Spatial Heterogeneity in Phytoplankton Responses to Marine Heatwaves in the Northeast Subarctic Pacific

Abstract The northeast subarctic Pacific Ocean (NESAP) has experienced several extreme marine heatwaves in recent years, with potentially significant ecosystem‐level effects. To date, the spatial variation in marine heatwave impacts on this ocean region remains underexplored, despite known differences in the physical and biogeochemical controls on phytoplankton dynamics in coastal and offshore waters, including a natural gradient of iron limitation. Using satellite‐derived estimates of chlorophyll‐ a concentrations and primary productivity spanning the past quarter century (1998–2023), supported by field observations and output from an ocean biogeochemical model, we found contrasting marine heatwave responses between shelf‐break and oceanic regions in the NESAP. During marine heatwaves, shelf‐break areas showed enhanced stratification with corresponding reductions in macro‐nutrient supply and primary productivity. In contrast, iron‐limited offshore waters with excess macro‐nutrients exhibited weaker marine heatwave responses. Our spatially‐resolved analysis suggests varying sensitivities to marine heatwaves across the NESAP in relation to underlying patterns of nutrient limitation. This result has implications for ecosystem responses under future warming scenarios.