Reduced and smaller phytoplankton during marine heatwaves in eastern boundary upwelling systems

Abstract Marine heatwaves have attracted great attention for their devastating impacts on marine organisms. However, the influence of marine heatwaves on phytoplankton community structure, a key aspect of pelagic ecosystems, remains elusive, especially in the biologically productive eastern boundary upwelling systems. Here, using a combination of multi-satellite observations and model outputs, we discover that phytoplankton in these upwelling systems exhibit not only a marked reduction in biomass (~50%), but also a significant shift toward smaller species in community structures during marine heatwaves, featuring a substantially increased/decreased dominance in picophytoplankton/microphytoplankton. This change in community structure is positively related to the intensity and duration of marine heatwaves, and is likely a result of community competition under reduced nutrient supply due to weakened coastal upwelling associated with poleward wind anomalies. These findings invite us to rethink the dominance of phytoplankton in the eastern boundary upwelling systems under extreme conditions, and may have important implications for local fisheries and carbon cycle under the ongoing warming.