Changes in the biomass and species composition of phytoplankton may reflect major shifts in environmental conditions.Based on the data of 13 cruises between 2002 and 2006, we investigated the changes of phytoplankton community structure in the Yangtze estuary and adjacent sea before and after the impoundment of the Three Gorges Dam. The relationships between phytoplankton communities and environmental factors were also studied with the correlation analysis and canonical correspondence analysis (CCA). The results showed that the phytoplankton community structure, salinity, nutrient concentrations and ratios changed significantly in the Yangtze estuary and adjacent sea,after the impoundment of the Three Gorges Dam.The percentage of diatom species to the overall species was significantly reduced from 77.09 % in autumn 2002 to 71.43 % in autumn 2006. Meanwhile,the percentage of dinoflagellate species increased significantly, from 17.32 % to 22.45 %. There were significant decreases in DSi, DIP and DIN concentrations, as well as significant increases in salinity and N: P ratio.DSi concentrations changed from 83.14 μmol/L in autumn 2002 to 42.83 μmol/L in autumn 2006; DIP concentrations decreased from1.40 μmol/L to 0.89 μmol/L; DIN concentrations decreased from 56.80 μmol/L to 49.18μmol/L;N: P ratio increased from 39.86 to 81.27, as well as salinity increased from 19.44 to 25.42.CCA indicated that salinity, N: P ratio, DIP and DSi had significant impacts on the phytoplankton community structure.After the impoundment of the Three Gorges Dam, the reduction of runoff and sediment discharge and the change in the seasonal distribution of runoff in the Yangtze River were the root causes for changes in the phytoplankton community structure in the Yangtze estuary and adjacent sea.%通过对长江口及其邻近海域2002-2006年13个航次的综合调查，研究了三峡水库蓄水前后长江口及其邻近海域浮游植物群落结构的变化，并应用相关性分析、典范对应分析（CCA）等方法对浮游植物群落结构变化的原因进行了探讨。结果表明：三峡水库蓄水后，长江口及其邻近海域的浮游植物群落结构、盐度及营养盐结构已发生显著变化。硅藻种类比例显著下降，由2002年秋季的77.09%下降至2006年秋季的71.43%；甲藻种类比例显著上升，由17.32%上升至22.45%。主要环境因子中，磷酸盐（DIP）、硅酸盐（DSi）、无机氮（DIN）浓度显著下降，盐度、N/P 比值显著上升。DSi 的降幅最大，从2002年秋季的83.14μmol/L 下降至2006年秋季的42.83μmol/L；DIP 从1.40μmol/L 下降至0.89μmol/L；DIN 从56.80μmol/L 下降至49.18μmol/L。而 N/P 比值从39.86上升至81.27；盐度则从19.44上升至25.42。CCA 及相关性分析表明，盐度、N/P 比值的升高，DIP、DSi 浓度的降低，均对浮游植物群落结构变化产生了显著的促进作用。三峡水库蓄水后，长江径流量、输沙量的减少以及径流季节分配的改变是造成长江口及其邻近海域浮游植物群落结构发生改变的根本原因。
展开▼
