Ecosystem impacts of marine heat waves in the northeast Pacific

Marine heat waves (MHWs) are a recurrent phenomenon in the northeast Pacific that impact regional ecosystems and are expected to intensify in the future. Prior work showed that these events, including the 2014-2015 "warm blob", are associated with widespread surface nutrient declines in the subpolar Alaska Gyre (AG) and the North Pacific Transition Zone (NPTZ) but reduced chlorophyll concentrations in the NPTZ only. Here we explain the contrast between these two regions using a global ocean biogeochemical model (MOM6-COBALT) with Argo float and ship-based observations to investigate how MHWs influence marine productivity. We find that phytoplankton and zooplankton production respond relatively modestly to MHWs in both regions. However, differences in the response to seasonal iron and nitrogen limitation between large (>10 μm) and small (<10 μm) phytoplankton size classes explain the differences in ecosystem response to MHWs across the two biomes. During MHWs, reduced nutrient supply limits large phytoplankton production in the NPTZ (-13 % annually) but has a limited impact on the already iron-limited large phytoplankton population in the AG (-2 %). In contrast, MHWs yield a springtime increase in small phytoplankton in both regions due to shallower mixed layers and weaker light limitation. These modest changes are in apparent contradiction with prior estimates suggesting a collapse in net community production during the warm blob. We show, however, that 70 % of the decline in net community production previously calculated from nitrate Argo data can be attributed to artifacts in the method and that only 30 % can be attributed to interannual variability, in line with our model-based results. Although modest, the primary production anomalies associated with MHWs modify the phytoplankton size distribution, resulting in a significant shift towards small phytoplankton production (i.e., lower large-to-small-phytoplankton ratio) and reduced secondary and export production, especially in the NPTZ.