.3 655.2 663.4 694.3 721.5 721.5 740.8 477.1 67.65 4.39 3.27 3.42 4.10 4.79 4.79 5.31 464.7 35.12 0.21 0.002 0.002 0.15 0.31 0.31 0.44 0.03 0.02 0.02 0.01 0.01 0.02 0.02 0.02 0.02 53.13 21.43 9.58 8.68 8.88 9.41 9.80 9.80 9.94 0.02 0.01 0 0 0 0 0 0 0 0.05 0.04 0.02 0.01 0.01 0.02 0.02 0.02 0.02

11 12 29.0 28.2 23.92 19.03 0.69 0.69 22.61 206.7 886.2 2248 9.79 67.65 1.85 35.12 0.02 0.02 11.64 21.43 0 0.01 0.03 0.04

612


5.5.5.2 Biogeochemical Process of Major Elements in Sinking Particulates of the Nansha Coral Reef Lagoons, the South China Sea The majority of major elements will be deposited in estuaries and coastal sediments after they have been dredged through precipitation and runoff. Several major elements are transferred to the open sea or to the seabed sediments as dissolved forms. By virtue of these detailed and deep researches, the distribution and variation rules of major elements in seawater and sediments have been revealed. However, no detailed information on the processes that take place when major elements are transferred from seawater to sediments is available now. After being transferred to the seabed sediments through the deposition of suspended particulates, the concentrations of major elements in particulate matter would generally differ from those in sediments. On the one hand, the transformation between the particulate form and dissolved form will change the major elements concentrations. The uptake and excretion processes of organisms also have an influence on the geochemistry characteristics of the major elements. When the major elements were transferred to the seabed sediments, they were redistributed, therefore their concentrations varied subsequently. Table 5.33 summarizes and tabulates their concentrations and fluxes. Table 5.33. The concentrations and fluxes of major elements in the sinking particulates in Nansha coral reef lagoons (Song, 1997a) Element Na K Cl Br I Ca Mg Ba Fe Sr Al

93-05 Concentrations Fluxes 4, 250 13, 940 8, 790 28, 831 1, 250 4, 063 192 630 73.8 242.1 450, 000 1, 476, 000 32, 900 107, 192 4.09 13.42 7.63 25.03 72.2 236.8 679 2, 227

94-04 Concentrations Fluxes 4, 200 19, 320 2, 000 9, 200 2, 146 9, 872 352 1, 619 74.1 340.9 442, 500 2, 035, 500 31, 780 146, 188 2.14 9.84 69.90 321.54 82.0 377.2 840 3, 864

93-94 Concentrations Fluxes 3, 170 72, 693 1, 786 40, 935 1, 264 28, 974 197 4, 526 52.0 1, 192.8 398, 700 9, 140, 219 26, 879 616, 342 0.58 13.21 59.13 1, 355.82 82.0 1, 880.0 236 5, 407

The concentrations and fluxes varied by 4 orders of magnitude among the 11 major elements. The sequence of the concentrations from high to low is: Ca>Mg>Na, K>Cl>Al>Br>I>Sr>Ba, so are the fluxes. With respect to sinking fluxes, Ca exceeds 1.4 g/(m2 ·d), and Mg exceeds 0.1 g/(m2 ·d). In particular, the concentrations and fluxes of Ca during the three cruises was about 13 factors higher than those of Mg. In the Nansha Sea area, a large number of coral reefs are developed, where reef-building corals live and produce structurally complex calcareous skeletons using inorganic Ca and Mg. Due to the majority of suspended particulate samples is coral crumb in Nansha coral reef, this raises the concentrations of Ca and Mg in suspended particulate matter. In addition, Br and I, as typical bio-modified elements,


613

have high sinking fluxes in the range of 0.63∼4.5 and 0.24∼1.19 g/(m2 ·d) respectively. Five forms are classified to reveal the major elements geochemistry characteristics as follows: ion exchangeable form (IEF), carbonate bound form (CF), iron and manganese oxides bound form (IMOF), organic matter and sulfide bound form (OSF), and silicate bound form (SIF). As shown in Table 5.34, the CF fluxes of Na, K, Ca, Mg, and Sr account for 97.8%, 93.0%, 99.8%, 99.6%, and 99.8% of all the sinking fluxes respectively, indicating that CF is the predominant form during their sinking process. Ca, Mg, and Sr are deposited on the bed sediments as CaCO3 , MgCO3 , and SrCO3 , while Na and K may be entrained in deposition. Al and Br are transferred from seawater to sediments with the major form of SIF, while Fe, I, and Ba are deposited mainly in IMOF. It is notable that the dominant forms of Cl in the sinking process are CF and SIF; OSF is the second important form for Br in its deposition; CF and SIF play equal roles followed by IMOF for I in deposition. The percentages of Cl, Br, and I forms suggest that Br is modified mainly in an organic form, while I is modified mainly in an inorganic form. Table 5.34. The concentrations, fluxes and percentages of four forms of the major elements in the sinking particulates (Song, 1997a) CF IMOF OSF C F R C F R C F R C Na 3,102 71,132 97.8 9.3 213 0.29 7.2 166 0.23 51.5 K 1,660 38,064 93.0 17.2 394 0.95 17.6 404 0.99 91.2 Cl 1,063 24,375 56.3 ND ND 0 ND ND 0 825 Br 44.1 1,012 22.4 7.7 175 3.87 60.6 1,390 30.7 85.0 I 18.1 414 22.9 43.4 994 54.9 ND ND 0 17.6 Ca 432,273 9,912,029 99.8 619 14,204 0.14 121 2,774 0.03 91.2 Mg 26,753 613,453 99.6 17.2 394 0.06 17.6 394 0.07 91.2 Ba 0.26 5.98 37.1 0.39 8.99 55.7 ND ND 0 0.05 Sr 84.31 1,933 99.8 0.13 3.03 0.16 0.01 0.18 0.01 0.03 Fe 22.11 507 31.0 45.3 1,038 63.4 3.21 73.7 4.50 0.78 Al 118.0 2,705 45.5 8.8 201 3.38 21.0 491 8.11 112 C, content; F, flux; R, relative ratio (%). The total IEF of major elements was not detected (ND) Element

SIF F 1,182 2,091 18,926 1,948 403 2,091 2,091 1.22 0.65 18.0 2,562

R 1.63 5.11 43.7 43.0 22.2 0.02 0.34 7.15 0.03 1.10 43.0

After being transferred downward, part of the major elements will be involved in recycling, and the other part will be eventually deposited. Except for the molecular ratio of Br/I, all the molecular ratios of other cognation elements in SPM were lower that those in the lagoon sediments. Taking the major element concentration in the sediments as the final concentration and the concentration of SPM as the final concentration, the difference between the final concentration and the original concentration was the part of major elements involved in the recycling process. Likewise, taking the concentrations in the block coral as the final concentrations and those in SPM as the original concentrations, the difference between them was the concentrations in the sandy detritus. Subsequently, the proportions of the recycling and buried concentrations were calculated and represented in Tables 5.35 and 5.36. Table 5.35 showed that the majority of Br, I, Al, and K were involved in the recycle; about half of Mg would be recycled. In contrast, Ca was finally

