ИСТИНА

On the Karelian coastline, as a result of post-glacial land uplift, bays are being gradually separated from the White Sea and turning into freshwater basins, passing a meromictic phase. Lake Kislo-Sladkoe and lake Elovoe under the investigation are at different stages of isolation. While stratified, they possess a five-layered hydrological structure where the desalinated surface layer, halocline, aerobic saline zone, chemocline, and hydrogen sulfide zone can be distinguished. Such conditions lead to the thriving of unique microalgal biota. The purpose of this study is to compare the dynamics of phytoplankton structure in both lakes in order to reveal any trends. To examine phytoplankton, water samples were taken from several horizons up to the hydrogen sulphide zone using a submersible pump, and concentrated by the reverse filtration method. Algal cells were identified, measured, and counted in a Najotte chamber under a light microscope. To calculate carbon biomass, biovolumes were determined by mathematical equations based on the geometric shapes of cells, and converted into carbon content. Similarity analysis of phytoplankton structure was performed via statistical nMDS, one-way ANOSIM, and SIMPER procedures. As a part of the study, 64 phytoplankton species were registered in Kislo-Sladkoe, while 70 species were catalogued in Elovoe, all belonging to Bacillariophyta, Myzozoa, Cyanobacteria, Ochrophyta, Chlorophyta, Charophyta, Cercozoa, Cryptophyta, and Euglenozoa. The phytoplankton structure in the lakes varied both in species composition and seasonal dynamics. The maximum biomass value for the growing season in Kislo-Sladkoe was significantly higher than that corresponding in Elovoe (149.8 mg C/m2 and 57.4 mg C/m2, respectively). Statistical analysis showed that most samples reliably clustered into four groups that were dissimilar by ≥91%. Group I includes samples of under-ice phytoplankton from Kislo-Sladkoe with Microcystis ichthyoblabe, Amphora sp. and cryptomonads as characteristic species. Group II consists of samples from mainly upper horizons of Kislo-Sladkoe in May-August where the determinative species are Cyclotella choctawhatcheeana and Microcystis ichthyoblabe. Group III comprises most samples from Elovoe where Gyrosigma obscurum, Melosira nummuloides and Desmodesmus armatus define similarity within the community. Group IV includes samples from chemocline of both lakes with Oxyrrhis marina acting as a characteristic species. Thus, at the beginning of the season the lakes differed in phytoplankton communities. However, later, due to the more intensive growth of organisms associated with chemocline as the most productive layer, Oxyrrhis marina formed a community typical for both lakes.