During summer of 2014,an investigation on the abundance and distribution of the three photosynthetic pi-coplankton groups of Synechococcus (Syn),Prochlorococcus (Pro)and pico-eukaryotes (Euk)in the northern South China Sea and their environmentally regulating mechanisms was carried out using flow cytometry BD Accuri C6.The results show that the average abundances of Syn,Pro and Euk were 5.13×103 cells/mL,3.27×104 cells/mL and 1.85×103 cells/mL,and the average abundances of carbon biomass were 1.19 μg/L,1.86 μg/L and 4.51μg/L,respectively.The cell abundance of each picoplankton group showed different distribution characteristics. The abundance of Syn,Pro and Euk was high nearshore and low off-shore.The highest abundances of Syn and Euk population both appeared in coast and continental shelf and upwelling waters.The highest abundance of Pro was observed along coast and continental shelf,while the lowest Pro population abundance was observed in upwelling waters.High values of Syn and Euk were mainly found at the surface,while high values of Pro mainly occurred at the bottom of euphotic layer.The variation in Euk distribution was less than those of Syn and Pro.The results of environmental factor correlation analysis indicated close negative correlations between picoplankton carbon biomass and nitrate concentration,silicate concentration and depth,and a significant positive correlation between picoplank-ton carbon biomass and phosphate concentration.%利用流式细胞仪 BD Accuri C6对2014年夏季南海北部超微型浮游植物进行了现场的观测研究，发现了3类超微型光合自养浮游植物，聚球藻（Synechococcus，Syn）、原绿球藻（Prochlorococcus， Pro）和超微型真核藻类（pico-eukaryotes，Euk），并对其丰度与分布以及环境因子影响进行了研究。结果表明，Syn、Pro 和 Euk 丰度总平均值分别为5．13×103个／mL，3．27×104个／mL 和1．85×103个／mL，碳生物量均值分别为1．19μg／L，1．86μg／L 和4．51μg／L。Syn、Pro 和 Euk 的丰度表现出不同的分布特征。Syn、Pro 和 Euk 丰度分布趋势呈现近海低而外海高，Syn 和 Euk 丰度高值区分别出现在沿岸带与陆架和上升流影响海域，Pro 丰度高值区出现在沿岸带与陆架，低值区出现在上升流影响海域。Syn、Euk 丰度高值区主要分布在次表层，Pro 丰度高值区主要分布在真光层底部，Euk 丰度垂直变化差异相对 Syn 和 Pro 较小。超微型浮游植物与环境因子的相关性分析结果表明，Syn、Pro 和 Euk的碳生物量均与硝酸盐、硅酸盐浓度和深度呈现负相关关系，Pro 的碳生物量与磷酸盐浓度呈现正相关关系。
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