Zooplankton community patterns in the Chukchi Sea during summer 2004

Zooplankton were sampled in the Chukchi Sea along three transects between Alaska and Russia, plus four high-speed transects across the axis of Herald Valley in August of 2004. A total of 50 holoplanktonic species, along with a prominent assemblage of meroplankton were encountered; most were of Pacific Ocean origin. Copepods represented the most diverse group with 23 species, and contributed the bulk (3100 ind. m−3, 30 mg dry weight m−3) of the total holozooplankton community abundance (3500 ind. m−3) and biomass (42 mg DW m−3) at most stations. Meroplanktonic larvae were, on average, almost as abundant (2260 ind. m−3) as the holozooplankton. Copepods were dominated numerically by four species of Pseudocalanus, Oithona similis, and the neritic copepods Acartia longiremis and Centropages abdominalis. The larger-bodied copepods, Calanus glacialis/marshallae and three Neocalanus species, equalled or exceeded the biomass of Pseudocalanus, followed by contributions from Metridia pacifica and Eucalanus bungii. Considerable abundance (256 ind. m−3) and biomass (42 mg DW m−3) of the larvacean Oikopleura vanhoeffeni was observed throughout the sampling area. The chaetognath Parasagitta elegans (4.8 mg DW m−3) and a diverse assemblage of cnidarians (∼1.2 mg DW m−3) comprised the dominant predators. Six major assemblages of zooplankton were identified, and each was closely tied to physical properties of water masses: Euryhaline species in the warm fresh Alaska Coastal Current, a Bering Sea assemblage of both shelf and oceanic species in cool salty Bering Sea Water, a transitional group between these two, a neritic Bering Sea assemblage in cold salty Bering Winter Water, and a small cluster of Arctic Shelf species in cold, fresh Resident Chukchi Water. Ongoing climate change may alter the boundaries, extent of penetration, size spectra, and productivities of these communities, thus warranting regular monitoring of the zooplankton communities of this gateway into the Arctic.