614


buried mainly in the sediments. The sequence of the major elements buried in the sediments from high to low was: Ca, Mg, I, Al, Br, and K. The ratio of the percentage of major elements in sandy detritus to those in block coral showed that Ba, Sr, and Fe predominately existed in block coral, while Na and Cl were distributed evenly in block coral and sandy detritus. Considering Tables 5.35 and 5.36 together, a series conclusions could be drawn, namely that: (1) the majority of Ca and Sr playing an important role in coral reef building were buried in the sediment eventually; (2) Br and I were biomodified and the majority were recycled; (3) the majority of Fe and Ba were buried in the sediments; (4) Na, K, Mg, and K were deposited mainly by coral detritus, and more than half of them would be involved in the recycling process. Table 5.35. The proportions of the major elements involved in recycling and burial in the sinking particles of the Yongshu Reef (Song, 1997a) Element Mg K Al Ca Br I

Recycling proportions (%)/buried proportions (%) 93-05 94-04 93-94 Average 62.3/37.7 61.0/39.0 53.9/46.1 59.1/40.9 98.1/1.9 91.6/8.4 90.6/9.4 93.4/6.6 93.2/6.8 94.5/5.5 80.4/19.6 89.4/10.6 17.9/52.1 16.5/83.5 7.4/92.6 13.9/86.1 90.1/10.9 94.0/6.0 90.4/10.6 96.8/9.2 77.0/23.0 77.1/22.9 67.3/32.7 73.8/26.2

Table 5.36. The proportions of the major elements in the sandy detritus and in the block coral of the Yongshu Reef (Song, 1997a) Element Cl Fe Na Sr Ba

93-05 33.8/66.2 4.6/95.4 58.2/41.8 1.4/98.6 45.7/54.3

Sandy detritus/block coral 94-04 93-94 58.0/42.0 34.2/65.8 42.1/57.8 35.6/64.4 57.5/42.5 43.4/56.6 1.6/98.4 1.6/98.4 23.9/76.1 6.4/93.6

Average 42.0/58.0 27.4/72.6 53.0/47.0 1.5/98.5 25.3/74.7

The deposition process of the major elements was strongly affected by the SST. Fig. 5.30 illustrates that the deposition processes of 11 major elements except K and Ba decreased exponentially with an increase in SST, while the deposition processes of K and Ba were SST free. The correlation between SST and the deposition fluxes was obtained based on the curvilinear regression as follows:


615

Na : F = 5.24 × 1033 e−2.38T + 1.35 × 104

(5.22)

Cl : F = 1.87 × 1025 e−1.72T + 4000

(5.23)

Ca : F = 4.58 × 1032 e−2.12T + 1.40 × 106

(5.24)

Mg : F = 1.86 × 1025 e−1.557 + 1.00 × 106

(5.25)

Sr : F = 1.17 × 1024 e−1.72T + 220

(5.26)

Br : F = 1.35 × 1035 e−2.60T + 700

(5.27)

I : F = 1.02 × 1023 e−1.67T + 240

(5.28)

Fe : F = 1.37 × 1032 e−2.39T + 10

(5.29)

Al : F = 3.09 × 1019 e−1.32T + 2400

(5.30)

The monthly monitor of the vertical fluxes and SST is represented in Table 5.37, in which it could be concluded that the sensitivities of major elements to SST are in the following sequence: Fe>Br>Sr>Ca>Na>Cl>Mg>I>Al (Song, 1997a). Table 5.37. The average vertical fluxes (μg/(m2 ·d)) of the major elements and SST in the Nansha Coral Reef Lagoon over 12 months (Song, 1997a) Month 1 2 3 4 5 6 7 8 9 10 11 12

SST Na Cl Ca Mg (◦ C) (×104 ) (×104 ) (×104 ) (×104 ) 27.6 16.45 5.31 17.75 5.90 27.5 20.51 6.23 21.61 6.73 28.2 4.97 2.15 5.97 2.94 29.6 1.48 0.56 1.63 1.22 30.6 1.36 0.43 1.43 1.05 30.2 1.38 0.46 1.47 1.09 29.7 1.45 0.53 1.59 1.19 29.5 1.51 0.59 1.69 1.26 29.5 1.51 0.59 1.69 1.26 29.4 1.56 0.62 1.76 1.30 29.0 1.89 0.84 2.24 1.56 28.2 4.97 2.15 5.97 2.94

Sr

Br

3,299 10,738 3,876 35,365 1,319 2,813 319 756 238 704 255 712 304 743 338 772 338 772 360 794 498 965 1,319 2,813

I 1,335 1,534 643 279 247 254 273 286 286 295 346 643

Fe 3,081 3,910 742 36 12 16 30 42 42 52 118 742

Al 7,188 7,863 4,570 2,742 2,492 2,555 2,700 2,791 2,791 2,846 3,155 4,570


2 F ( m g/(m d))

616

.

Fig. 5.30. The relationships among the vertical fluxes (μg/(m2 ·d)) of major elements and SST (◦ C) (Song, 1997a)

5.5.6 Sulfide (−2 Valence) in Lagoon and Off-Reef Sediment Interstitial Waters The concentrations of H2 S, HS− , and S2− were used to calculate the total −2 valence sulfur (sulfide) concentrations ( S(–II)) according to the following equation: (5.31) S(−II) = H2 S + HS− + S2− The vertical profiles of −2 valence sulfide ( S(−II) and HS− ) in the pore water from the in- and off-reefs are presented in HS− was the predominant content of total S(−II), exceeding 90%. The concentrations of S(−II) and HS− increased with depth except the 94-8, as Fig. 5.31 shows. Results of the two year monitor showed that the average S(−II) of the pore water from in-reef lagoon sediments was much higher than that from offreef sediments. S(−II) of in-reef lagoon pore water was higher by about 2


10 0

11

12

SS(-II) (mmol/L) 23 0

24

25

Depth (m)

(a) 10

20

20 30

40

0

(c) 10

26 (b)

10

20 0

617

8.0

8.5

9.0

9.5 (d)

10

SS(-II) 20

20

HS-

Fig. 5.31. Vertical profiles of sulfide (–2 valence) in off-reef sediment interstitial waters in the water region of the Nansha Islands, South China Sea (Song and Li, 1996b). (a), (b), (c), and (d) illuminated the vertical distribution of –2 valence sulfur in the water sample from 94-8, 94-12, 94-16, and 94-23 stations respectively. The solid and dashed lines represented the ΣS(−II) and HS− respectively

factors than that in off-reef lagoons in 1993 and by over 4 factors in 1994. The reason might be that in-reef lagoons were characterized by a high nutrients level and high productivity, and the vital activities of organisms living there, such as growth, propagation, and excretion, could raise the reducibility of the sediment environment. In particular, Eh values of the sediments from the Banyue Reef and the Xinyi Reef were 5 mV and −26 mV, respectively, indicating the high reducibility of the sediment environments. Considering the whole study area, S(−II) of the pore water from the in-reef lagoon declined southward with latitude in general. In the investigation performed in March∼April 1994, S(−II) at 97-Y located in the Yongshu Reef, the most northerly station, was as high as 353.12 μmol/L and declined sharply to 3.90 μmol/L at 94-10H, the most southerly station in the study area. In an east-west direction, S(−II) declined with the increase in longitude. For the pore water from the off-reef sediments, S(−II) was distributed in a radial pattern, i.e., the higher S(−II) was, the closer the station was to the coral reef. For example, the S(−II) detected in 93-6 (the closest station to the Yongshu Reef, 135.14 μmol/L) and 93-6A (the closest station to the Sanjiao Reef, 36.55 μmol/L) were quite a bit higher than those in other stations. In the investigation performed in March∼April 1994, the maximum value occurred at 94-16 (29.92 μmol/L), close to the Banyue Reef.

