Sinking rates of phytoplankton in the Bohai and Yellow Seas: Insights into the regulatory role of the Yellow Sea cold water mass on carbon export.

Phytoplankton sinking is a key process in the biological carbon pump, driving vertical transport of organic carbon in the ocean. The Yellow Sea Cold Water Mass (YSCWM) is a prominent hydrological feature in the Yellow Sea in warm seasons, yet its association with phytoplankton sinking rates and subsequent carbon export remains poorly understood. Here, we investigated phytoplankton sinking and carbon export in the Bohai Sea and in waters inside and outside the YSCWM through field and statistical analyses. Phytoplankton sinking rates, determined using the settling column (SETCOL) method based on chlorophyll a redistribution after water collection, ranged from -0.20 to 1.64 m d-1. Both phytoplankton carbon biomass and flux exhibited pronounced spatial heterogeneity, and their spatial patterns were similar. In the Bohai Sea, sinking rates were statistically associated with multiple variables, whereas outside the YSCWM, chemical and biological variables were identified as important predictors. Inside the YSCWM, variance partitioning indicated that measured variables explained only a small fraction of the variation, suggesting that sinking rates in this stratified region are associated with more complex mechanisms. Structural equation modeling indicated that stratification was indirectly and negatively associated with carbon flux inside the YSCWM, whereas the nitrogen to phosphorus ratio was identified as an important indirect predictor outside the YSCWM. However, such statistical associations do not necessarily imply independent ecological mechanisms. These findings provide insights into phytoplankton-mediated carbon export in temperate shelf seas under climate warming.