Diel Vertical Distribution Patterns of Zooplankton along the Western Antarctic Peninsula

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The Western Antarctic Peninsula (WAP) is a highly dynamic region of the Southern Ocean that has undergone significant change over the past several decades due to unprecedented increases in air and sea surface temperature, and decreases in sea ice cover. The ongoing Palmer Antarctica Long-Term Ecological Research (PAL LTER) study shows that these environmental changes are significantly affecting the marine pelagic ecosystem along the WAP, including shifts in relative abundance of the dominant zooplankton. The goal of this study was to analyze diel vertical distribution patterns of zooplankton along a north to south gradient of the WAP. Diel vertically migrating zooplankton move between surface waters at night where they feed, and the mesopelagic zone where they reside during the day. However, during the austral summer in polar regions, since there is only a brief daily period of (or no) darkness, zooplankton may not exhibit vertical migration behavior. We determined occurrence and magnitude of zooplankton diel vertical migration by conducting day and night Multiple Opening and Closing Net and Environmental Sensing System (MOCNESS) tows at discrete depth intervals through the epipelagic and mesopelagic zones from 2009-2015, and with epipelagic net tows from 1993-2013. Night densities in surface waters for each taxon were compared to day, and the mean night:day ratio (N:D) was determined for each taxon. Taxa that consistently exhibited diel migration in all years included calanoid copepods (e.g., Metridia gerlachei and Calanoides acutus) and ostracods. Other taxa migrated only during some years (e.g., Limacina helicina and Euphausia crystallorophias). The Antarctic krill Euphausia superba did not vertically migrate, but the abundant gelatinous salp, Salpa thompsoni, did with a N:D ratio of 2. We discuss our results in the context of inter-annual and north-south variation in environmental parameters such as sea ice, as well as the implications of this variation for food-web dynamics and zooplankton-mediated export of carbon to depth.