Comparing zooplankton community structure and diel vertical migration in the subarctic Northeast Pacific and Atlantic oceans

Zooplankton community structure plays a key role in determining the fate of net primary production and export of organic matter to the deep sea. We compared diel changes in zooplankton (>200 µm) vertical structure in the subarctic NE Pacific and Atlantic oceans during late summer 2018 and spring 2021, respectively, by analyzing biomass size structure and abundance of major taxa in discrete depth intervals between 0-1000 m. Diel vertical migration (DVM) was pronounced at both sites, with most taxa migrating from daytime depths of ~300-500m into the surface 0-100m at night. Total dry weight biomass was 2 to 5-fold higher in the NE Atlantic than the NE Pacific, except for depths >200m at night. Zooplankton biomass during month-long sampling at both sites was relatively unchanged in the NE Pacific and influenced by passage of four storms in the NE Atlantic. Biomass in the NE Pacific was mostly in the larger (>2mm) size fractions due to large, seasonally migrating copepods Neocalanus and Eucalanus spp., and salps, versus in the NE Atlantic in the epipelagic zone where smaller (<2 mm) size classes comprised most of the biomass. Night:day biomass and abundance ratios in the epipelagic zone and migration amplitudes increased with increasing animal size, due to effects of size on susceptibility to predation regulating DVM behavior. Calanoid copepod genera at both sites that were strong, long-distance migrators included Metridia and Pleuromama spp., and in the NE Pacific there was a short-distance DVM from 50-100m in the day into the top 0-50 m at night by Neocalanus cristatus. Themisto compressa amphipods comprised a sizable component of the DVM community in the NE Atlantic. Large gelatinous zooplankton such as the salp Salpa aspera in the NE Pacific and the pteropods Cymbulia peroni and Clio recurva in the NE Atlantic were also conspicuous diel migrators. This diversity in zooplankton taxa, life history, and size, along with trophic ecology and physiology, can be used to improve predictions of future changes in carbon flows and export.