Distribution of Zooplankton on the Continental Shelf off Marguerite Bay, Antarctic Peninsula, during Austral Fall and Winter, 2001

Carin J. Ashjian*, Gaelin A. Rosenwaks¹, Peter H. Wiebe, Cabell S. Davis, Scott M. Gallager, Nancy J. Copley, Gareth L. Lawson, Philip Alatalo

Department of Biology, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

¹Present Address: Nichols School of the Environment, Duke University, Durham, NC

* Corresponding Author

cashjian@whoi.edu

The Marguerite Bay region of the Western Antarctic Peninsula is known to support a large population of krill during the summer and is hypothesized to be a site of successful overwintering of krill. The distributions of zooplankton biomass, abundance, and taxonomic composition at six locations on or near the continental shelf at Marguerite Bay were described during fall and winter, 2001, as part of the Southern Ocean GLOBEC program. Water column zooplankton were sampled using a 1-m² MOCNESS. Samples were analyzed for taxonomic composition, size, and estimated biomass using silhouette analysis. Characteristics of four taxonomic categories (small and large copepods, small and large krill) were described in detail. Both total biomass and abundance were markedly reduced in winter relative to fall. The vertical distributions of total abundance and biomass and of taxonomic composition were associated with the hydrographic structure of the water column. Larger larval krill were observed at all locations during winter than during fall, suggesting that growth (molting) had occurred between the sampling times. Krill were relatively unimportant to total biomass and abundance at most locations while copepods frequently dominated both biomass and abundance. IN fall, greatest abundance and biomass of larval krill in fall were observed on the continental shelf in association with a persistent clockwise gyre located to the west of Adelaide Island. During winter much lower abundances of larval krill were present throughout the study region. High abundance and biomass of juvenile and adult krill were observed only in Laubeuf Fjord (Marguerite Bay) during fall. Retention of larval krill in the study area did not appear to take place in the water column, but may have taken place within the pack ice throughout the area.

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