Carin J. Ashjian, Cabell S. Davis, Scott M. Gallager, and Peter H. Wiebe
Department of Biology
Woods Hole Oceanographic Institution
Woods Hole, MA 02543
Marguerite Bay is known to be a region where persistent populations of krill are found, even throughout the Antarctic winter. The Southern Ocean GLOBEC program seeks to describe how circulation and hydrography influence the distribution of krill on the shelf and how this contributes to successful overwintering by populations of krill in the region. The distribution of plankton, including larval krill, and hydrography were described during two cruises to Marguerite Bay, Antarctic Peninsula, in April-May (fall) and July-August (winter) 2001. The taxonomic and size composition of the zooplankton community and coincident hydrography were quantified using a Video Plankton Recorder and environmental sensors mounted on the BIOMAPER-II oceanographic surveying instrument and a 1-m2 MOCNESS. Broad-scale surveys of the Marguerite Bay region, including the continental shelf and slope, were conducted by towyoing the BIOMAPER-II from near-surface to near-bottom or 250 m. Larval and adult euphausiids, copepods, pteropods, polychaetes, and marine snow were observed with the video cameras. Abundances of all plankton were much reduced during the winter cruise, when the region was nearly completely ice covered, relative to during the fall cruise. The most striking and surprising finding of the fall cruise was that larval krill were distributed across the entire shelf from the offshore edge adjacent to the Antarctic Circumpolar Current to the coast. Greatest abundances of larval krill were observed subsurface in association with the pycnocline. Elevated abundances of adult krill were observed at depth within Marguerite Bay and in coastal waters west of Alexander Island. It is clear from the distributions of larval krill that they had spread across the shelf prior to the onset of winter.