Biological Response to the Interaction of a Mesoscale Eddy and the River Plume in the Northern South China Sea

The Pearl River delivers a large amount of freshwater, sediments and nutrients to the northern shelf of the South China Sea (SCS). In June 2015, an anomalously strong phytoplankton bloom was captured by satellite images in the slope region of the northern SCS, which was associated with the southeastward spreading of the river plume on the shelf and a southwestward-moving eddy along the slope. In this study, the underlying dynamics triggering the bloom was investigated using a coupled physical-biogeochemical model. Results show that the nutrients supporting the bloom were not directly sourced from the Pearl River, but were transported locally from subsurface. The eddy cross-slope current advected low salinity water from the Pearl River plume, which interacted with eddy edge and enhanced frontal dynamics with vertical motions. The front-induced upwelling injected nutrients from subsurface to surface layer and stimulated phytoplankton bloom in the upper layer. Overall, the phytoplankton bloom was attributable to the interaction of freshwater plume on top and the eddy edge induced anomaly in the subsurface. These findings suggest that the eddy-entrained freshwater could have significant biological consequences through modifying local dynamics in the plume-influenced region.

Automated summary

This study found that the anomalously strong phytoplankton bloom in the slope region of the northern South China Sea was triggered by the interaction between locally transported nutrients from subsurface and the eddy edge, which led to enhanced frontal dynamics and injection of nutrients from subsurface to surface layer, stimulating phytoplankton growth in the upper layer. The findings suggest that the eddy-entrained freshwater could have significant biological consequences by modifying local dynamics in the plume-influenced region.