SURFACE-LAYER SEA ICE ASSEMBLAGES IN ANTARCTIC PACK ICE DURING THE AUSTRAL SPRING - ENVIRONMENTAL-CONDITIONS, PRIMARY PRODUCTION AND COMMUNITY STRUCTURE

Favorable conditions for algal growth in sea ice develop during early spring as daylength and solar radiation increase. In the pack ice regions surrounding Antarctica, 'Surface-layer' and 'Internal' assemblages are the characteristic ice algal assemblages, whereas bottom-layer assemblages usually predominate in nearshore land-fast ice. Seasonal measurements of chlorophyll a have indicated little change in the algal biomass of internal assemblages, but very high concentrations (exceeding 400-mu-g Chl a l-1) have been reported for surface layer assemblages in late summer. These observations suggest that most of the pack ice-associated production may depend on the formation and development of the surface-layer assemblages. In the austral spring (Nov), we measured chlorophyll a concentrations up to > 54-mu-g Chl a l-1 in surface-layer assemblages from pack ice floes in the Weddell Sea. Production rates reached > 1.0 mg C l-1 d-1 with growth rates of algae ranging from 0.2 to 0.6 doublings d-1. Two experiments in a variable light regime indicated the algal assemblages were adapted to high light conditions. Well-developed heterotrophic populations (e.g. flagellates, ciliates, micrometazoans) were also present with heterotrophic biomass averaging 26% (range 1 to > 93%) of the total biomass among samples. Correlation analysis suggested shading by snow cover, and grazing possibly influenced growth and biomass accumulation. The observed growth rates are sufficient to account for the development of surface-layer assemblages found in spring into the very high concentrations reported in late austral summer. Calculations of algal biomass accumulation based on available nutrients, however, indicated that net nutrient transport from water into the ice assemblage would be necessary to reach such high densities. A pattern of high algal biomass along the edge ice floes, with biomass decreasing toward the floe center, suggest nutrient transport, but the role of light entering ice floes from the side and producing this distribution cannot be ruled out. Estimates of ingestion and respiration by heterotrophic members of the ice biota suggest that grazing could affect community development and succession. Field observations indicate the development of surface-layer assemblages is extremely patchy. It follows that accurate estimates of ice-associated production depend on a better understanding of their ecology.