海洋生态与环境科学重点实验室知识库

    近年来，长江口邻近海域富营养化程度加剧，有害藻华频发，低氧现象频繁出现，生态系统的稳定性明显下降。已有的研究结果表明，浮游植物产生的大量颗粒有机碳向底层输送，并在底层进行化学和生物氧化是长江口外低氧区形成的主要原因。然而，目前尚缺乏关于该海域浮游植物群落及低氧区耦合机制的连续、系统的定量化研究。本论文基于2013年及2015年在长江口邻近海域的多学科综合调查，系统分析了该海域浮游植物时空变化及其碳沉降对低氧区形成的影响。主要研究结果如下：

    1. 2013年长江口邻近海域叶绿素a的分布受长江冲淡水的影响，在8月较高，平均值为5.18μg/L，最高值达32.05μg/L（现场发现海水变色现象）；与同期历史资料相比，叶绿素a浓度呈现出波动增长趋势。3月共发现浮游植物3门41属80种（不包括变种和变型），以硅藻为主，主要优势种为具槽帕拉藻（Paralia sulcate）；8月共发现浮游植物4门67属135种（不包括变种和变型），甲藻物种数量和细胞丰度均升高，主要优势种为东海原甲藻（Prorocentrum donghaiense）和骨条藻（Skeletonema sp.）；盐度及浊度影响该海域浮游植物的空间分布，浮游植物群落组成受营养盐结构影响：随着N/P比值升高，甲藻的相对丰度升高，硅藻的相对丰度逐渐降低。

    2. 2015年该海域 浮游植物的分布在各调查月存在明显的差异，叶绿素a浓度范围介于0.17到20.44μg/L之间，4月、7月及10月整体较高，且在4月发生了东海原甲藻藻华现象。全年共发现浮游植物5门94属265种，其中硅藻64属169种，占浮游植物物种数的63.78%，以中心硅藻为主，甲藻24属88种，占浮游植物物种数的33.21%，硅鞭藻1属3种，绿藻1属1种，蓝藻2属3种；以沿岸广温型为主，一些偏暖水性种类也有出现。

    3. 与其他测量浮游植物沉降速率的方法相比较：SETCOL方法由于其现场仪器操作简单且系统误差较小，广泛应用于现场调查实验中。应用此方法测量的该海域浮游植物周年沉降速率结果显示：全年沉降速率介于0.02~3.49m/d，9月、10月整体较高。浮游植物沉降速率与环境因子无显著的相关关系，而浮游植物群落组成直接影响其沉降速率：以链状硅藻占优的浮游植物群落的沉降速率比以粒径较小的甲藻占优的群落高，更易沉降至底层。

    4. 2013年8月及2015年7月到10月该海域均出现了溶解氧低值区（< 3mg/L）与低氧区（< 2mg/L），与历史资料相比较出现了位置北移、面积扩大、溶解氧最低值下降的趋势。同时，该海域低氧区位置的变动受浮游植物生物量高值区及沉降速率变化的影响，与浮游植物碳通量呈负相关关系。因此，浮游植物旺发所引起的大量海源有机质的沉降分解对低氧区的产生具有重要作用。

    综上所述，本论文通过分析长江口邻近海域浮游植物分布及其群落演替特征，首次定量研究了浮游植物沉降速率并估算其碳沉降通量，综合评估了浮游植物有机碳沉降对底层溶解氧消耗的量化贡献，系统研究了该海域浮游植物时空变化及其  对于低氧区形成的影响。相关结果为研究该海域的生态系统结构提供了基础资料，并为深入揭示浮游植物在低氧区形成中的作用奠定了基础。

    In recent years, the Changjiang Estuary (CE) and its adjacent waters has been suffering from serious eutrophication and seasonal hypoxia. Studies have shown that hypoxia in CE and its adjacent waters is closely associated with stratification of the water column and the sinking and decomposition of organic matter from surface phytoplankton. There have been various snapshots, annual or seasonal observations of phytoplankton community, however, long-term continuous record of phytoplankton community is in need to study its correlation with dissolved oxygen (DO) in the hypoxic zone of the CE and its adjacent waters. Accordingly, based on the multi-disciplinary investigations in 2013 and 2015, the distribution and seasonal variations of phytoplankton carbon flux and the relationship with formation of the hypoxia were studied. The major findings were listed as following: 

    1. Influenced by the Changjiang Diluted Water (CDW), the Chl a concentration was higher in August, with an average of 5.18μg/L and the highest of 32.05μg/L. Compared to historical data, the Chl a concentrations showed a fluctuate growth trend in the flood period. The phytoplankton assemblage was influenced by the salinity and turbidity during the survey. In March, 80 taxa belonging to 41 genera and 3 phyla were identified. Paralia sulcate was the most dominant specie. While, 135 taxa belonging to 67 genera and 4 phyla were identified in August, the main dominant species were Prorocentrum donghaiense and Skeletonema sp..The N/P ratio, instead of Si/N ratio, played a significant role in regulating the phytoplankton community structure. With the increase of N/P ratio, the diatom became the dominant phytoplankton assemblage.

    2. The Chl a concentration varied among ten months in 2015, with the values ranging from 0.17 to 20.44 μg/L. The Chl a concentration was relatively higher in April, July and October. In total, 265 taxa were identified during ten cruises, belonging to 5 phyla and 94 genera. Diatoms (accounted for 63.78%) and dinoflagellates (accounted for 33.21%) were the dominant groups, most of which were eurythermal species. Due to the influence of TWC, some warm-water species were found.

    3. The SETCOL method, with the technologically simple operation and reliable determined results, has been widely used to measure phytoplankton sinking rates in field. The sinking rates of the phytoplankton community varied clearly among months in the survey area. Phytoplankton sinking rates were relatively higher in September and October. There was no significant correlation between the sinking rates of the phytoplankton community and its ambient environmental parameters, especially the major nutrient components. While, a significant correlation between phytoplankton sinking rate and the phytoplankton community structure was found. The diatom-dominated community had higher sinking rates than the dinoflagellate-dominated community.

   4. Low DO (< 3 mg/L) and hypoxia (< 2 mg/L) were observed at the near-bottom waters in August of 2013 and July to October of 2015. Compared to the historical records, the hypoxic zone expanded and moved northward, with the minimum DO value decreased significantly. The lower DO zone in near-bottom water was in accordance with the surface water with higher Chl a. The statistical analysis showed that there was significantly negative correlation between phytoplankton carbon flux and DO in the near-bottom water. Thus, the station with higher phytoplankton carbon flux tended to be the hypoxia. Therefore, the flourishing of phytoplankton in the surface water may contribute directly to the formation of low-oxygen zone in near-bottom water.

    In conclusion, based on the study of variation of phytoplankton and its carbon flux, and quantitative contribution of phytoplankton carbon sinking to the formation of hypoxia, the inherent relationship among environmental parameters, phytoplankton sinking rates, and the formation of hypoxia in the CE and its adjacent waters were revealed. The major findings of the dissertation will help further understand on the role of phytoplankton in the formation of hypoxia in the CE and its adjacent waters.