Changes in the sea-ice brine community during the spring-summer transition, McMurdo Sound, Antarctica. II. Phagotrophic protists

Abstract

The land-fast sea-ice brine contains a diverse phagotrophic protist assemblage consisting of <5 pm heterotrophic flagellates, Cryothecomonas spp., heterotrophic dinoflagellates, and heterotrophic and rnixotrophic ciliates. Fine-scale horizontal spatial variability is a feature of this assemblage; samples taken within 1 m of each other can be dominated by different heterotrophic protists. Many of the larger heterotrophic protists found in the brine are also found in the water column. The photosynthetic ciliate Mesodinium rubrum is also common. In mid to late austral spring, the heterotrophic assemblage accounts for ca 10% of the total protist biomass in the brine and is dominated by Cryothecornonas spp. This flagellate can reach densities of over 106 cells I-' of brine. In the early austral summer, ciliates (primarily Strornbidium spp., Mesodiniurn rubrum and Didinium spp.) and heterotrophic dinoflagellates (primarily a small Gyrnnodinium sp. and Polykrikos sp.) increase in abundance in the brine. Ciliate densities of 2 3 X 103 1-' and heterotrophic dinoflagellate densities of 104 cells 1-' are common in the brine during early summer. By the end of January Cjust prior to ice decay and break-out), heterotrophic flagellates and ciliates can account for 50 % of the protist biomass.