Low seasonal variability in community composition of sediment bacteria in large and shallow lake.

The community composition of bacteria with highly dynamic abundance and activity was observed to be with low variability in a shallow lake sediment with frequent physical disturbance. This suggests that physical disturbance did not create more niches and did not lead to highly variable bacterial community. The major part of the bacterial community was homogeneous, with 40% of phylotypes being ubiquitous and present in all samples. A minor part was responding to two contrasting periods - permanent ice cover and open lake. During the period of ice cover the total number of phylotypes decreased by ∼ 10%, the productivity of sediment bacteria varied by 15-fold (decreased by 40% under ice) and the abundance of bacteria by up to sixfold (decreased by 20%), suggesting that community of sediment bacteria with stable species composition might be highly dynamic in numbers and activity. Phylotypes identified by 16S rRNA gene sequencing were close to those observed in similar environments. All sequences were closely related only to uncultured phylotypes. Proteobacteria, particularly of the Beta subgroup, were the most common identified species in Lake Võrtsjärv sediment samples. A few other phylotypes, mostly those typical of anoxic sediments, were observed, but were uncommon.