ECOLOGICAL STRUCTURE AMONG MIGRANT AND RESIDENT SEABIRDS OF THE SCOTIA-WEDDELL CONFLUENCE REGION

1. We quantitatively assess seasonal changes in community structure an d habitat selection among seabirds in the Scotia-Weddell Confluence re gion, Antarctica. Additionally, we discuss the biological and physical factors underlying the patterns. Data were derived from strip-transec ts on closely-coordinated multidisciplinary cruises that characterized the physics and biology during spring 1983, autumn 1986 and winter 19 88. 2. We describe for the first time ever for the Southern Ocean seas onal changes in seabird communities in terms of composition, using clu ster analysis, as well as relative density and diversity among species . Sea-surface temperature, distance to the pack ice edge and ice type, all physical characteristics of habitat, were the most important envi ronmental variables that affected assemblage composition. We identifie d three recurrent assemblages of species. One persistent assemblage, p resent year round, was associated with the pack ice; another was assoc iated with open waters immediately adjacent to the ice; and a third wa s a far-from-ice assemblage. Only the two open-water assemblages chang ed markedly on a seasonal basis. 3. Close similarity of patterns in th e spring 1983 data with those collected during spring 1976 in the Ross Sea, on the other side of Antarctica, supported our contention that w e were comparing seasonal and not interannual differences in community structure. 4. In spite of a major reduction in the number and density of species in the open-water assemblages during winter, the pack-ice assemblage exhibited no habitat expansion, which might be expected if competition affected community structure and habitat selection. The pa ck ice, in contrast to the adjacent open water, is a habitat in which food is abundantly available year round for an assemblage of species s pecialized to exploit the opportunities present. Unspecialized species vacate the region when food becomes sparse.