Abstract. Analysis of long-term observations of dominant diatom species of spring phytoplankton of Lake Baikal Synedra acus subsp. radians, Aulacoseira baicalensis, Aulacoseira islandica, Stephanodiscus meyeri and Nitzschia graciliformis was demonstrated. It is shown, how the abundance of these species varied in Southern, Central and Northern Baikal in 2007-2016 compared to other observation periods in 1964-1968 and 1969-1984. It is found out that, species ratio changed in 2007-2016. The abundance of S. acus subsp. radians increased in Southern and Central Baikal, while for other species - A. baicalensis, A. islandica, N. graciliformis and S. meyeri, it significantly decreased. In Northern Baikal, the ratio of species in 2007-2016 was comparable to 1969-1984. Despite the change in the ratio of species over the past decade, the contribution of these five dominant species to the total abundance of spring phytoplankton of Lake Baikal has comparable values in all studied periods.

Abstract. The review examines the work on biodiversity in the reservoirs of Eastern Siberia of a wide, but poorly studied group of organisms forming siliceous scales and bristles these are scaled chrysophytes, chrysophycean stomatocysts and heterotrophic protists: rotosphaerids, colorless free-living thaumatomonad flagellates, centrohelid heliozoans. The difficulty in studying these objects is the need to use electron microscopy for their species identification. High biodiversity of silica scaled Protozoa has been revealed in the reservoirs of Eastern Siberia and the relationship of their species composition with the parameters of the aquatic environment, including in areas of local anthropogenic impact, has been shown. The recorded enrichment of the mouths of Arctic rivers with boreal species is important for predicting changes in aquatic ecosystems in the context of GCC. The proposed scenario of the settlement of siliceous chrysophytes at the beginning of the Holocene may be valid for other small planktonic organisms. The high degree of preservation of siliceous stomatocysts in sediments allows them to be used as an additional signal of changes in lake ecosystems in the past, this is based in particular on the reconstruction of the ecosystem and changes in the level of Lake Vorota (Yakutia) in the Holocene-Upper Pleistocene.