Offshore phytoplankton biomass increase and its oceanographic causes in the South China Sea

Abstract

High phytoplankton biomass often occurs in plumes near river mouths or in eutrophic coastal waters for short time periods. However, we observed an increased phytoplankton biomass in a narrow jet-shaped protrusion into the western South China Sea (SCS) using satellite chlorophyll a (chl a) data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and concurrent ship measurements in 1999. In June, regional phytoplankton biomass appeared as a large jet shape extending from the coastal waters of Vietnam eastward towards the SCS, about 200 km northeast of the mouth of the Mekong River; this feature intensified in the form of a large jet or gyre from July to September, decayed in October, and disappeared entirely in November. The gyre was about 400 km in diameter with chl a concentrations from 0.5 to 2 mg m-3. Data on sea surface temperature (SST), winds, and sea surface height anomalies indicated a strong offshore upwelling during a period of strong southwesterly winds alongshore. The upwelling coincided with the regional increase in phytoplankton biomass in terms of shape, timing, and location. We observed this phenomenon during every southwesterly monsoon season from 1997 to 2002 using chl a images from 3 satellite ocean color sensors - SeaWiFS, Ocean Color and Temperature Scanner (OCTS), and Ocean Color Imager (OCI) - and it appeared to be related to the cross-shelf upwelling that delivers nutrients to surface waters. The phytoplankton then flowed with a large anticyclonic gyre into the western SCS.