Phytoplankton are simultaneously involved in the processes of carbon fixation, sequestration and sinks in the oceans, and are able to respond rapidly to changes in the marine environment. The direct sinking behavior of phytoplankton is an important way for marine carbon pumping and an accurate estimation of the sinking rates of phytoplankton is necessary for a comprehensive understanding of the ocean carbon cycle. Phytoplankton cell size and community structure play an important role in physiological performance along vertical light gradients and influence sinking rates and nutrient uptake by single species. The high diversity of phytoplankton shapes exhibited represents an evolutionary compromise for increased grazing resistance and optimized sinking rates. In this study, three South China Sea (SCS) cruises were selected for a comprehensive study of phytoplankton sinking rates using the SETCOL method. The main purpose of this study is to quantify the sinking rates of individual species phytoplankton (φ_indiv) at the euphotic zone (0-200 m) in the SCS. The carbon flux values contributed by direct phytoplankton sinking were also estimated for the three sea areas. Finally, we examined the effects of different environments and the size and shape of phytoplankton on φ_indiv。
Our study suggested that, cell abundance and carbon biomass of phytoplankton were both highest in the northeastern South China Sea (NeSCS). The cell abundance was lowest in the western South China Sea (WeSCS) and the carbon biomass was lowest in the central South China Sea (CeSCS).  The highest φ_indiv was observed in WeSCS area and the lowest φ_indiv occurred in NeSCS area (Fig. 3B). We made rough estimates of the carbon fluxes contributed through direct phytoplankton sinking in the euphotic zone of the three sea areas (Fig. 3C). The WeSCS area has the highest daily downward export of carbon by phytoplankton, which is 4 × higher than the CeSCS area and 13 × higher than the NeSCS area. Finally, we analyzed the regulation of the φ_indiv by cell size and shape of phytoplankton. In the CeSCS, the φ_indiv of cylinder-shaped and rhombus-shaped cells in diatoms and fusiform-shaped and ellipsoid-shaped cells in dinoflagellates were significantly modulated by cell size. In the NeSCS, the long rod-shaped diatoms sank fastest and the elliptical dinoflagellates sank significantly slower after being affected by tropical storms. The φ_indiv correlated extremely well with cell size in normal sea conditions in a positive linear fashion and slowed down with increasing carbon density.