Summer microbial ecology off East Antarctica (80-150 degrees E): protistancommunity structure and bacterial abundance

We present a comprehensive protistan data set for the southern Indian secto r of the Southern Ocean, sampled during the Austral summer (January-March) of 1996, Cells from discrete water bottle samples were examined to gain bot h qualitative and quantitative information on the composition, distribution and abundance of protistan organisms, together with bacterial abundance an d distribution data. A total of 92 protistan taxa were identified by shipbo ard and transmission electron microscopic observations, including 52 diatom , 13 dinoflagellate and 24 nano-planktonic species. The diatom species asse mblage was indicative of mature summer conditions. The most widespread spec ies were Fragilariopsis curta and Nitzschia lecointei. Nitzschia spp, and r elated genera (Fragilariopsis spp., Psuedonitzschia spp. and Cylindrotheca spp.), were also numerically dominant throughout the survey area (mean cell counts of 1.6 x 10(6) 1(-1) for cells ( 20 mu m and 1.8 x 10(6) 1(-1) for cells > 20 mu m). The protistan community was determined to be biomodal, wi th a variable net-planktonic community superimposed on a stable nano-plankt on base. The impact of macro and protozooplankton grazing on a protistan co mmunity of this nature is discussed. Both protistan and bacterial abundance decreased in the west of the survey area, coupled with an increase in taxo nomic diversity. Changes in abundance-based community structure in the east are attributed to mesoscale variations in the availability of macronutrien ts. The decrease in abundance across the entire survey area was concurrent with the onset of winter conditions (decreased light availability and incre ased mixed layer depths), although evidence is presented to suggest that de pressed biological activity in this region may be a persistant feature. (C) 2000 Elsevier Science Ltd, All rights reserved.