Shifting responses of phytoplankton to atmospheric and oceanic forcing in a prolonged marine heatwave

Marine heatwaves are becoming a severe challenge for marine ecosystems. However, current understanding of their impacts on phytoplankton, especially biomass and community structure, is still deficient or fragmentary. Here, we focus on the response of phytoplankton to a prolonged (372 d) marine heatwave in the southeastern tropical Indian Ocean in 2015-2016. Despite a similar surface warming throughout the marine heatwave, we found two distinctively different changes in chlorophyll a (Chl a) concentration, based on which the marine heatwave was divided into two phases of nearly equal duration. During Phase 1, the Chl a concentration and phytoplankton biomass increased slightly, while during Phase 2 they decreased dramatically, with a marked shift in phytoplankton community toward smaller-size species. This difference mainly resulted from the change in dominant drivers of the marine heatwave, which shifted from atmospheric forcing (enhanced net air-sea heat flux) in Phase 1 to oceanic process (downwelling Kelvin waves) in Phase 2. The intensified near-surface stratification and upwelling Kelvin waves led to a slight surface nutrient increase in Phase 1, while the downwelling Kelvin waves suppressed the seasonal upwelling in Phase 2, resulting in a substantial decrease in nutrient supply from the subsurface ocean, and hence dramatic decreases in Chl a concentration and biomass. Our results demonstrate the complex responses of phytoplankton to marine heatwaves and highlight the importance of distinguishing the driving mechanisms of marine heatwaves when evaluating their ecological impacts.