Ecosystem impacts of marine heat waves in the northeast Pacific

[摘要] Marine heat waves (MHWs) are a recurrent phenomenon inthe northeast Pacific that impact regional ecosystems and are expected tointensify in the future. Prior work showed that these events, including the2014–2015 “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. Herewe explain the contrast between these two regions using a globalocean biogeochemical model (MOM6-COBALT) with Argo float and ship-basedobservations to investigate how MHWs influence marine productivity. We findthat phytoplankton and zooplankton production respond relatively modestly toMHWs in both regions. However, differences in the response to seasonal ironand nitrogen limitation between large ( > 10   µ m) and small( 10   µ m) phytoplankton size classes explain the differencesin ecosystem response to MHWs across the two biomes. During MHWs, reducednutrient supply limits large phytoplankton production in the NPTZ ( − 13 %annually) but has a limited impact on the already iron-limited largephytoplankton population in the AG ( − 2 %). In contrast, MHWs yield aspringtime increase in small phytoplankton in both regions due to shallowermixed layers and weaker light limitation. These modest changes are inapparent contradiction with prior estimates suggesting a collapse in netcommunity production during the warm blob. We show, however, that 70 % ofthe decline in net community production previously calculated from nitrateArgo data can be attributed to artifacts in the method and that only 30 %can be attributed to interannual variability, in line with our model-basedresults. Although modest, the primary production anomalies associated withMHWs modify the phytoplankton size distribution, resulting in a significantshift towards small phytoplankton production (i.e., lower large-to-small-phytoplankton ratio) and reduced secondary and export production, especiallyin the NPTZ.

[学科分类] 大气科学