|Place of Conferral||中国科学院海洋研究所|
|Keyword||浮游植物 群落结构 沉降速率 碳通量 低氧|
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。
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。
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.
|First Author Affilication||中国科学院海洋研究所|
|李照. 长江口邻近海域浮游植物群落特征及其碳沉降研究[D]. 中国科学院海洋研究所. 中国科学院大学,2018.|
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