The impact of marine heatwaves on surface phytoplankton chlorophyll-a in the South China Sea.

According to previous studies, marine heatwaves (MHWs) significantly suppress the phytoplankton chlorophyll-a concentration (Chl a) in tropical oceans. However, pre-MHW Chl a has rarely been considered as a reference value. In this study, the Chl a for the periods preceding and during MHWs events was used to explore the impact of MHWs on Chl a from 1998 to 2022 in the South China Sea (SCS). The Chl a response to MHWs in different regions was further discussed based on the Chl a variation characteristics. The results showed that the Chl a response to MHWs exhibited regional variability. Interestingly, there was a large proportion of positive Chl a anomalies (∼0.55) in the estuary and offshore regions during MHWs; however, Chl a anomalies were mostly negative in the upwelling regions. These different response patterns are related to background conditions, including nutrient concentrations, wind-driven dynamics, and light availability. In upwelling regions, negative Chl a anomalies were primarily due to the weakening of wind speeds, Ekman pumping velocities, and upwelling intensities. In estuarine regions, positive Chl a anomalies were caused by enhanced light availability, whereas in offshore regions, there were attributed to the increased atmospheric wet deposition. These results have improved our understanding of the impact of MHWs on marine ecosystems.