618


Anthropogenic activities would increase the reducibility of the sediment environment to some extent. For example, the Zhubi Reef and the Yongshu Reef were explored in recent years; therefore, the S(–II) values of the two reefs were 1∼2 orders of magnitude higher than other reefs, as high as 235.06 and 353.13 μmol/L, respectively (Table 5.38). A large amount of sewage was discharged into the lagoons, where the organic matter decomposed, thus the reducibility of the sediment environment increased and −2 valence sulfur was released. Table 5.38. Sulfur of interstitial water in lagoon and off-reef sediments in the Nansha Islands waters, South China Sea (Song and Li, 1996b) P S(−II) HS− (μmol/L) (μmol/L) Lagoon 21.03∼235.06 20.78∼232.0 (93.02) (91.88) 1993 Off-reef 19 25.93∼135.14 25.93∼133.46 (43.03) (43.03) Lagoon 15 3.90∼353.12 3.84∼349.96 (82.75) (81.86) 1994 Off-reef 4 8.61∼29.92 8.48∼29.46 (18.76) (18.41) All data expressed as range (average) Year

Stations

Sample number 6

H2 S (μmol/L) 0.12∼2.93 (1.14) 0.30∼1.68 (0.64) 0.04∼3.16 (0.89) 0.13∼0.50 (0.35)

PS0

SO2− (g/L) 4

10.41∼11.35 (10.91) 10.86∼11.31 (11.17) 10.00∼12.28 (11.04) 11.41∼11.83 (11.70)

2.60∼2.80 (2.68) 2.59∼2.76 (2.70) 2.57∼2.91 (2.75) 2.66∼2.84 (2.74)

5.5.6.1 Diffusion Fluxes of Sulfur across the Sediment-Seawater Interface The diffusion fluxes of sulfur were calculated based on Fick’s first role and shown in Table 5.39. Comparing the results of 1993 and 1994, it was found that the average diffusion fluxes of HS− in the pore water from in-reef sediments did not vary significantly in the two years, which were 60.56 and 60.12 μmol/(m2 ·d), neither did SO2− 4 . However, no well-defined difference occurred between the average diffusion fluxes of S2− and SO2− 4 of the pore water from off-reef sediments in 1993 and 1994. Nevertheless, the diffusion fluxes of HS− decrease from 22.64 μmol/(m2 ·d) in 1993 to 7.27 μmol/(m2 ·d) in 1994. 5.5.6.2 Thermal Dynamic Balance of (−2 Valence) Sulfur The −2 valence sulfur in the seawater and sediment derived from the reductive product of SO2− 4 . The reaction formula was as follows: −→ H2 S + 2HCO− 2CH2 O + SO2− 4 3 bacteria

(5.32)

where the CH2 O presents organic matter. The correlation between total or ganic carbon contents with S(−II) was studied, as Fig. 5.32 shows. S(−II) concentrations were in linearly inverse proportion to TOC contents, which


619

Table 5.39. Diffusion fluxes of sulfur across sediment-seawater interface in the Nansha coral reef lagoon waters, South China Sea (Song and Li, 1996b) Year

Station

1993

Lagoon

1994

Lagoon

1993

Off-reef

1994

Off-reef

H2 S (μmol/(m2 ·d)) −0.90∼181.60 (60.56) −5.32∼297.32 (62.12) 3.55∼96.46 (22.64) −1.31∼16.81 (7.27)

S2− (pmol/(m2 ·d)) −1.90∼0.28 (0.70) −52.70∼80.96 (14.70) −1.88∼6.67 (0.36) −1.22∼1.21 (0.22)

SO2− 4 (mmol/(m2 ·d)) −1.08∼0.63 (−0.39) −1.71∼1.35 (−0.43) −1.17∼0.36 (−0.36) −1.08∼0.35 (−0.33)

All data expressed as range (average)

0.8

TOC (%)

0.7 0.6 0.5 0.4 0.3 15

20

S

25

30

35

S( - II) ( m mol/L)

P Fig. 5.32. Relation of organic carbon (Corg ) and S(−II) of interstitial waters of the off-reef sediments in the Nansha Islands waters, South China Sea (Song and Li, 1996b)

suggested a new piece of evidence to show that S(−II) derived from SO2− 4 reduction (Song and Zhao, 2001). The pH value detected in Nansha Islands ranged from 7.28 to 8.67, and the Eh value was in the range of −242∼333 mV. SO2− 4 was the predominant content, accounting for 90%∼99.8% of all the sulfur. The reduction reaction (+6 valence) to S2− (−2 valence) under bacterial activities was a of SO2− 4 2− complex process, with the intermediate products of S2 O2− 5 (+4), SO3 (+4), 2− 2− S4 O5 (+2), S2 O3 (+2), and S (0) produced incidentally. In different situations, this reduction reaction would occur step by step, or skip some intermediate reaction steps to a lower valence product. It was assumed that sulfur in the pore water from Nansha Islands was reduced to S2− from elementary substance form sulfur, whose reduction reaction could be expressed as follows:

620


S + 2e → S2−

(5.33)

The calculated Eh based on the equation and the detected Eh values are shown in Table 5.40. The calculated Eh values were close to the corresponding detected Eh values. Therefore, S2− in the pore water from the Nansha Islands was dominated by the balance of the S and S2− transformation. Moreover, elementary substance form sulfur existed in the sediment from the Nansha Islands. However, elementary substance form sulfur was a kind of unstable substance and inclined to co-precipitate FeS to FeS2 , which had be improved in the sediments from Nansha Island in the previous literature (Song and Li, 1996b). Table 5.40. Comparison of Eh values calculated from S/S2− redox pair (Ehc ) and Eh values measured (Ehd ) (Song and Li, 1996b)

Ehc Ehd

Zhubi Banyue Ren’ai 93-6 93-7 93-8 93-6A 94-12 Reef Reef Reef −0.175 0.050 0.055 0.050 0.102 0.260 0.340 0.225 −0.119 0.010 0.025 0.060 0.061 0.203 0.327 0.163

94-16

94-23

0.210 0.196

0.235 0.292

References Bates NR, Takahashi T, Chipman DW, Knap AH (1998) Variability of PCO2 on diel to seasonal timescales in the Sargasso Sea. J Geophys Res 103(C8):1556715585 Bilger RW, Atkinson MJ (1995) Effects of nutrient loading on mass-transfer rates to a coral-reef community. Limnol Oceanogr 40(2):279-289 Cai PH, Huang YP (2002) Measurement of new production in Nansha Trough sea area by 228 Ra-NO3 method. Sci Chin (Ser D) 32(3):249-254 (in Chinese with English abstract) Callahan J, Dai M, Chen RF, Li X, Lu Z, Huang W (2004) Distribution of dissolved organic matter in the Pearl River Estuary, China. Mar Chem 89(1-4):211-224 Cao JJ, Lee SC, Ho KF, Zou SC, Fung K, Li Y, Watson JG, Chow JC (2004) Spatial and seasonal variations of atmospheric organic carbon and elemental carbon in Pearl River delta region, China. Atmos Environ 38(27):4447-4456 Chao SY, Shaw PT, Wu SY (1996a) El Ni˜ no modulation of the South China Sea circulation. Prog Oceanogr 38(1):51-93 Chao SY, Shaw PT, Wu SY (1996b) Deep water ventilation in the South China Sea. Deep Sea Res Part I 43(4):445-466 Chen CTA (1996) The Kuroshio intermediate water is the major source of nutrients on the East China Sea continental shelf. Oceanol Acta 19(5):523-527 Chen CTA, Huang MH (1995) Carbonate chemistry and the anthropogenic CO2 in the South China Sea. Acta Oceanol Sin 14(1):47-57 Chen CTA, Wang SL (1999) Carbon, alkalinity and nutrient budgets on the East China Sea continental shelf. J Geophys Res 104(C9):20675-20686

