Phytoplankton patchiness during spring intermonsoon in western coast of South China Sea

Jet-like phytoplankton blooms usually occur off the southwestern coast of the South China Sea (SCS) caused by strong winds during summer monsoons. However a jet-like phytoplankton patch was observed in the western SCS in the spring intermonsoon of 2010 in both field and remote sensing data. The present study investigated the biological processes associated with this spring phytoplankton patchiness. The data showed that chlorophyll a concentrations increased in the surface water, extending out to the SCS, and the depth of the subsurface chlorophyll maximum uplifted from 75 m to 50 m depth; low dissolved oxygen, low pH and nutrient enrichment (nitrate+nitrite and soluble reactive phosphate) were observed in the subsurface water (50 to ~200 m depth). Data analysis showed that variations in chlorophyll a, nutrients and temperature in the water column were related to wind-stress curl: the spatial distribution pattern and vertical structure of the phytoplankton patchiness were controlled by vertical flux of nutrients caused by curl-driven upwelling through Ekman pumping. There was a high correlation between chlorophyll a concentration and wind-stress curl where the influence of nutrient influx from the coast was limited. This study shows the importance of wind-stress curl in providing nutrients to support phytoplankton growth during the spring intermonsoon along the western coast of SCS. It may help to better understand the role of wind in marine biological processes.