Indirect immunofluorescence was used to quantify the seasonal variation of four halophilic aerobic Antarctic bacteria in Antarctic saline lakes from July 1990 to January 1991. Antibodies were raised against type strains from the Australian Collection of Antarctic Microorganisms. During summer, all four serogroups were identified in the aerobic waters of lakes with total dissolved salts above 61"/00. Maximal abundances of Halomonas meridiana, H. subglaciescola and Flavobacterium gondwanense

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... roups were observed at discrete depths within the water column in the two most hypersaline lakes at about midsummer, coincident with the time of maximum sunlight and the commencement of the summer thaw. At this time the Halornunas spp. serogroups comprised up to 40% of the total bacteria and the F. gondwanense serogroup up to 10% of the total bacteria. The F. salegens serogroup was in low numbers (>2% of total bacteria) in some aerobic waters. Up to 2% of the total bacterial populations in the lakes were autofluorescent or stained non-specifically. Dissolved organiccarbonvalues were measured throughout the sampling period and correlated well with total bacterial numbers but not with changes in species composition. Change in species abundance, as indicated by imrnunofluorescence, was not reflected in the total bacterial count, indicating compositional change of the total bacterial population.