Diatoms are key players in the silicon (Si) cycle in aquatic ecosystems. Diatom cells assimilate dissolved Si and utilize it to synthesize their silica frustules. This study aimed to assess the activity of cells of different diatom species in both cell division and biogenic silica deposition, under experimental conditions and in natural populations in Lake Baikal across seasons. We employed a staining technique using PDMPO and Lumitracker LysoGreen dyes, which specifically label newly formed frustules. The percentage of stained cells for a given species was defined as the Cell Division and Silica Deposition (CDSD) index. When culturing Ulnaria acus and Fragilaria radians diatom strains, we demonstrated that Si deficiency in the medium caused a decrease in CDSD. In the natural environment no correlation was found between the CDSD index and dissolved Si concentrations. Observations revealed the highest CDSD values (up to 95% of cells) during the ice-covered period. In June, U. acus cell activity resumed and CDSD ranged from 9 to 24%. During direct stratification in July, diatom abundance and CDSD were low. Diverse patterns of abundance and species composition were observed throughout the lake during homothermy in June 2022 and 2023. CDSD could vary significantly with the simultaneous development of different species. When abundances of U. acus, Nitzschia graciliformis, and Stephanodiscus meyeri were high, the CDSD index did not exceed 42%. Conversely, its peak values in 2022 (78–84%) were recorded at stations with low species abundances. These findings advance our understanding of diatom life cycle dynamics in natural environments.

Lakes located in the north temperate zone may be covered with ice for long periods. Under the ice, a habitat is created, different from the open water period, which is characterized by low temperature and reduced light due to ice and snow cover. We investigated phytoplankton in sub-ice communities (SI) at the ice-water interface and the 0-25 m under-ice water column communities (UW) in the pelagic zone of Lake Baikal. Community structure was assessed using microscopy and metabarcoding of fragments of 18S rRNA gene. Flagellate diversity included 34 taxa from 64 microalgae identified by microscopy and 48 operational taxonomic units (OTUs) from 56 identified by metabarcoding. In conditions of complete snow-covered ice, high diversity and abundance of flagellates (up to 14 million cells L-1) were observed, with an advantage due to their mobility and different feeding modes. Both microscopy and metabarcoding data show that the taxonomic structure of SI and UW is different. SI communities of Lake Baikal showed mass development of a mixed group of nanoflagellates, which consisted of "Spumella- and Chlamydomonas-like flagellates". SI communities were less taxonomically diverse, suggesting that available resource gradients on the ice bottom are more tightly constrained, and taxa have to be ice bottom specialists to survive there. The relative spatial heterogeneity of SI communities is reflective of greater homogenization of habitat parameters in the winter water column and more dynamic conditions on the ice bottom. The data obtained expand the understanding of the diversity and abundance of organisms in under-ice habitats in north temperate latitudes.

Microbial communities are key drivers of ecological processes in freshwater ecosystems, yet their organization and dynamics in boreal reservoir cascades, especially in the Angara River system, remain insufficiently characterized by modern molecular methods. The Angara River, originating from oligotrophic Lake Baikal and flowing through four large reservoirs—Irkutsk (IR), Bratsk (BrR), Ust-Ilimsk (UR), and Boguchany (BgR)—provides a valuable system for investigating microbial biogeography under hydrological regulation.This study presents the first comprehensive metabarcoding survey of bacterial and microeukaryotic communities across the entire Angara cascade. Samples were collected during hydrological spring (May–June 2024), with complementary data from SB and IR obtained in June 2023. Microbial assemblages showed pronounced spatial differentiation, forming two main clusters: an upstream group encompassing SB and IR and a downstream group comprising the northern reservoirs. These sharp ecological discontinuities likely result from the combined effects of intensive tributary inflow, anthropogenic pressure, and strong hydrological contrasts in water retention and lateral inflow.Interannual comparison between 2023 and 2024 further revealed that temporal shifts were more pronounced in the reservoir than in the lake, indicating that the hydrological setting modulates the pace of seasonal succession. While bacterioplankton communities showed strong regional specificity aligned with individual reservoirs, microeukaryotic assemblages exhibited cross-reservoir associations linked to similar hydromorphological features, such as bays and confluences.These findings demonstrate that microbial community structure in large river-reservoir cascades is governed primarily by local limnological context, hydrodynamic conditions, and landscape-level inputs rather than by downstream connectivity or geographic distance.