References

621

Chen CTA, Wang SL, Wang BJ, Pai SC (2001) Nutrient budgets for the South China Sea basin. Mar Chem 75(4):281-300 Chen SJ, Luo XJ, Mai BX, Fu JM, Sheng GY, Zeng EY (2006) Distribution and mass inventories of polycyclic aromatic hydrocarbons and organochlorine pesticides in sediments of the Pearl River Estuary and the northern South China Sea. Environ Sci Technol 40(3):709-714 Chen YLL (2005) Spatial and seasonal variations of nitrate-based new production and primary production in the South China Sea. Deep Sea Res Part I 5(2):319340 Chou WC, Shen DDD, Chen CTA, Wang SL, Tang CM (2005) Seasond variability of carbon chemistry at the SEATS time-series, northern South China Sea between 2002 and 2003. Terr Atmos Ocean Sci 16(2):445-465 Chu PC, Li R (2000) South China Sea isopycnal-surface circulation. J Phys Oceanogr 30(9):2419-2438 Collister JW, Rieley GR, Stern B, Eglinton G, Fry B (1994) Compound-specific δ 13 C analyses of leaf lipids from plants with differing carbon dioxide metabolisms. Org Geochem 21(6-7):619-627 Coynel A, Seyler P, Etcheber H, Meybeck M, Orange D (2005) Spatial and seasonal dynamics of total suspended sediment and organic carbon species in the Congo River. Glob Biogeochem Cycles 19(4):GB4019 Deines P (1980) The isotopic composition of reduced organic carbon. In: Fritz P, Fontes JC (eds.) Handbook of Environmental Isotopic Geochemistry. Elsevier Science, Amsterdam, pp.29-406 Doval MD, Pérez FF, Berdalet E (1999) Dissolved and particulate organic carbon and nitrogen in the Mediterranean. Deep Sea Res Part I 46(3):511-527 Duan Y, Wang ZP (2002) Evidence from carbon isotope measurements for biological origins of long-chain n-alkanes in sediments from Nansha Sea, China. Chin Sci Bull 47(7):578-581 Duan Y, Cui MZ, Ma LH, Song JM, Zhou SX, Luo BJ (1997) Organic geochemical studies of sinking particulate material in China sea area (II) Geochemical significance of compositional features of ketone, aldehyde and alcohol compounds. Chin Sci Bull 42(22):1894-1898 Duan Y, Song JM, Cui MZ, Luo BJ (1998) Organic geochemical studies of sinking particulate material in China sea area (I) Organic matter fluxes and distributional features of hydrocarbon compounds and fatty acids. Sci Chin (Ser D) 41(2):208-214 Duan Y, Song JM, Zhang H (2004) Carbon isotopic studies of individual lipids in organisms from the Nansha sea area, China. Sci Chin (Ser D) 47(7):593-598 Fang GH, Wei ZX (2002) The South China Sea branch of the Indo-Pacific through flow and its oceanographic implication. In: Proceedings of the 5th Cross-Strait Conference on Oceanography of Water of Taiwan Island and Its Adjacent Areas, pp.1-210 Friis K, Kfrtzinger A, Wallace DWR (2003) The salinity normalization of marine inorganic carbon chemistry data. Geophys Res Lett 30(2):1085 Gan HH, Wu SH, Fan XD (2003) Soil organic carbon reserves and characteristic of spatial distribution in Guangdong Province. Chin J Appl Ecol 14(9):1499-1502 (in Chinese with English abstract) Gan ZJ, Cai SQ (2001) Geographical and seasonal variability of Rossby radii in South China Sea. J Trop Oceanogr 20(1):1-9 (in Chinese with English abstract)

622


Gao RZ, Zhou FX, Wang DX (2002) Multi-scale variability of subsurface temperature in the South China Sea. Acta Oceanol Sin 21(2):165-173 Gao XL, Chen SY, Xie XL, Long AM, Ma FJ (2007) Non-aromatic hydrocarbons in surface sediments near the Pearl River Estuary in the South China Sea. Environ Pollut 148(1):40-47 Gao XL, Chen SY, Long AM (2008) Composition and sources of organic matter and its solvent extractable components in surface sediments of a bay under serious anthropogenic influences: Daya Bay, China. Mar Pollut Bull 56(6):10661075 Gismondi MM, Giani M, Savelli F, Boldrin A, Rabitti S (2002) Particulate matter in northern and central Adriatic. Chem Ecol 18(1):27-38 Hallock P (2005) Global change and modern coral reefs: New opportunities to understand shallow-water carbonate depositional processes. Sediment Geol 175(14):19-33 Han WY (1998) Chemical Oceanography in the South China Sea. Science Press, Beijing, pp.30-40 Hansell DA, Peltzer ET (1998) Spatial and temporal variations of total organic carbon in the Arabian Sea. Deep Sea Res Part II 45(10-11):2171-2193 He Y, Guan C, Gan Z (1997) Interannual and interdecadal variations in heat content of the upper ocean of the South China Sea. Adv Atmos Sci 14(2):271276 Hinz K, Schluter HU (1985) Geology of the dangerous grounds, South China Sea, and the continental margin off Southwest Palawan: Results of SONNE cruises SO-23 and SO-27. Energy Citations Database 10(3-4):297-315 Hu J, Peng P, Jia G, Mai B, Zhang G (2006) Distribution and sources of organic carbon, nitrogen and their isotopes in sediments of the subtropical Pearl River Estuary and adjacent shelf, southern China. Mar Chem 98(2-4):274-285 Huang JJ, Wang SM, Chen YL (2007) Biogeochemical controls on distributions and flux of dissolved and particulate organic carbon in the northern South China Sea. Deep Sea Res Part II 54(14-15):1486-1503 Huang LM (1991) Studies of the distributional features of photopigment and primary productivity in Nansha Islands sea area. In: Paper Volume on the Studies of Organisms in Nansha Islands and Adjacent Sea Area. Scientific and Technological Publishing House, Wuhan, pp.34-49 Huang LM, Chen QC (1997) Distribution of chlorophyll a and primary production of Nansha Islands sea area in winter. In: Study on Ecological Processes of Nansha Islands Sea Area I. Science Press, Beijing, pp.102-115 (in Chinese) Huang XP, Huang LM, Yue, WZ (2003) The characteristics of nutrients and eutrophication in the Pearl River Estuary, South China. Mar Pollut Bull 47(16):30-36 Hung JJ, Chen CH, Gong GC, Sheu DD, Shiah FK (2003) Distributions, stoichiometric patterns and cross-shelf exports of dissolved organic matter in the East China Sea. Deep Sea Res Part II 50(6-7):1127-1145 Ingall E, Jahnke R (1994) Evidence for exchange phosphorus regeneration from marine sediment overlain by oxygen deplete water. Geochim Cosmochim Acta 58(11):2571-2575 ¨ Axelman J, Sundberg H (2003) Global accounting of J¨ onsson A, Gustafsson O, PCBs in the continental shelf sediments. Environ Sci Technol 37(2):245-255

