Summer microbial ecology off East Antarctica (80–150°E): protistan community structure and bacterial abundance

We present a comprehensive protistan data set for the southern Indian sector of the Southern Ocean, sampled during the Austral summer (January–March) of 1996. Cells from discrete water bottle samples were examined to gain both qualitative and quantitative information on the composition, distribution and abundance of protistan organisms, together with bacterial abundance and distribution data. A total of 92 protistan taxa were identified by shipboard and transmission electron microscopic observations, including 52 diatom, 13 dinoflagellate and 24 nano-planktonic species. The diatom species assemblage was indicative of mature summer conditions. The most widespread species were Fragilariopsis curta and Nitzschia lecointei. Nitzschia spp. and related genera ( Fragilariopsis spp., Psuedonitzschia spp. and Cylindrotheca spp.), were also numerically dominant throughout the survey area (mean cell counts of 1.6×10 6 l −1 for cells 20 μm). The protistan community was determined to be biomodal, with a variable net-planktonic community superimposed on a stable nano-plankton base. The impact of macro and protozooplankton grazing on a protistan community of this nature is discussed. Both protistan and bacterial abundance decreased in the west of the survey area, coupled with an increase in taxonomic diversity. Changes in abundance-based community structure in the east are attributed to mesoscale variations in the availability of macronutrients. The decrease in abundance across the entire survey area was concurrent with the onset of winter conditions (decreased light availability and increased mixed layer depths), although evidence is presented to suggest that depressed biological activity in this region may be a persistant feature.