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On a model natural object, the Lake Baikal–Angara River–Irkutsk Reservoir (IR), we studied changes in the qualitative and quantitative characteristics of phytoplankton communities over three seasons in 2023 depending on seasonal changes in habitat parameters. Of the 151 identified taxa, Chrysophyta (57), Chlorophyta (41) and Bacillariophyta (24) predominated in diversity. Over the entire observation period, the highest values of total biomass and total abundance were detected in the IR in June (hydrological spring) at a water temperature of 10.0–12.7 °C, and the lowest in August, despite the fact that the water warmed up to 20 °C. No mass blooms of Cyanobacteria were observed. Statistical analysis of species abundance profiles revealed that phytoplankton community structure varied across time and space. The direct effect of cold lake waters on the structure of phytoplankton in the reservoir was observed only in early June. In summer and autumn, the structures of phytoplankton in the lake and in the reservoir differed, even at the same water temperature. Low concentrations of phosphates and nitrates, high species diversity, the presence of cold-water species and species with a wide range of temperature preferences formed a dynamic spatiotemporal structure of IR phytoplankton, distinct from other temperate reservoirs, including Lake Baikal. The results obtained are important for understanding the mechanisms of formation of the flora of artificial reservoirs of temperate latitudes and for their monitoring, taking into account seasonal dynamics and the context of global climate warming. [ABSTRACT FROM AUTHOR]

In the context of global climate changes, it is important to assess the sustainability perspective of aquatic ecosystems based on marker organisms. In this work, we analyzed seasonal dynamics of silica-scaled chrysophytes in freshwater communicating environments which have considerable differences in water temperature between two ecosystems: the deep and cold oligotrophic Lake Baikal versus the shallower and warmer downstream Irkutsk Reservoir having mesotrophic features. During three seasons of the open water period of 2023, 38 species of silica-scaled chrysophytes were observed at 17 stations using scanning and transmission microscopy. The distribution of silica-scaled chrysophytes was shown to correlate with the water temperature. The greatest species richness was observed in the spring season in a large bay of the Irkutsk Reservoir (23 species), the smallest in the cold spring waters of Southern Baikal (up to 7 species). Widespread species living in Southern Baikal continued to grow in warmer waters of the reservoir. Using the example of silica-scaled chrysophytes, the stability of the high-latitudinal freshwater ecosystems affected by climate change is discussed. Continuous increment of the water temperature can lead to an increased abundance of widespread species and the displacement of psychrophilic species, affecting the overall biodiversity in such ecosystems. [ABSTRACT FROM AUTHOR]

Two new species of silica-scaled chrysophytes (order Synurales) from the genus Mallomonas and sections Striatae Mallomonas baicalensis sp. nov. and M. grachevii sp. nov. found at the bottom surface of the ice of Lake Baikal, with a structure of siliceous scales, are described using electron microscopy. The main and unique distinctive feature of M. baicalensis is its dome with a long spine and the slightly asymmetrical shape of its scales, regardless of their position on the cell. We could not find the bristles, and if there are spines on the dome, we can assume that they may be missing. The main distinguishing feature of M. grachevii is the presence of a secondary layer on the shield except at the angle of the V-rib, in which an area without the secondary layer, a "window", is present with numerous pores, and the first transverse rib closest to the dome is thickened. Among the Mallomonas species from the section Striatae, only M. siveri and M. baicalensis have a group of rimmed pores in the corner of the V-rib. As a result of our research, the number of Mallomonas species of the section Striatae found in Lake Baikal has increased to eight, of which, in addition to the new species, only one species is widespread, and the rest are rare, previously foundin oligotrophic/mountain water bodies. [ABSTRACT FROM AUTHOR]

Heterotrophic protists Rotosphaerida, Thaumatomonadida, and centrohelid heliozoans are among the less studied silicified protists in terms of their biogeography and ecology. These organisms inhabit fresh and brackish water, and leave behind siliceous structural elements after death that are species‐specific and amenable to electron microscopic analysis. This paper is the first to present data on species richness and taxonomic structure of silica‐scaled heterotrophic protists—rotosphaerids, colorless free‐living thaumatomonad flagellates and centrohelid heliozoans—in the large continuous water system of Siberia connecting Lake Baikal to the Kara Sea. In the study area, electron microscopy revealed 21 centrohelid heliozoan species from the genera Raphidiophrys (1), Acanthocystis (7), Choanocystis (3), Raineriophrys (2), Raphidocystis (6), and Pterocystis (2), seven rotosphaerid species from the genera Pinaciophora (3), Turriplaca (2), Rabdiophrys (1), and Pompholyxophrys (1), and one thaumatomonad flagellate species Thaumatomastix. Two species of rotosphaerids, Rabdiophrys cf. anulifera and Pinaciophora tridentata, and two species of centrohelid heliozoans, Acanthocystis cf. tubata, and A. cf. cornuta, were found in the waters of Russia for the first time. The most widespread species in fresh water from Lake Baikal to the Lower Yenissei River were Pinaciophora fluviatilis and Raineriophrys cf. fortesca. These species disappeared from the protist assemblages of the Yenissei gulf of the Kara Sea due to higher salinity, with only three species, Acanthocystis cf. mylnikovi (at 3.12‰ salinity) and A. pectinata and Raphidocystis sp. 2 (at 8‰ salinity), found in the area. The sensitivity of silica‐scaled heterotrophic protists to changes in habitat parameters suggests that these microeukaryotes could be new indicator organisms. [ABSTRACT FROM AUTHOR]

The present study examined the taxonomic diversity of silica-scaled chrysophytes in the estuaries of the Arctic watershed of Yakutia in the context of global climate change, as these aquatic organisms are highly sensitive to environmental changes. Previously, 41 species of silica-scaled chrysophytes were recorded in the waters of Yakutia. In the present study, we supplemented this list with 55 species. We observed a high species richness (82 taxa) of silica-scaled chrysophytes in the study area. Of these, eight species were recorded in the waters of Russia for the first time. At present, the study area represents the northernmost habitat for most species observed during the study. The diversity of silica-scaled chrysophytes in Arctic rivers is comparable to or even greater than the diversity of chrysophycean flora at the middle latitudes. Most of these chrysophytes are represented by polyzonal and ubiquitous species. During 2008–2010, we noted increased relative numbers of boreal species in northern waters compared with data obtained during the previous 30 years. Overall, the species richness of silica-scaled chrysophytes in the studied rivers increased with increase in water transparency, temperature, and magnesium ion concentration, but decreased with increase in surfactant concentration. The origin of samples from a certain river; distance of the sampling site from the estuary; water temperature, transparency, and colour; and concentration of carbon dioxide, magnesium ions, total iron, surfactants, and oil products affect the species composition of silica-scaled chrysophytes in the study area. Our findings are fundamental to evaluate the current status of Arctic aquatic microflora and its further monitoring in the context of anthropogenic and climatic impacts. [ABSTRACT FROM AUTHOR]