References

623

Kuo NJ, Zheng QA, Ho CR (2000) Satellite observation of upwelling along the western coast of the South China Sea. Remote Sens Environ 74(3):463-470 Laws EA, Popp BN, Bidigare RR (1997) Effect of growth rate and CO2 concentration on carbon isotopic fractionation by the marine diatom Phaeodactylum tricornutum. Limnol Oceanogr 42(7):1552-1560 Li L, Su JL, Xu JP (1997) Detached Kuroshio rings in the South China Sea. J Trop Oceanogr 16(2):42-57 (in Chinese with English abstract) Li XL (2000) Research Report on Pollutants Flux in Pearl River Estuary from East Four Channels and the Influence on Water Quality. Guangdong Science and Technology Press, Guangzhou, pp.28-36 (in Chinese) Liss PS, Merlivat L (1986) Air-sea gas exchange rates: Introduction and synthesis. In: Buat-Menard P (ed.) The Role of Air-Sea Exchange in Geochemical Cycling. NATO ASI Ser C: Math Phys Sci 185:113-128 Liu ZY, Yang HJ, Liu QY (2001) Regional dynamics of seasonal variability in the South China Sea. J Phys Oceanogr 31(1):272-284 Long AM, Chen SY, Zhou WH, Xu JR, Sun CC, Zhang FQ, Xu HZ, Zhang JL (2006) Distribution of macro-nutrients, dissolved oxygen, pH and Chl a and their relationships in northern South China Sea. Mar Sci Bull 25(5):9-16 Monson KD, Hayes JM (1982) Carbon isotopic fractionation in the biosynthesis of bacterial fatty acid: Ozonolysis of unsaturated as a means of determining the intramolecular distribution of carbon isotopes. Geochim Cosmochim Acta 46(2):139-149 Morton B, Blackmore G (2001) South China Sea. Mar Pollut Bull 42(12):1236-1263 Muscatine L, Porter JW (1977) Reef corals: Mutualistic symbioses adapted to nutrient-poor environments. Bioscience 27(7):454-460 Ni HG, Lu FH, Luo XL, Tian HY, Zeng EY (2008) Riverine inputs of total organic carbon and suspended particulate matter from the Pearl River delta to the coastal ocean off South China. Mar Pollut Bull 56(6):1150-1157 Onstad GD, Canfield DE, Quay PD, Hedges JI (2000) Sources of particulate organic matter in rivers from the continental USA: Lignin phenol and stable carbon isotope compositions. Geochim Cosmochim Acta 64(20):3539-3546 Peng Z, Chen T, Nei B, Head MJ, He X, Zhou W (2003) Coral 18 O records as an indicator of winter monsoon intensity in the South China Sea. Quat Res 59(3):285-292 Popp BN, Laws EA, Bidigare RR, Dore JE, Hanson KL, Wakeham SG (1998) Effect of phytoplankton cell geometry on carbon isotopic fractionation. Geochim Cosmochim Acta 62(1):69-77 Qin YC, Weng HX (2006) Silicon release and its speciation distribution in the surficial sediments of the Pearl River Estuary, China. Estuar Coast Shelf Sci 67(3):433-440 Qu T, Mitsudera H, Yamagata T (2000) Intrusion of the North Pacific waters into the South China Sea. J Geophys Res 105(C3):6415-6424 Richey JE, Devol AH, Wofsy SC, Victoria R, Riberio MNG (1988) Biogenic gases and the oxidation and reduction of carbon in Amazon River and floodplain waters. Limnol Oceanogr 33(4):551-561 Richey JE, Melack JM, Aufdenkampe AK, Ballester VM, Hess LL (2002) Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2 . Nature 416(6881):617-620

624


Risatti JB, Rowland SJ, Yon D (1984) Stereochemica studies of acyclic isoprenoids. VII. Lipids of methanogenic bacteria and possible contributions to sediments. In: Schenck, PA (ed.) Advance in Organic Geochemistry. Pergamon Press, Oxford, pp.93-103 Schouten S, Breteler WCMK, Blokker P, Schogt N, Rijpstra WIC, Grice K, Baas M, Damste JSS (1998) Biosynthetic effects on the stable carbon isotopic composition of algal lipids: Implications for deciphering the carbon isotopic biomarker record. Geochim Cosmochim Acta 62(8):1397-1406 Shaw PT, Chao SY, Fu LL (1999) Sea surface height variations in the South China Sea from satellite altimetry. Acta Oceanol Sin 22(1):1-17 Simoneit BRT (1977) Diterpenoid compounds and other lipids in deep sea sediments and their geochemical significance. Geochim Cosmochim Acta 41(4):463476 Smith SV, Kinsey DW (1976) Calcium carbonate production, coral reef growth, and sea level change. Science 194(4268):937-939 Song JM (1997a) Biogeochemical process of major elements in sinking particulate of Nansha coral reef lagoons, South China Sea. Acta Oceanol Sin 16(4):557-562 Song JM (1997b) Chemistry of Sediment-Seawater Interface of the China Seas. China Ocean Press, Beijing, pp.1-222 (in Chinese) Song JM (1999) The new viewpoint of the high productivity supporting Nansha coral reef ecosystem-sediment drift-net theory. Stud Mar Sin 41:79-85 (in Chinese with English abstract) Song JM (2000) Marine Chemistry in China. China Ocean Press, Beijing, pp.1-210 (in Chinese) Song JM (2004) Biogeochemistry of China Seas. Shandong Science and Technology Press, Jinan, pp.1-591 (in Chinese) Song JM, Li PC (1996a) Iron and manganese in interstitial waters and sediment environments of Nansha Islands, South China Sea. Acta Sci Circum 16(3):294301 (in Chinese with English abstract) Song JM, Li PC (1996b) –2 valance sulfur in sediment interstitial waters of Nansha Islands waters. Oceanol Limnol Sin 27(6):642 (in Chinese with English abstract) Song JM, Li PC (1996c) Studies on characteristics of nutrient diffusion fluxes across sediment-water interface in the district of Nansha islands, South China Sea. Mar Sci 20(5):43-50 (in Chinese with English abstract) Song JM, Li PC (1998a) Vertical transferring process of rare earth elements in coral reef ecosystem of Nansha Islands waters, South China Sea. Stud Mar Sin 40:125-130 (in Chinese with English abstract) Song JM, Li PC (1998b) Vertical transferring process of rare elements in coral reef lagoons of Nansha Islands, South China Sea. Sci Chin (Ser D) 41(1):42-48 (in Chinese with English abstract) Song JM, Zhao WD (2001) A new biogenic sulfide chemical sensor for marine environmental monitoring and survey. Chin J Oceanol Limnol 19(3):282-286 Song JM, Ma HB, L¨ u XX (2002) Nitrogen forms and decomposition of organic carbon in the southern Bohai Sea core sediments. Acta Oceanol Sin 21(1):125133 (in Chinese with English abstract) Song JM, Xu YF, Hu WP, Ni LY (2008) Biogeochemistry of Carbon in China Seas and Lakes. Science Press, Beijing, pp.1-553 (in Chinese)

