EFFECTS OF SEASONAL PACK ICE ON THE DISTRIBUTION OF MACROZOOPLANKTON AND MICRONEKTON IN THE NORTHWESTERN WEDDELL-SEA

The presence of mesopelagic organisms in the guts of surface-foraging seabirds feeding in open areas within seasonal pack ice in the Antarct ic has given rise to questions regarding the effects of pack ice on th e underlying mesopelagic community. Bottom-moored free-vehicle acousti c instruments were used in concert with midwater trawls and baited tra ps to examine the abundance, size distribution and vertical distributi on of pelagic organisms in the uppermost 100 m of the water column dur ing the austral spring of 1992 in two areas of the northwestern Weddel l Sea, one covered by seasonal pack ice and the other free of ice cove r. Acoustic targets were more abundant and significantly larger at the open-water station than beneath pack ice. However, targets at the ice -covered site exhibited a pronounced diel pattern, with the largest ta rgets detected only at night. Samples from night trawls at the ice-cov ered site contained several species of large, vertically-migrating mes opelagic fishes, whereas these species were absent from trawls taken d uring the day. In addition, baited traps deployed in pack ice just ben eath the ice-water interface collected large numbers of scavenging lys ianassoid amphipods, while deeper traps beneath the ice and traps at t he open-water station were empty, indicating the presence of a scaveng ing community associated with the undersurface of the ice. These resul ts support the idea that mesopelagic organisms migrate closer to the s urface beneath pack ice than in open water, exposing them to possible predation by surface-foraging seabirds.