Driven by the southwest monsoon, an offshore jet is usually formed in the western South China Sea (SCS) and sandwiched by a cyclonic eddy in the north and an anticyclonic eddy in the south, which affects the ecosystem of the region. Using in-situ and satellite data in September 2014, we analyzed the impact of this jet with two eddies on phytoplankton size structure in this region. The data showed that picophytoplankton (0.2~2 μm) dominated the surface, averaged at 76.7% of the total chlorophyll. The contributions of nanophytoplankton (2~20 μm) and microphytoplankton(20~200 μm)in the jet area were respectively higher, and had a positive relationship with the total chlorophyll. Comparatively higher percentage of microphytoplankton appeared in the anticyclonic eddy at surface (average 10.3%) than in the cyclonic eddy (average 3.6%). The results suggested that physical processes significantly influenced summertime surface phytoplankton size structure in the western South China Sea. Both the jet and eddies can affect phytoplankton size structure by increasing the contribution of microphytoplankton. Surface horizontal advection of phytoplankton by the northeastward jet formed the coastal upwelling area and was the main source of microphytoplankton in the open sea. The interactions of convergence and divergence in the eddies with the jet formed a chlorophyll front and increased the microphytoplankton component. Upwelling in the center of the cyclonic eddy brought up nutrients, which increased the percentage of the microphytoplankton component.%受夏季西南季风影响,南海西部会形成一支离岸急流,该急流通常与冷涡和暖涡组成偶极结构,共同影响该海域生态环境.本研究利用2014年9月现场和卫星数据,首次结合南海西部急流和冷暖涡过程,分析它们对该海域浮游植物粒径结构的影响.数据显示,表层以微微型浮游植物(0.2~2μm)为主,占总叶绿素a的平均比例为76.7%.在急流区中,微型浮游植物(2~20μm)和小型浮游植物浓度(20~200μm)占总叶绿素a平均比例相对较高,且与总叶绿素浓度呈线性正相关.暖涡区小型浮游植物比例(平均10.3%)高于冷涡区(平均3.6%).结果表明,物理动力过程是南海西部夏季表层浮游植物粒径结构的主要影响因素.急流和冷暖涡可以提升小型浮游植物的比例,改变表层的粒径结构.急流对沿岸上升流区浮游植物的水平输送是表层小型浮游植物的主要来源.涡旋的辐合、辐散与急流的相互作用,导致各粒径浮游植物在冷涡边缘出现锋面分布,并且提高了暖涡区表层小型浮游植物的比例.冷涡的垂直抽吸作用加强了营养盐供给,也提高了冷涡中心表层小型浮游植物的比例.
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