| Summary: | The variability of size-fractionated chlorophyll <i>a</i> (Chl <i>a</i>) biomass and related environmental variables was investigated in a survey conducted in the northeastern South China Sea (NESCS), to assess their responses to upwelling, Kuroshio, and nutrients in the summer of 2017. The depth-weighted total Chl <i>a</i> concentration varied from 0.057 to 0.496 μg/L. The contribution of size-fractionated Chl <i>a</i> to total Chl <i>a</i> biomass was determined, with the contributions of pico-Chl <i>a</i> (0.2–2 μm), nano-Chl <i>a</i> (2–20 μm), and micro-Chl <i>a</i> (>20 μm) found to be 52.87%, 38.54%, and 8.59%, respectively. We found that nano-Chl <i>a</i> contributes more to total-Chl <i>a</i> than pico-Chl <i>a</i> in areas affected by upwelling. In addition, the results indicated an upward shift in the deep chlorophyll maximum (DCM) content in the upwelling area and the Kuroshio intrusion area. The Chl <i>a</i> biomass was increased in the physical process. Overall, correlation analysis showed that environmental variables (temperature, salinity, phosphate, silicate, nitrate, nitrite, ammonium) significantly influenced the phytoplankton biomass. However, physical processes determined the changes in environmental variables, with upwelling and Kuroshio being the main factors regulating the distribution of Chl <i>a</i>.
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