Algal and bacterial processes in platelet ice during late austral summer

The biota inhabiting layers of platelet ice were investigated in the Weddell Sea during late austral summer. Due to meltwater release, the salinity of the interstitial water between platelets was reduced. Algae and bacteria accumulated within this ice environment attaining concentrations of up to 500 μg in total pigments (chlorophyll a plus phaeopigments) and 2 mg in bacterial biomass per liter. Pennate diatoms of the genusFragilariopsis were most common in the platelet layer, while ice-free water was dominated by autotrophic nanoflagellates. Protozoa contributed only 5% or less to the total protistan (microalgae plus protozoa) cell concentration in the ice, compared to about 10% in open water, thus suggesting a low grazing pressure within the platelet habitat. The bulk of bacterial biomass occurred within the dense assemblages of pennate diatoms that grew attached to the ice platelets. Algal and bacterial concentrations in the interstitial water between platelets were much lower. Measurements of bacterial growth showed that substantial heterotrophic potential can be established within assemblages inhabiling late summer platelet ice. Small-scale analyses of bacterial activity patterns revealed that those bacteria that were closely associated with ice and/or algae showed considerably less biomass-specific substrate uptake than cells that occurred freely suspended in the interstitial water, indicating that their physiological state differed.