Marine heatwaves are shaping the vertical structure of phytoplankton in the global ocean

Marine heatwaves, characterized by extremely high ocean temperatures, significantly impact marine ecosystems. To date, the vertical structure of phytoplankton responses to marine heatwaves remains poorly understood. Here we use 17 years of global profiles from Biogeochemical-Argo float data to show that marine heatwaves drive distinct phytoplankton responses at different depths, with subsurface signals sometimes stronger than surface ones. These responses are classified into intensified, weakened, and subsurface-reversed chlorophyll-a types, each showing distinct latitudinal patterns and driven by changes in light, temperature, and nutrient supply during marine heatwaves. Intensified responses dominate high latitudes, while weakened and subsurface-reversed responses are more common in mid-to-low latitudes. Marine heatwaves also promote the development of deep chlorophyll maxima and reshape subsurface chlorophyll-a distributions. Our findings highlight the critical role of vertical marine heatwaves in shaping phytoplankton dynamics and underscore the need for enhanced subsurface monitoring to better assess ecological and biogeochemical impacts under ocean warming. Marine heatwaves promote the development of a deep chlorophyll maximum, which reshapes subsurface chlorophyll and therefore vertical phytoplankton distributions, according to analyses of Biogeochemical Argo float data.