Sinking rates of phytoplankton in response to cell size and carbon biomass: A case study in the northeastern South China Sea

Abstract

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. After comparing all the methods of studying cell sinking behavior for nearly 40 years, we gave a reason for using the SETCOL method. To study the effect of Kuroshio Current (KC), and Tropical Storms (TS) on the sinking rates, we analyzed 672 samples collected from the northeastern part of the South China Sea (neSCS), where the marine environments were highly diverse. KC could increase the total sinking rates, TS transit caused the phytoplankton to sink slower. The effect of nutrients on the sinking rates was minimal in the absence of abnormal sea conditions. Chl-a concentrations showed negative regulation on the sinking rates of KC-affected stations. Nutrients positively regulated the TS-affected stations, and dinoflagellates abundance has negative feedback regulation on the sinking rates. In addition, long rod-shaped diatoms sank fastest and the elliptical dinoflagellates sank significantly slower after being affected by TS. Under the influence of KC, the cell with an equivalent sphere diameter (ESD) of around 10–12 μm, and carbon content per unit volume (CCV) of about 0.5 Pg C cell⁻¹ μm⁻³ sank fastest. TS resulted in the fastest sinking rates of cells at 15–17 μm. The sinking rates correlated extremely well with ESD in normal sea conditions in a positive linear fashion and slowed down with increasing CCV. We thus suggested that the sinking rates of cells under different marine conditions was closely related to cell size and carbon biomass. In future oceans, it is possible that most phytoplankton in the euphotic layer will be able to maintain themselves in an optimal growth environment consistently by reducing its size or increasing the proportion of biological carbon content in cells.