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渤海底层低氧区的空间特征与形成机制
Spatial characteristics and formation mechanisms of bottom hypoxia zone in the Bohai Sea during summer

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张华 *   李艳芳   唐诚   邹涛   于靖   郭凯  
文摘 近海低氧区的发生是影响海洋生态系统健康的重要因素,也是海洋科学的研究热点.通过对2014年渤海夏季(8月末)航次观测数据的深入分析及与春(4月末)、秋(11月)两季观测的对比,研究了半封闭的渤海底层低氧区的分布特征、形成机制、及其与温盐结构的关系.结果表明,春季和秋季渤海底部溶解氧(DO)>8 mg/L,但在夏季出现大范围底部DO低值区,其中DO<3 mg/L的低氧区总面积约为4.2×10~3 km~2,具有南、北“双核”结构,空间特征与双中心冷水结构基本一致.渤海中部断面分析表明渤海浅滩南北两侧洼地在夏季出现明显垂向分层,导致底部冷水团DO含量下降与明显酸化(pH<7.8).冷水区稳定的沉积环境使其表层沉积物总有机碳(TOC)和总有机氮(TON)含量明显高于其他海区.对数据综合分析表明,渤海中部海水季节性层化及其对溶氧的阻滞作用是低氧区产生的关键物理机制,沉积物中累积的有机质在夏季的矿化分解是产生底部低氧和酸化环境的重要原因.该低氧区的产生是渤海生态系统剧变的结果和集中体现,与赤潮等生态灾害频繁发生和渔业资源衰退可能存在密切关系.
其他语种文摘 Formation of hypoxia zone in coast seas is a critical factor influencing the health of marine ecosystem and has becoming a focus of ocean science. The Bohai Sea in northeastern China is a semi-enclosed shallow sea receiving tremendous amount of terrestrial input of nutrients and sediments from several large rivers as well as coastal municipalities and has suffered serious eutrophication in the last two decades. Seasonal hypoxia has been reported on some locations in the Bohai Sea but relative few studies have been carried out to reveal its physical and biological mechanisms. Based on analysis of observation data from cruises conducted in the Bohai Sea in spring (May), summer (August), and autumn (November) of year 2014, this study investigated the spatial characteristics, formation mechanisms of bottom hypoxia zone in the Bohai Sea and its relationship with the seasonal thermocline. The results demonstrated that bottom dissolved oxygen (DO) was > 8 mg/L in Spring and Autumn of the Bohai Sea, but a large zone with low bottom DO appeared in Summer, among which are hypoxia zones (DO<3 mg/L) with a total area approximately 4.2×10~3 km~2. The hypoxia zone displayed a south-north “dual core” structure and the spatial characteristic is similar to that of the dual-core structure of cold bottom water in summer. The vertical profile of the cross section at the middle of Bohai Sea exhibited clear vertical stratification at the south and north depression basin alongside the ridge in the central Bohai Sea, which caused the decreasing DO and substantial acidification (pH<7.8) of bottom cold water. Regression analysis showed clear positive correlation between DO and water temperature (r~2=0.91), DO and pH (r~2=0.93), as well as DO and chlorophyll a(r~2=0.70). The results revealed that inter-annual stratification hinders vertical exchange of oxygen and is the predominant physical mechanism responsible for the depletion of DO in bottom sea water. To determine chemical mechanisms related to hypoxia, content of total organic carbon (TOC) and total organic nitrogen (TON) in surface sediment (0-5 cm) was analyzed at 99 locations. It revealed that the relatively stable sedimentation environment in the bottom cold water zone has resulted in the substantial higher content of TOC and TON in the surface sediment at the south and north depression basin than other areas. Measured ratios of C/N were in the range of 5-10.5, which indicates that organic matter were from both terrestrial and marine sources. The elevated level of organic matter content in these areas might enhance microbial activities and result in higher oxygen consumption rate in bottom sea water. In addition, mineralization of organic matter can release carbonates and contribute to severe acidification observed in bottom sea water. It is suggested that enhanced terrestrial loading of nitrogen and phosphorus from rivers and coastal cities might be the trigger of hypoxia in the Bohai Sea. In conclusion, the seasonal stratification in central Bohai Sea and its retardation effect on exchange of DO is the critical physical mechanism for the formation of hypoxia zone. The mineralization of organic matter accumulated in surface sediment is an important driving force causing the low bottom DO and substantial acidification. Appearance of hypoxia zone reflects the dramatic change of the ecosystem in the Bohai Sea and might closely linked with the frequent hazardous algae bloom (HAB) and degradation of fisheries. However, a lack of systematically collected data has impeded the establishment of a clear relationship between eutrophication, spring phytoplankton bloom, summer hypoxia, and continuous degradation of benthic and aquatic ecosystem. Further interdisciplinary studies are warranted to elucidate the coupled physical, chemical, and biological processes that drive the depletion of oxygen and to develop solutions to facilitate the ecological recovery in the Bohai Sea.
来源 科学通报 ,2016,61(14):1612-1620 【核心库】
DOI 10.1360/N972015-00915
关键词 渤海 ; 低氧 ; 层化 ; 有机质 ; 生态灾害
地址

中国科学院烟台海岸带研究所, 烟台, 264003

语种 中文
文献类型 研究性论文
ISSN 0023-074X
学科 环境污染及其防治
基金 中国科学院战略性先导科技专项 ;  中国科学院重点部署项目 ;  青年“千人计划”项目资助
文献收藏号 CSCD:5715302

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引证文献 23

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