ALGAL AND BACTERIAL PROCESSES IN PLATELET ICE DURING LATE AUSTRAL SUMMER

The biota inhabiting layers of platelet ice were investigated in the W eddell Sea during late austral summer. Due to meltwater release, the s alinity of the interstitial water between platelets was reduced. Algae and bacteria accumulated within this ice environment attaining concen trations of up to 500 mu g in total pigments (chlorophyll alpha plus p haeopigments) and 2 mg in bacterial biomass per liter. Pennate diatoms of the genus Fragilariopsis were most common in the platelet layer, w hile ice-free water was dominated by autotrophic nanoflagellates. Prot ozoa contributed only 5% or less to the total protistan (microalgae pl us protozoa) cell concentration in the ice, compared to about 10% in o pen water, thus suggesting a low grazing pressure within the platelet habitat. The bulk of bacterial biomass occurred within the dense assem blages of pennate diatoms that grew attached to the ice platelets. Alg al and bacterial concentrations in the interstitial water between plat elets were much lower. Measurements of bacterial growth showed that su bstantial heterotrophic potential can be established within assemblage s inhabiting late summer platelet ice. Small-scale analyses of bacteri al activity patterns revealed that those bacteria that were closely as sociated with ice and/or algae showed considerably less biomass-specif ic substrate uptake than cells that occurred freely suspended in the i nterstitial water, indicating that their physiological state differed.