Global Responses of Phytoplankton Size Structure to Marine Heatwaves

Abstract Marine heatwaves (MHWs) are receiving fast‐growing attention due to their devastating ecological and socioeconomic impacts under global warming. Despite studies on the response of phytoplankton abundance to MHWs, their effects on phytoplankton size structure (PSS), a crucial property of phytoplankton community, are poorly understood. Here, we present a global assessment of PSS changes during MHWs based on multi‐satellite observations. We find that MHWs tend to increase the dominance of smaller‐size phytoplankton in mid‐low latitudes, while shifting PSS toward larger‐size species in high latitudes. Compared with the high‐latitude oceans, PSS are more susceptible to MHWs in mid‐low latitudes (ML). Hotspots of PSS shifting to smaller species are observed in the tropical oceans, the eastern boundary upwelling systems, and the mid‐latitude transition zones, mainly due to reduced nutrient availability associated with changes in oceanic processes. In contrast, the shift of PSS to larger size classes in the high‐latitude oceans is likely attributed to increased light exposure during MHWs. The potential recovery time of these PSS disturbance exhibits marked spatial heterogeneities and suggests a more elastic phytoplankton community during MHWs in ML. These results extend our understanding of the phytoplankton responses to MHWs and have important implications for global fisheries and carbon cycling.