Ecosystem impacts of marine heat waves in the northeast Pacific

Abstract. 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.
