The interactive roles of zooplankton grazing (top-down) and nutrient (bottom-up) processes on phytoplankton distribution in a temperate estuary were investigated via dilution and nutrient addition experiments. The responses of size-fractionated phytoplankton and major phytoplankton groups, as determined by flow cytometry, were examined in association with zooplankton grazing and nutrient availability. The summer bloom was attributed to nanoplankton, and microplankton was largely responsible for the winter bloom, whereas the picoplankton biomass was relatively consistent throughout the sampling periods, except for the fall. The nutrient addition experiments illustrated that nanoplankton responded more quickly to phosphate than the other groups in the summer, whereas microplankton had a faster response to most nutrients in the winter. The dilution experiments ascribed that the grazing mortality rates of eukaryotes were low compared to those of the other groups, whereas autotrophic cyanobacteria were more palatable to zooplankton than cryptophytes and eukaryotes. Our experimental results indicate that efficient escape from zooplankton grazing and fast response to nutrient availability synergistically caused the microplankton to bloom in the winter, whereas the bottom-up process (i.e., the phosphate effect) largely governed the nanoplankton bloom in the summer.

中文翻译：

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自上而下和自下而上过程在光阳湾大小分级浮游植物分布中的不同作用

通过稀释和营养添加实验研究了浮游动物放牧（自上而下）和养分（自下而上）过程对温带河口浮游植物分布的交互作用。通过流式细胞术确定的大小分级浮游植物和主要浮游植物群的反应与浮游动物放牧和养分可用性相关。夏季水华归因于纳米浮游生物，而微型浮游生物是冬季水华的主要原因，而微型浮游生物生物量在整个采样期间相对一致，除了秋季。营养添加实验表明，纳米浮游生物在夏季对磷酸盐的反应比其他组更快，而微型浮游生物在冬季对大多数营养物质的反应更快。稀释实验表明，与其他组相比，真核生物的放牧死亡率较低，而自养蓝藻比隐生植物和真核生物更适合浮游动物。我们的实验结果表明，有效逃离浮游动物放牧和对养分可用性的快速反应协同导致微型浮游生物在冬季大量繁殖，而自下而上的过程（即磷酸盐效应）在很大程度上控制了夏季微型浮游生物的大量繁殖。