References

625

Summons RE, Jahnke LL, Roksandic Z (1994) Carbon isotopic fractionation in lipids from methanotrophic bacteria: Relevance for interpretation of the geochemical record of biomarkers. Geochim Cosmochim Acta 58(13):2853-2863 Sun DH, Gagan MK, Cheng H, Scott-Gagan H, Dykoski CA, Edwards RL, Su RX (2005) Seasonal and interannual variability of the mid-Holocene East Asian monsoon in coral 18 O records from the South China Sea. Earth Planet Sci Lett 237(1-2):69-84 Tans PP, Fung IY, Takahashi T (1990) Observational constraints on the global atmospheric CO2 budget. Science 247(4949):1431-1438 Taylor B, Hayes DE (1980) The tectonic evolution of the South China Sea Basin. In: Hayes DE (ed.) The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands. Geophys. Monogr Ser, Vol. 23. AGU, Washington, DC, pp.110-123 Thompson PA, Calvert SE (1994) Carbon isotopic fractionation by a marine diatom: The influence of irradiance, daylength, pH, and nitrogen source. Limnol Oceanogr 39(8):1835-1844 Tolosa I, Bayona JM, Albaigés J (1996) Aliphatic and polycyclic aromatic hydrocarbons and sulfur/oxygen derivatives in northwestern Mediterranean sediments: Spatial and temporal variability, fluxes, and budgets. Environ Sci Technol 30(8):2495-2503 van Dongen BE, Schouten S, Sinninghe, Damsté JS (2002) Carbon isotopic variability in monosaccharides and lipids of aquatic algae and terrestrial plants. Mar Ecol Prog Ser 232(3-4):83-92 Volkman JK, Johns RB, Gillan FT (1980) Microbial lipids of an intertidal sediment: I. fatty acids and hydrocarbons. Geochim Cosmochim Acta 44(8):11331143 Wang DX, Xie Q, Du Y, Wang WQ, Chen J (2002) The 1997-1998 Warm Event in the South China Sea. Chin Sci Bull 47(9):711-716 Wang WQ, Wang DX, Qi YQ (2000) Large scale characteristics of interannual variability of sea surface temperature in the South China Sea. Acta Oceanol Sin 22(4):8-16 (in Chinese with English abstract) Wang WQ, Wang DX, Shi P (2001) Annual and interannual variations of largescale dynamic field in South China Sea. J Trop Oceanogr 20(1):61-68 (in Chinese with English abstract) Wanninkhof R (1992) Relationship between wind speed and gas exchange over the ocean. J Geophys Res 97(C5):7373-7382 Wen WY, Huang XP, Zhang GX (1999) Discussion on water pollution across multiregional in the Pearl River Estuary. In: Lin JZ, Zhu T, Hu EB (eds.) Proceedings of the China Mainland-Hong Kong Symposium on Regional Environmental Impact Assessment. Open University of Hong Kong Press, Hong Kong, China, pp.471-478 (in Chinese) Wu CR, Shaw PT, Chao SY (1998) Seasonal and interannual variations in the velocity field of the South Sea. J Oceanogr 54(4):361-372 Yang HJ (2000) The Study on the Temporal-spatial Structure and Mechanism of the Circulation in the South China Sea. PhD Thesis, Ocean University of Qingdao, China, pp.14-21 (in Chinese) Yu HB, Xu XZ (1994) A preliminary analysis of tide and sea current features in Yongshu Reef and Zhubi Reef. In: Proceedings of Geophysics and Reef Geography in Nansha Islands and Neighbouring Waters. Science Press, Beijing, p.89 (in Chinese)

626


Yu KF, Chen TG, Huang DC, Zhao HT, Zhong JL, Liu DS (2001) The highresolution climate recorded in the delta O-18 of Porites lutea from the Nansha Islands of China. Chin Sci Bull 46(24):2097-2102 Yu KF, Zhao JX, Xei GL, Cheng XR, Chen TG, Felise T, Wang PX, Liu TS (2005) δ 18 O, Sr/Ca and Mg/Ca records of Porites lutea corals from Leizhou Peninsula, northern South China Sea, and their applicability as paleoclimatic indicators. Palaeogeogr Palaeoclimatol Palaeoecol 218(1-2):57-73 Yu KF, Zhao JX, Wang PX, Shi Q, Meng QS, Collerson DK, Liu TS (2006) Highprecision TIMS U-series and AMS 14 C dating of a coral reef lagoon sediment core from southern South China Sea. Quatern Sci Rev 25(17-18):2420-2430 Yuan DL (2002) Dynamics of the Luzon Strait transports. Acta Oceanol Sin 21(2):175-185 Zhai WD, Dai MH, Cai WJ, Wang YC, Wang ZH (2005) High partial pressure of CO2 and its maintaining mechanism in a subtropical estuary: The Pearl River Estuary, China. Mar Chem 93(1):21-32 Zhao YY, Yan MC (1994) Geochemistry of sediments of the China shelf sea. Science Press, Beijing, pp.50-155 (in Chinese) Zheng GX, Song JM, Sun YM, Dai JC, Zhang P (2008) Characteristics of nitrogen forms in the surface sediments of southwestern Nansha Trough, South China Sea. Chin J Oceanol Limnol 26(3):280-288 Zhu YZ, Sha QA, Guo LF, Yu KF, Zhao HT (1997) Cenozoic Coral Reef Geology of Yongshu Reef. Nansha Islands. Chinese Science Press, Beijing, pp.1-134 (in Chinese)

6 Prospects for Marine Biogeochemical Process Research in China

Abstract: In this chapter the main key biogeochemical processes in China marginal seas, the methods, concepts, and focus on marine biogeochemical process research in China are presented. Then we put forward the prospects for biogeochemistry in China marginal seas. Marine biogeochemical process research in China has made progress in the past 20 years. The research methods and concepts of marine biogeochemical processes have been established. Some key processes, including river input, biological action, and others, have been illustrated to some degree (Song, 2004). These processes play an important part in biogenic elements and other chemical components cycling.

6.1 Marine Biogeochemical Process Research in China The methods and concepts of marine biogeochemical processes are very important although our knowledge of marine biogeochemical processes in the marginal seas is still sketchy and we do not yet have a clear outline sketch of the marginal seas’ biogeochemical processes. Therefore, it is necessary to summarize the methods and concepts of marine biogeochemical processes. 6.1.1 Introduction Biogeochemistry is the study of the interactions of the biology, chemistry, and geology of the Earth. In the case of a large body of water such as the ocean, biogeochemistry can be thought of as a huge experiment or a set of reactions. Instead of happening in a clean glass beaker, the reactions have the ocean floor as the container. Oceans cover 71% of the Earth’s surface area and contain more than 97% of all water on the planet. Their capacity to store and redistribute heat and water around the globe is of profound importance in maintaining the Earth’s environment.

