Phytoplankton Response to the Record-Breaking Marine Heatwave in the Summer of 2020 in the South China Sea

A record-breaking marine heatwave (MHW) occurred in the South China Sea (SCS) during the summer of 2020, causing substantial ecological impacts, while its influence on phytoplankton dynamics remains unknown. Here we employed a regionally optimized physical-biogeochemical model to examine the phytoplankton response to this event. An analysis of the mixed-layer heat budget in the MHW region revealed that the MHW was driven by enhanced shortwave radiation and reduced horizontal advection of cold water, associated with weakened upwelling off the coast of Vietnam. For phytoplankton, average surface chlorophyll-a (Chla) concentrations during the MHW experienced a significant decline in June and July, and a slight reduction in September compared to normal conditions, which can be attributed to reduced horizontal advection of nutrients associated with weakened upwelling. Furthermore, we found that the reduction in depth-integrated Chla in the euphotic zone was much smaller during the MHW due to the increase in subsurface Chla, which was also attributable to the changed lateral transport. This study highlights the importance of non-local effects of MHWs on phytoplankton distributions in the SCS.