Microbial community structure in the marginal ice zone of the Bellingshausen Sea

Phytoplankton, bacteria and microzooplankton were investigated on a transect in the Bellingshausen Sea during the ice melt period in November-December 1992. The transect along the 85 degrees W meridian comprised seven stations that progressed from solid pack-ice (70 degrees S), through melting ice into open water (67 degrees S). The abundance, biomass and taxonomic composition were determined for each component of the microbial community. The phytoplankton was mostly dominated by diatoms, particularly small (< 20 mu m) species. Diatom abundance ranged from 66000 cells l(-1) under the ice to 410000 cells l(-1) in open water. Phytoplankton biomass varied from <1 to 167 mg C m(-3), with diatoms comprising 89-95% of the total biomass in open water and autotrophic nanoflagellates comprising 57% under the ice. The standing stocks of autotrophs in the mixed layer ranged from 95 mg C m(-2) under the pack-ice to 9478 mg C m(-2) in open waters. Bacterial abundance in ice-covered and open water stations varied from 1.1 to 5.5 x 10(8) cells l(-1). Bacterial biomass ranged from 2.4 mg C m(-3) under pack-ice to an average of 14 mg C m(-3) in open water. The microzooplankton consisted mainly of aloricate oligotrich ciliates and heterotrophic dinoflagellates and these were most abundant in open waters. Their biomass varied between 0.2 and 54 mg C m(-3) with a minimum at depth under the ice and maximum in open surface waters. Microheterotrophic standing stocks varied between 396 mg C m(-2) under pack-ice and 3677 mg C m(-2) in the open waters. The standing stocks of the total microbial community increased consistently from 491 mg C m(-2) at the ice station to 13155 mg C m(-2) in open waters, reflecting the productive response of the community to ice-melt. The composition of the microbial community also shifted markedly from one dominated by heterotrophs (82% of microbial stocks) at the ice station to one dominated by autotrophs (73% of microbial stocks) in the open water. Our estimates suggest that the microbial community comprised > 100% of the total particulate organic carbon (POC) under the ice and 62-66% of the measured POC in the open waters.