Background nutrient concentration determines phytoplankton bloom response to marine heatwaves

Ocean temperature extreme events such as marine heatwaves are expected to intensify in coming decades due to anthropogenic global warming. Reported ecological and economic impacts of marine heatwaves include coral bleaching, local extinction of mangrove and kelp forests, and elevated mortalities of invertebrates, fishes, seabirds, and marine mammals. In contrast, little is known about the impacts of marine heatwaves on microbes that regulate biogeochemical processes in the ocean. Here we analyze the daily output of a near-global ocean physical-biogeochemical model simulation to characterize the impacts of marine heatwaves on phytoplankton blooms in 23 tropical and temperate oceanographic regions from 1992 to 2014. The results reveal regionally-coherent anomalies of shallower mixed layers and lower surface nitrate concentrations during marine heatwaves. Strengthened stratification is the plausible cause of such responses that exert counteracting effects on phytoplankton growth through light and nutrient limitation. Consequently, the responses of phytoplankton blooms are mixed, but can be related to the background nutrient conditions of the study regions. With one exception, blooms are weaker during marine heatwaves in nutrient poor waters, whereas in nutrient rich waters, the heatwave blooms are stronger. The corresponding analyses of sea-surface temperature and chlorophyll a concentration based on satellite observations support this relationship between phytoplankton bloom anomalies and background nitrate concentration. Given that nutrient poor waters are projected to expand globally in the twenty-first century, this study suggests increased occurrence of weaker blooms during marine heatwaves in coming decades, with implications for higher trophic levels and biogeochemical cycling of key elements.