Due to postglacial isostatic uplift many stratified lakes, at different stages of isolation, are located along the shores of the White Sea. In five lakes, located near the White Sea Biological Station of Moscow State University, salinity, temperature, pH, concentration of dissolved oxygen, redox potential, and illuminance were measured. Distribution of microorganisms and spectral properties of water layers were also studied. All the lakes had a narrow bright coloured layer in the redox zone caused by mass development of phototropic microorganisms. Light absorption and fluorescence spectra indicated algae containing chlorophyll a predominate in the red water layers while the colouration of green and brown layers is caused by green sulphur bacteria with bacteriochlorophylls d and e. Sunlight is completely absorbed in the redox zone because of the high density of algae and/or bacteria, resulting in aphotic conditions below. Coloured layers act as a specific biotope for special communities of microorganisms. Eukaryotes identified by the 18S rRNA gene included different species of mixotrophic algae and ciliates resistant to anoxia. The water layer colour and spectral characteristics (i.e. light absorption and fluorescence) of water in the redox zone can be considered indicators of the stage of lake isolation from the sea, with the red colour caused by cryptophyte alga Rhodomonas sp. bloom found in earlier stages and brown and green colours caused by green sulphur bacteria in later stages.

Bloom of a cryptomonad alga Rhodomonas sp. (Cryptophyta, Pyrenomonadaceae) was observed in the chemocline of saline basins separating from the White Sea, resulting in red coloration of the relevant water layer. According to the sequence of the 18S nuclear rRNA gene, this species was identical to Rhodomonas sp. RCC2020 (GenBank accession no. JN934672) from the Beaufort Sea. The presence of the red layer formed by mass development of Rhodomonas sp. is considered an indicator of a certain stage of separation of a basin from the sea.

Based on the analysis of scientific literature for the period from the end of the XIX century to the present, as well as the author’s own research, the information about meromictic waterbodies in Russia has been collected. The list includes 54 water bodies: 31 coastal water bodies of marine origin and 22 inland waterbodies, as well as the Black Sea, the world largest meromictic waterbody. An overview of the main ecological features of the most numerous category of Russian meromictic waterbodies, the coastal waterbodies of marine origin, is presented.

A non-extraction method for determining the concentration of chlorosomal bacteriochlorophylls (Bchl), photosynthetic pigments of green sulfur bacteria has