Long-term Changes in Zooplankton Community Structure along the West Antarctic Peninsula

The West Antarctic Peninsula (WAP) is one of the most rapidly warming regions on Earth, and where a high apex predator biomass is supported in large part by macrozooplankton. The structure of zooplankton communities has a significant impact on higher trophic level production and on vertical flux of organic material and cycling of elements in the sea.  Thus, climate-induced changes in zooplankton abundance and species composition have the potential to dramatically affect the WAP food web and biogeochemical cycling. We are examining long-term (1993 to the present) trends in macrozooplankton abundance and species composition as part of the Palmer, Antarctica Long-Term Ecological Research Program (PAL LTER).  Zooplankton are collected along a sampling grid on the continental shelf of the WAP including inshore, mid-shelf, and at the shelf break.  Net tows are conducted in the top 120 m of the water column, and samples analyzed ship-board for species composition and biovolume.   Our results suggest that Antarctic krill (Euphausia superba) are decreasing in the northern part of the PAL LTER study region, and shifting closer inshore in the south.   This may be attributable to a decrease in the northern WAP in sea ice important to krill larval stages, or to changes in phytoplankton community structure.   Conversely, warmer water and ice-free conditions favor gelatinous salps (Salpa thompsoni), which are increasing over time, and expanding over the shelf in the north and south. Interestingly, calcium carbonate-shelled peteropods (Limacina helicina) are increasing in abundance and expanding their range as well. Whether ocean acidification will begin to affect these pteropod populations in the near future will be monitored by the LTER.  Other significant trends include an increase in carnivorous zooplankton (e.g., Themisto sp. amphipods, chaetognaths, and Tomopteris sp. polychaete worms). We will continue to explore how these long-term changes in zooplankton community composition in the WAP may affect energy transfer to higher trophic levels, and alter export of organic matter.