The bacteria from different phylogenetic groups were studied in surface microlayer (SM, up to 100 [mu]m ) versus subsurface water (SW - 20 cm) in eutrophic lake from spring to autumn of 2007. Abundance of bacteria was determined using a combination of direct counting of 4', 6-diamidino-2-phenylindole and the phylogenetic diversity was determined in fluorescence in situ hybridization (FISH) method with group-specific, fluorescently labeled oligonucleotide probes. The numbers of DAPI bacteria varied between 4.75 and fluorescence in situ hybridization (FISH) demonstrated that Eubacteria constituted the majority of the whole bacterial population and their percentage share ranged from 59 to 75%. Abundances of alpha- beta-Proteobacteria and Cytophaga-Flavobacteria groups varied across seasons, layers, and lacustrine zones. The lowest number of alpha-Proteobacteria group bacteria was observed in spring (SM - 0.2 x 10[^6], SW - 0.16 x 10[^6] cells cm[^-3]), whereas the highest in autumn (SM - 0.62 x 10[^6], SW - 1.6 x 10[^6] cells cm[^-3]). The percentage share of these groups of bacteria in the Eubacteria domain was lower in spring (20.50%) than in summer and autumn (from 65 to over 80%). No fixed difference between the composition of SM and SW bacteria was noticed. Seasonally occurred changes are similar in both layers.