The escalating impact of climate change requires comprehensive monitoring of aquatic microeukaryotic communities. Specifically, it is essential to study their successional dynamics under natural thermal fluctuations and identify organisms particularly sensitive to these changes. During the hydrological spring (early June), the watercourses of the Angara region differ in the intensity of water warming and can serve as promising model systems for studying such variations. Chrysophytes, which form siliceous scaled shells, serve as indicators of water temperature changes. In this study, we analyzed the influence of the main tributaries in the upper reaches of the Angara River–Irkut, Kuda, Kitoy, Belaya, and Ida–on the species richness of silica-scaled chrysophytes in the river channel, which is mainly affected by the cold, oligotrophic waters of Lake Baikal. During research performed in June 2024, a total of 57 silica-scaled chrysophytes species were identified in the study area using scanning and transmission electron microscopy. Among these, 11 species are potentially new to science, while one rare species Paraphysomonas capreolata and the form Synura petersenii f. columnata were recorded for the first time in Russian water bodies. The analysis demonstrated a positive correlation between the species diversity of silica-scaled chrysophytes and both water temperature and silicon concentration, alongside a negative correlation with pH levels. Statistical analysis of silica-scaled chrysophytes occurrence revealed two main profile groups: those from the main channel of the Angara River and those from its tributaries. Thus, it was determined that tributaries considerably increase the species richness of silica-scaled chrysophytes communities in the large river system, thereby increasing the overall stability of the aquatic ecosystem. This research is important for understanding the underlying mechanisms of the formation of biodiversity in freshwater ecosystems.

Phosphorus is one of the major biogenic elements. Its inflow facilitates eutrophication of lake water. In aquatic ecosystems, phosphorus is present mostly in organic compounds. Ability of aquatic microorganisms to assimilate phosphorus from organophosphorous compounds results from activity of alkaline phosphatases; activity of these enzymes may be an indicator of the state of the ecosystem, phosphate load, and water quality. In the present work, alkaline phosphatase activity (APA) and abundance of phosphatase-active bacteria (PAB) in Lake Baikal pelagic zone and in the mouths of its major tributaries was studied. In the pelagic zone, APA and PAB abundance decreased with depth, indicating that the main processes of phosphate generation occurred in the trophic layer of the lake. In the main tributaries, both APA and PAB abundance were considerably higher than in the pelagic zone. These results indicate active biochemical processes of transformation of organophosphorous compounds occur in the estuarine zones of the rivers. The degradation processes result in regeneration of phosphates, which are completely incorporated in the biological turnover, providing for phytoplankton development.

This article presents the 2023 data on the snow cover in the northern basin of Lake Baikal. We estimated the spatial distribution of the main chemical elements in the snow cover. The high relative concentration of nitrates and hydrogen ions was a characteristic of the ionic composition of snowmelt water in comparison with the industrial areas of the Baikal region. We calculated the accumulation of major ions and biogenic elements in the snow cover. A comparative analysis of the obtained experimental data with regional background values and results of the similar previous studies revealed a trend towards a decrease in mineralization and an increase in the acidity of snow melt waters over the past 11 years. Low values of the total amount of ions and pH in the snow cover of some areas of the northern basin of Lake Baikal allowed us to classify them as a background for the entire Baikal region.

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.

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.

The results of describing the spatial distribution of methane concentration in the surface water of Lake Baikal in the spring are presented. The basis was the measurements of CH4 content which were first carried out in the round-the-clock continuous mode along the entire route of the passage of research vessel in the complex expeditions of Limnological Institute, Siberian Branch, Russian Academy of Sciences, in the spring seasons of 2013, 2016, 2017, 2018, 2021, and 2022. Based on the results of six expeditions, a merged data array was compiled; it included 12 100 segments (with a step of 0.005° in latitude and 0.01° in longitude) which covered the total area 4466.7 km2, or 14

