Impact of physical and biogeochemical forcing on particle export in the South China Sea

Abstract

This study examines temporal variations of 234Th-based particle export based on time-series observations conducted from 2004 to 2014 at the South-East Asian Time-series Study (SEATS) site in the basin area of the Northern South China Sea. It shows significant variability in particle export at both seasonal and interannual time scales. A significant feature of our data as observed by prior studies is that particle export was high during the NE monsoon in winter, but low during the inter-monsoon and SW monsoon. Particle flux increased with increasing mixed layer depth (MLD), primarily in winter due to the presence of a strong NE monsoon, in addition to influence by mesoscale features quantified by eddy kinetic energy values. Particle export was generally elevated when phytoplankton biomass was abundant, as shown by the significant correlation between particle fluxes and concentrations of chlorophyll (Chl) a and/or particulate organic carbon (POC) between 0 and 100 m. This is also supported by the negative correlation between 234Th residence time and Chl a concentration, suggesting that increased phytoplankton biomass accelerated downward particle sinking. The correlation disappeared however at Chl a values < 0.11 mg m−3, implicating other factors (e.g., phytoplankton community composition) as playing a more important role when biomass is relatively low. Satellite Chl a-based POC export estimated using various established algorithms agreed well with our measured fluxes when the export horizon was defined as the bottom of the mixed layer, but differed compared to estimated fluxes at 100 m depth. This is likely due to the fact that satellite-derived Chl a measurements only reflect surface values, suggesting that reliable algorithms must be used after considering the biological processes in subsurface waters.