Vertical structure of chlorophyll-a during marine heatwaves in the California Current Ecosystem

In the past decades, the California Current Ecosystem has experienced intense marine heatwaves, which have induced significant disruptions to local phytoplankton communities. Here, using 30-year cruise observations, we identify a previously undocumented vertical structure in chlorophyll-a concentration response to marine heatwaves, characterized by reductions in the surface layer coupled with increases in the subsurface. By integrating observations and coupled physical-biogeochemical model products, we demonstrate that declines of surface chlorophyll-a are primarily attributed to suppressed nutrient upwelled to the upper ocean. Although surface irradiance increased modestly (+3.5%), light availability in the subsurface layer improved substantially (+21.7%) due to reduced phytoplankton shading at the surface. Concurrent with enhanced lateral nutrient transport, phytoplankton growth at depth was promoted during heatwave events. This study highlights the pivotal role of subsurface phytoplankton dynamics in shaping the vertical chlorophyll-a concentration structure and its variability under extreme events. During marine heatwaves in the California Current, chlorophyll-a reduces at the surface but phytoplankton growth is strengthened at depth, according to combined nutrient observations and coupled physical-biogeochemical modelling.