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

Abstract

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°40'S, 68°25'E). Autotrophic biomass was low throughout the year ( 20 mg m~2) was associated with increased water column stability. During summer blooms, the >10 um size fraction contributed 60% to total integrated biomass. Large autotrophic dinoflagellates, mainly Prorocentrum spp., were associated with the summer phyto- plankton 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 Gyro- dinium spp. contributed a high proportion of total carbon biomass. During winter, the <10 um 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 106 cells I1.