Diatom assemblages in sediments of Lake Juusa, Southern Estonia with an assessment of their habitat

Diatom analysis of surface sedimentsand two sediment cores from different sedimentation areas of a small closed lake was undertaken with the aim of acquiring knowledge on the dependence of the distribution of diatom assemblages on lake bathymetry. Lake Juusa was selected for the study because we have for this lake a large data set about the lithological composition of sediments and macrofossil and cladoceran records for the Holocene. A high carbonate content (20-60%) in the sediment sequence indicates high carbonacity and relatively stable pH values during the Holocene. On the basis of comprehensive analysis, abrupt water-level fluctuations and changes in the trophic status were established. Results of this study showed that the fluctuations of the water-level were the leading factor determining the habitats of diatom assemblages in the lake. In the surface sediment samples planktonic species such as Cyclotella spp., Stephanodiscus spp. and Aulacoseira spp. had a depth optimum at 3-4 m and the most abundant periphytic taxa were distributed mostly at depths shallower than 3.5 m. The same regularity was established in sediment cores where a good correlation between planktonic species and lake water depth was found in sediments accumulated at water depths > 4 m. Lake Juusa appears to be a proper site for detailed environmental reconstructions over the Holocene, and the results will give us a good opportunity to analyse the history of water-level fluctuations in other small Estonian lakes.