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Marine biogeochemistry is challenging because chemical concentrations and transformation rates are determined by complex combinations of transportation and reaction. To a first approximation, the chemical structure of the oceans is determined by the ubiquitous sinking of biogenic particles and local upwelling of solutes regenerated from the chemical and biological dissolution of such particles, but the details are far more intriguing than what this broad-brush summary can reveal. The distribution and circulation of biogenic elements such as carbon (C), nitrogen (N), phosphorus (P), silicon (Si), and sulfur (S) in the ocean are regulated by both physical transportation processes and biochemical transformation by various organisms. These elements may occur in volatile, dissolved, or particulate forms, and thus their biogeochemical cycles in the ocean are closely linked with those in the atmosphere and the lithosphere (Song, 1997; 2000a). Because of its large capacity, the ocean plays a crucial role in maintaining the global cycles and balance of these elements. The upper ocean is the interface between the deep ocean, the atmosphere and terrestrial components of the planet. It plays a crucial role in the regulation of the global environment. The surface water is an essential component in the “conveyor belt” of waters flowing through the ocean basins and is also a major reservoir of life on the planet. Associated exchanges of gases, organic matter biominerals, and trace elements between the atmosphere and deep ocean have profound effects on the global cycles of many important elements. Marine snow is a critical component of marine biogeochemical cycles because it transports organic and inorganic material from the water column to the ocean floor. The oceanographers use bongo nets to collect the falling debris, which is faintly visible and somewhat resembles falling snow on land (Song, 1997). The surface of the water is open to the air, and every day more dust and dirt from land blows over the ocean and falls in. Moreover, the surface of the water contains many small plant forms that are continually growing and being consumed by animals that are themselves consumed by other animals. As this life and death drama continues, the scraps and leftovers drift downward towards the ocean floor like a snowfall, hence the name “marine snow”. Around the edges of the ocean, rivers empty water and sediment to the ocean, and mud-dwelling creatures wait for the arrival of their next meal from the falling biological debris (marine snow). These events are linked to each other, to the history of life on the Earth, and to variations in the Earth’s climate. Scientists who study biogeochemistry usually consider the cycling of materials through the different parts of the system. To do this, they deal with reservoirs of materials and the fluxes of a substance from one reservoir to another. For example, they examine reservoirs such as the surface ocean water versus the deep ocean water, or the transfer of masses of materials per unit time (fluxes). An example of this kind of approach to biogeochemical cycles in the ocean can be seen in the Joint Global Ocean Flux Study (JGOFS) results, where the reservoirs represented are the atmosphere, lithosphere, terrestrial (land-based) biosphere, surface ocean, phytoplankton, and deep ocean. The

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JGOFS results show the global carbon cycle, a network of interrelated processes that transport carbon between different reservoirs on the Earth. Most scientific studies have focused on the carbon cycle. Carbon, after all, is the basis of life on the Earth, and its gaseous form, carbon dioxide, is linked to the greenhouse effect and changes in the Earth’s climate over time. For these reasons, understanding the carbon cycle has been the focus of several large research programs such as the JGOFS, GLOBal ocean ECosystems dynamics (GLOBEC), Surface Ocean-Lower Atmosphere Study (SOLAS), Integrated Marine Biogeochemistry and Ecosystem Research (IMBER), and Global Carbon Program (GCP). The biogeochemical processes in the oceans, such as CO2 uptake and release, exchange of biogenic trace gases, fluxes of particulate and dissolved organic carbon to different depths, variable bacterial activities, and phytoplankton blooms, are critical to the global biogeochemical cycling. The extreme heterogeneity, a characteristic of the coastal zone, which harbors more than 50% of the human population, influences the carbon cycling and carbon storage on a global scale. Human-driven changes in nutrient availability are known to increase the frequency of toxic algal blooms, development of hypoxia and anoxia, and changes in biomass and productivity. Bays are of particular interest for understanding the linkages between land and sea, as they are heavily impacted by human activity. Since the dynamics of the marine ecosystems are closely related to climate variability, changes in climate are bound to have a significant effect on marine ecosystems. For example, an increasing load of CO2 in the atmosphere will lead to increased CO2 concentration in the upper layer and consequently will change the carbonate chemistry, affecting adversely the reef organisms. Also, changes in temperature and circulation patterns will affect the geographical distribution of fishes, their prey, and their predators. Other substances also have well-studied cycles. Water, of course, is constantly moving into, through, and out of the ocean. Some of the atmospheric gases such as oxygen and carbon dioxide are vitally important to life. Nutrient elements such as nitrogen, phosphorus, and silicon are necessary to the phytoplankton, and form the basis of the oceanic food web. The presence of life forms on the Earth is tremendously important in the cycling of elements through the major reservoirs. Take the ocean as an example. If one focuses on the impact of a single diatom on the ocean, the following story emerges. Diatoms are a group of algae living by the millions in each cubic centimeter of surface ocean water. In this water each alga has access to the sunlight needed for photosynthesis, the CO2 (carbon dioxide), N (nitrogen), and P (phosphorus) needed to make its soft tissue, the Si (silicon) needed for its shell-like covering, and a number of rare or trace substances in sea water, including Cu (copper) and Fe (iron). To reproduce, it undergoes cell division. Its life processes produce O2 (oxygen) that can be used by other organisms, organic tissue that becomes food for the next higher creatures in the food web, and often an exudate or slime.

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Once the diatom has been consumed by an animal (a copepod, for example), its life is over, but its effect on the ocean is not. The copepod digests and derives energy from the diatom’s soft tissue, and then packages the remains into a fecal pellet that is discharged as waste to become part of the falling debris (marine snow) headed for the ocean floor. The pellet lands on the ocean floor, forming a site for bacteria to live as well as food for them to consume. The inorganic part of the diatom that remains (the silicon shell) will begin to dissolve on the way to the ocean bottom, and Si taken out of the surface water is returned to deeper water as the shell dissolves. Decomposition of sinking organic matter by bacteria returns N, C, and P to the water and removes dissolved O2 . Ocean water itself is changed by life processes. During the growth of diatoms and the consumption of diatoms by zooplankton, carbon is removed from ocean water and in turn from the atmosphere as the diatoms use it to grow. The transfer of this carbon towards the ocean floor and its partial burial in the sediments are often referred to as the carbon pump; it is one of the processes that slow the accumulation of CO2 in the atmosphere (Song, 2000b). The silicon (Si) used in the diatom shell enters the ocean from rivers, from the hot springs along mid-ocean ridges and by diffusion from deep-sea sediments. Diatoms remove Si so efficiently from the ocean surface water that it is a very scarce element there, and mixing and upwelling processes are necessary to redistribute enough Si back to the surface to provide vertical profiles that illustrate the changing concentration with depth. As common constituents in the ocean, silicon dioxide (SiO2 ), nitrogen as nitrate (NO− 3 ), phosphorus as phosphate (PO3− 4 ), and oxygen (O2 ) are important for diatom growth and are largely consumed. For that reason, Si, N, P, and other biologically important elements are in low concentrations in the surface water of the ocean, and increase with depth. Another consequence of ocean biogeochemistry can be seen in the distribution of O2 with depth. The oxygen content at the surface is relatively high (about 6 ml/L) and is replenished from the air. Deeper in the water, the O2 content begins to decrease with depth, until at about 1,000 m, the value reaches a minimum. The reason for the decrease is the consumption by bacteria of the rain of organic debris (marine snow) falling through the water. The process requires O2 , and below the surface there is no immediate source to return the O2 being used up. The exact amount of O2 at the O2 minimum varies with location in the ocean; below the minimum, O2 content begins to increase again with depth (Song, 2004). The increase is related to water circulation in the ocean. The deep water in the ocean starts out at the surface in polar regions, where it becomes very dense because of the extreme cold, and sinks to great depths in the ocean, carrying with it dissolved oxygen from the surface waters. This cold, dense, deep water flows along the ocean floor close to the bottom, well beneath the depths of the O2 minimum. These factors combine to give the observed shapes of O2 profiles in the ocean.


