INFLUENCE OF PHYSICAL AND BIOLOGICAL MESOSCALE DYNAMICS ON THE SEASONAL DISTRIBUTION AND BEHAVIOR OF EUPHAUSIA-SUPERBA IN THE ANTARCTIC MARGINAL ICE-ZONE

The influence of day length, currents, sea ice presence, seawater temperature and salinity, chlorophyll a, particulate organic carbon, and predators was investigated in relation to the distribution and behavior of life history stages of Euphausia superba Dana in the marginal ice zone of the Weddell and Scotia Seas during autumn, winter, and spring. Physical processes control the extent of ice cover, the magnitude and location of food, and the distribution of pack ice predators; however, physical processes did not appear to directly affect krill. Instead, the seasonal distribution and behavior of krill was interpreted to be a function of the need to acquire food and avoid predators. These 2 factors also are hypothesized to be the proximate cause of swarming during our study. Seasonal sea ice plays an integral role in the ecology of krill. Ice-edge blooms are an important and predictable food supply, particularly for reproducing adults and first-feeding larvae. Ice floes provide protection for larvae and juveniles, and sea ice biota, a widespread food source, are important to the survival of larvae during winter. In the marginal ice zone, overwintering strategies of adults included regression to an immature (sub-adult) stage, reduction of metabolic rate, and omnivorous feeding in the water column. Adults were not observed feeding on the undersurface of ice floes probably because of increased risk of predation from pack ice predators. However, adult krill may migrate deeper into the pack ice in winter and also feed on ice biota. We conclude that sea ice biota act as a stabilizing mechanism against extreme seasonal oscillations of food supply for overwintering krill, thus contributing to the persistence of populations of E. superba.