Seasonal and interannual variability of size-fractionated phytoplankton biomass and community structure at station Kerfix, off the Kerguelen Islands, Antarctica

Time series of phytoplankton biomass and taxonomic composition have been obtained for the 3 years 1992, 1993 and 1994 in the northern part of the :Southern Ocean (station Kerfix, 50 degrees 40'S, 68 degrees 25'E). Autotrophic biomass was low throughout the year ( <0.2 mg m(-3)), except during a short period in summer when a maximum of 1.2 mg chlorophyll Chl) a m(-3) was reached, During winter, the integrated biomass was low (<10 mg m(-2)) and associated with deeply mixed water, whereas the high summer biomass (>20 mg m(-2)) was associated with increased water column stability. During summer blooms, the >10 mu m size fraction contributed 60% to total integrated biomass. Large autotrophic dinoflagellates, mainly Prorocentrum spp., were associated with the summer phytoplankton maxima and accounted for >80% of the total autotroph carbon biomass. In November and December, the presence of the large heterotrophic dinoflagellates Protoperidinium spp. and Gyrodinium spp. contributed a high proportion of total carbon biomass. During winter, the <10 mu m size fraction contributed 80% of total Chi a biomass with domination of the picoplankton size fraction. The natural assemblage included mainly naked flagellates such as species of the Prasinophyceae, Cryptophyceae and Prymnesiophyceae. During spring, picocyanobacteria occurred in sub-surface water with a maximum abundance in September of 10(6) cells l(-1).