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There are other processes that play a role in determining the nature of the ocean. For example, hydrothermal activity at mid-ocean ridges results in significant changes in the chemistry of ocean water. The water that comes out of these hot springs comes from normal deep-ocean water that runs down into deep cracks on the ocean floor alongside the ridges. As the water penetrates into the oceanic crust, it becomes heated to very high temperatures, and reacts with the rocks. The water that comes out of the vents is very hot. It contains sulfide (S2− ) instead of sulfate (SO2− 4 ), contains no Mg (magnesium) or O2 (oxygen), and contains large amounts of Si (silicon). Because the entire volume of the ocean circulates through the mid-ocean ridge system every 10 million years, these changes are of great significance to the oceans and the organisms that live in them. 6.1.2 Focus on Marine Biogeochemical Process Research in China The biogeochemical study of biogenic elements in China marginal seas started relatively late in the world. However, there has been much progress in recent decades (Song, 2004). Research into ecosystem dynamics in the Bohai Sea, particulate fluxes of the East China Sea, biogeochemical processes of biogenic elements in the Taiwan Strait, carbon fluxes in the South China Sea, and the coral reef ecosystem in the Nansha Islands, has pushed forward marine biogeochemical studies in China (Song, 2000a). 6.1.2.1 Biogeochemical Research on Biogenic Elements in China Marginal Seas Biogenic elements include carbon, nitrogen, phosphorus, silicon, oxygen and sulfur, and so on. They have a very close relationship with organisms in the sea, and have always been the focus of study in the biogeochemistry of China marginal seas. With the implementation of global and national biogeochemical programs, the biogeochemistry of biogenic elements in China marginal seas has made much progress over the last 20 years. (1) Carbon biogeochemistry At present, research on the intensity of the carbon source/sink of China marginal seas mainly focuses on the Yellow Sea and the East China Sea. Because of the different area and methods used in the study, different researchers have different results. However, all of the Bohai Sea, the Yellow Sea, the East China Sea, and the South China Sea are the sink of CO2 , and they can absorb 284×104 , 1,665×104 , 896×104 , and 188×104 t/yr carbon from the atmosphere, respectively. The intensity of the carbon source/sink of China marginal seas has the characteristic of great seasonal variation. In spring and winter, all of the Bohai Sea, the Yellow Sea, and the East China Sea are the sink of CO2 ; these three seas altogether can absorb 769×104 and 1,356×104 t carbon in spring and winter, respectively. In summer, all of China marginal

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seas are the source of CO2 to the atmosphere, and they can release 459×104 t carbon. In autumn, the Bohai Sea and the North Yellow Sea are the sink of CO2 , and they can absorb 27×104 t carbon from the atmosphere. However, the South Yellow Sea and the East China Sea are the source of CO2 in autumn, and they can release 324×104 t carbon to the atmosphere (Song et al., 2008). Thus, all of eastern China marginal seas release CO2 to the atmosphere in autumn. In China, the cycling of organic carbon in the open ocean has received a lot of attention over the last two decades. Organic carbon profoundly influences the marine biogeochemical cycle of carbon. Organic carbon is mainly composed of dissolved organic carbon (DOC) and particulate organic carbon (POC). DOC and POC represent 89% and 11% of the total organic carbon in the ocean, respectively. The average DOC inventory was about 10 times that of the POC in the southern East China Sea, and DOC represented the major portion of total organic carbon (80%∼95%) in the northern South China Sea. In the seawater DOC mainly comes from decomposition products of organisms, excrement during metabolism and terrestrial input. It can be transformed into POC through biological activities, and plays an important role in the vertical transportation of carbon. The concentrations of DOC in China marginal seas are higher in coastal waters than in the open seas. The mean DOC concentration in the surface water of the South Yellow Sea was the highest in China marginal seas with a value of 2.02 mg/L. DOC concentrations were relatively high (>85 μmol/L) in inner shelf and slope waters but low (ca. 65 μmol/L) around the shelf break in the East China Sea (Hung et al., 2000; Song et al., 2008). The DOC concentration in the South Yellow Sea was characterized by low concentration in the central offshore area, which was controlled mainly by the dilution effect of a low DOC current from the East China Sea. POC comprises living and non-living POC. Living POC is composed of micro-phytoplankton, bacteria, fungus, zooplankton, small fish and shrimp, and so on; non-living POC, i.e., organic detritus, mainly comes from biological activities. The proportions of living and non-living POC in the total organic carbon have been investigated in China marginal seas. Living and non-living POC represent about 2% and 9% of the total organic carbon in the ocean, respectively. In the East China Sea, living POC occupies 10% of the total organic carbon in spring, while in autumn it occupies only 4% of the total organic carbon (Liu et al., 1997). Vertical transportation of POC is a very important process. Different parts of China marginal seas have different concentrations of POC in the water, different sedimentation rates, and different hydrodynamic environments. Thus, characteristics of transportation of POC vary among different parts of China marginal seas. In the Yellow Sea, the bottom settling particulate matter comes mainly from the resuspension of sediments with the resuspension rate up to 90%∼96% (Song, 2004; Zhang et al., 2004). In the northern South China Sea, most POC is dissolved before being buried in sediments; organic carbon in sediments mainly comes from lateral transition and the contribution of verti-


633

cal transition is limited. In the East China Sea, the POC export ratios were about 48%∼77% and 15%∼21% in the upwelling zone and the middle continental shelf, respectively; the resuspension ratios in the bottom layer averaged 66.50% and 88.52% at the stations in the upwelling zone and the middle continental shelf, respectively (Zhang YS et al., 2006). The downward fluxes of total organic carbon were nearly balanced by new productivity derived from upward fluxes of nutrients in the northern South China Sea. In addition, the net export of POC from the East China Sea shelf was estimated to be 106 Gmol C/yr (Hung et al., 2000), and the shelf export of POC was equivalent to Hg>Zn>Cu>Cd; then the dissolved heavy metals were transformed to particulate forms by biological activities. After 27 d, Cd and Cu existed mostly in the dissolved form, most of Pb and Hg were transported to the bottom, and about half of Zn was transported to the bottom. The release of heavy metals from the sediments was determined by both the nature of the metals themselves and the environmental conditions. An anoxic environment is beneficial to the release of heavy metals. In an oxidizing environment, release fluxes of Cu and Zn were 1∼2 order(s) of magnitude higher than those of Cd and Pb. There has been much research on organic carbon metabolism, suspended particulate materials, and the grazing pressure of zooplankton on bacteria in a shrimp cultural enclosure ecosystem. In the shrimp cultural enclosure ecosystem, about 35% of the total organic carbon income was mineralized through the process of plankton community metabolism during the cultural period (Liu et al., 2002). Particulate organic matter occupies 62% of the total suspended particulate materials. The percentage of humic substances in particulate organic matter can reach up to 98%, and the remaining 2% are plankton among which the biomass of phytoplankton is 4.5 times larger than that of zooplankton (Liu et al., 1999). Bacteria can be preyed upon by zooplankton through the microbial loop, which is a very important part of the biological pump. The grazing pressure of zooplankton can reach up to 73%∼175% of the total biomass of bacteria. The feeding amount of zooplankton (

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