Temporal Change of Export Production at Xisha of the Northern South China Sea

Abstract

AbstractPhysical‐biological interaction within the upper ocean influences fixed organic carbon with various efficiencies leading to spatial and temporal change of carbon export in the ocean, which is important for global biogeochemical dynamics but is inadequately understood in the northern South China Sea (NSCS). Here the temporal variations of primary and export production are investigated using a 1‐D physical‐biogeochemical coupling model at a pelagic station of Xisha (XS) in the NSCS. At this station, the model reproduces the satellite sea surface observations, the depth‐integrated primary production, and the sinking flux of particulate organic carbon at 500 m from a moored sediment trap. By synthesizing these results, we investigate the long‐term dynamics of the biological pump with the focus on export production. We compared our results at XS with that at A Long‐Term Oligotrophic Habitat Assessment (ALOHA) in the North Pacific subtropical gyre. While the surface export at XS is greater than ALOHA, the stronger subsurface remineralization at XS has resulted in a similar week biological pump as ALOHA. The high surface export and subsurface flux attenuation at XS are caused by its physical dynamics that support a stronger upward nitrate flux than ALOHA. We also find a time lag of about 20 days between primary production and export production at XS, which has led to the large variability of export ratios in the subsurface layers. Further analyses suggest that the temporal variations of primary and export productions are largely driven by the change of turbulent mixing and Ekman pumping that control the vertical nutrient fluxes into the upper ocean.