Аннотация:The meromictic coastal water bodies have a specific vertical structure consisting of the layers with different hydrological, hydrochemical parameters and plankton communities, which have been poorly investigated, especially protist component. Here we report the first data on the species richness and taxonomic composition of protists in the Arctic coastal meromictic Lake Trekhtzvetnoe (Kandalaksha Bay, White Sea). This lake is the second meromictic water body on the coast of the White Sea, where the protist communities have been comprehensively studied using light, transmission and scanning electron microscopy. A total of 189 species and forms of heterotrophic, mixotrophic and phototrophic protists assigned to different taxonomic groups have been recorded, including 50 species new for the White Sea region. The supergroup TSAR was the richest, with Stramenopliles and Alveolata showing the largest number of species. A taxonomic list and micrographs of cells for observed protists are provided. Most species of protists are characterized by a wide geographical distribution. Half of the found species is freshwater. Taxonomic composition of the protist communities varied in the main hydrological zones (myxolimnion, chemocline and monimolimnion) according to the vertical stratification of hydrological and hydrochemical parameters. The similarity and differences of protist communities have been compared with those of another Arctic meromictic Lake Kislo-Sladkoe.
