CHANGES IN THE SEA-ICE BRINE COMMUNITY DURING THE SPRING-SUMMER TRANSITION, MCMURDO SOUND, ANTARCTICA .1. PHOTOSYNTHETIC PROTISTS

During the austral spring, a characteristic microbial community develops in the subsurface brine pockets and channels of the annual land-fast sea-ice in McMurdo Sound. This community is distinct from the diatom-dominated community that develops in the channels at the base of the sea-ice, at the seawater/ice interface, and in the platelet layer. The photosynthetic biomass in the brine pockets is dominated by athecate dinoflagellates. Chrysophyte statocysts (sometimes known as archaeomonads) and < 5-mu-m photosynthetic flagellates are also characteristically found in this assemblage. In December, chlorophyll a content and biomass peak, and photosynthetic gymnodinioid dinoflagellates can reach densities of over 10(3) ml-1 of brine. The photosynthetic dinoflagellates form cysts (hypnozygotes) during late December and early January, and chrysophyte statocysts also become abundant. During austral summer, total autotrophic biomass in the upper ice brine decreases due to dilution by melt water, flushing of brine into the water column, and grazing. By late summer, the annual sea-ice in McMurdo Sound has broken out. The yearly decay and retreat of sea-ice introduces a characteristic set of brine protists and their cysts into McMurdo Sound.