Ecosystem impacts of marine heat waves in the Northeast Pacific

Abstract. Marine heatwaves (MHWs) are a recurrent phenomenon in the Northeast Pacific that impact regional ecosystems and are expected to intensify in the future. These events, including the 2014–2015 “warm blob,” are associated with widespread surface nutrient declines across the subpolar Alaskan Gyre (AG) extending south into the North Pacific Transition Zone (NPTZ) with reduced chlorophyll concentrations confined to the NPTZ only. Here we explain the contrast between these two regions using a coupled global ocean-biogeochemical model (MOM6-COBALT) with Argo float and ship-based observations to investigate how the MHWs influence the productivity of the two primary phytoplankton size classes (large > 10 μm, small < 10 μm) and the subsequent ecosystem response. Differences in seasonal iron and nitrate limitations between the AG and NPTZ explain the differences in ecosystem response to MHWs between the two biomes. The reduced nutrient supply during MHWs most strongly influences large phytoplankton in the NPTZ (-13 % annually), whereas it has a limited impact on the climatologically iron-limited large phytoplankton population in the AG (-2 %). Contrastingly, we find that MHWs yield a springtime increase in small phytoplankton population in both regions due to shallow mixed layers and lower light limitation. These primary production anomalies modify the allometric phytoplankton distribution, resulting in a 2 % decrease in the ratio of large to small phytoplankton in both regions. This shift in the assemblage towards small phytoplankton production is associated with reduced secondary and export production especially in the NPTZ.