Eutrophication is a major ecological problem and affects and endangers freshwater bodies, making assessment of the trophic status of water bodies crucial for their restoration and sustainable use. Lake Baikal is affected by a number of environmental stressors, including coastal eutrophication. Daily measurements of concentrations of nutrients, dissolved oxygen (DO), chlorophyll-a (Chl-a), weekly measurements of algae abundance and biomass in the open water season in June-December 2020, and measurements of concentrations of nutrients at 2–7-day intervals in June-October 2021 were made in the littoral of the South Baikal for the first time. It was shown that nitrate and phosphate concentrations decreased by July-August, their minimum content was maintained until September, concentrations began to increase in October and reached a maximum in December. The maximum abundance and biomass of algae and chlorophyll concentrations were only observed in early July. Storm situations increased the content of nitrogen, phosphorus and DO in water, the duration of their influence was not more than 2 days. A correlation matrix revealed significant positive correlations of NO3−-DO, phosphate (SPR)-NO3−, SRP-DO and biomass-Chl-a and strong negative correlations between water temperature (Tw)-DO, Tw-NO3−, Tw-total nitrogen (TN) and Tw-SRP. Based on SRP and NO3− concentrations and TN:TP ratios, it was concluded that algal development was limited to nitrogen and phosphorus in summer. The trophic status of the Southern Baikal littoral zone was assessed using classifications based on TN, TP, NO3−, SRP, Chl-a content and algal biomass, as well as the Carlson index (TSI) and probabilistic assessment. The results of assessments using different methods of trophic status determination showed that the Baikal littoral zone in the study area belongs to the oligotrophic type with minor elements of mesotrophy. According to the saprobity index, water purity of littoral waters varies within the oligosaprobic and β-mesosaprobic zones and corresponded to quality classes II and III (clean and moderate purity); the system demonstrates a high capacity for self-purification.

Studying correlations between phytoplankton communities and environmental factors is critical for understanding how aquatic ecosystems function. The high sensitivity of phytoplankton to changes in these factors makes it possible to control the state of the ecosystem of water bodies. Artificial lakes often demonstrate increased trophic status, inducing changes in phytoplankton structure. In this paper, we studied phytoplankton in June 2023 (hydrological spring) in two ecosystems, South Baikal and the Irkutsk Reservoir, that are connected by a water course but have different environmental parameters. The gradient of environmental parameters from the lake towards the reservoir revealed peculiarities in the distribution of some microalgae species. Microscopy and statistical analysis showed that water temperature was the most important factor affecting the structure of the communities. The warmer water of the reservoir, in contrast to the lake, demonstrated a twofold increase in species number, abundance, and biomass. Downstream from the reservoir, we observed a succession in the dominating Baikal species complex, its supplementation, and replacement with other species typical of the summer period and Baikal bays. The trophic status of the reservoir during the study may be described as oligotrophic, with local traits of mesotrophicity; its water refers to Class I and Class II and may be qualified as clean.

Abstract. This research was investigated the seasonal changes in phytoplankton, bacterioplankton and autotrophic picoplankton as well as environmental parameters in the photic layer of Lake Baikal. Using microscopy methods, 43 species belonging to Bacillariophyta, Dinophyta, Chlorophyta, Chrysophyta, Cryptophyta, Haptophyta, Charophyta and Cyanoprokaryota have been identified. Bacillariophyta and Chlorophyta were dominating throughout the year. The Shannon, Simpson, InvSimpson diversity indices, and evenness index showed that microalgae communities varied seasonally. In late spring these indices were significantly lower than in the under-ice period, summer and autumn that is due to the fact of the dominance of one species Ulnaria acus (K tzing) Aboal in the community. The abundance and biomass of phytoplankton also varied significantly depending on the season. Five different periods of phytoplankton development were identified, which were regulated by physicochemical factors.

The gross and net primary production is estimated for the most characteristic seasonal life cycles of Baikal plankton based on long-term measurement series of carbon dioxide concentrations in the near-water atmosphere and surface and bottom water and “water–air” carbon dioxide fluxes. The calculations are carried out according to the scheme similar to the diel technique. With regard to the conditions of the coastal zone, the methodological issues are analyzed, neglect of which can lead to a significant uncertainty in these characteristics. We show that it is impossible to correctly estimate the net primary production only from CO2 concentration measurements in this technique. For periods of open water, the net primary production is calculated from the daily average CO2 flux. The estimates of the productivity for individual series of measurements in different periods quite adequately show the main features and are consistent with long-term observations.

One of the little-studied ways that climate warming or temperature increases in aquaculture could affect aquatic animals is through accelerated aging. This study is dedicated to understanding the principles of molecular and cellular aging in the target tissues of juvenile whitefishes (Yenisei hump-snout whitefish and its hybrid) under the influence of acute heat stress (up to 26 °C), and the effects of thermal preconditioning as pre-adaptation. Non-adapted stressed hump-snout whitefish showed a higher induction threshold for functionally active mitochondria in the blood and a decrease in telomerase activity in the liver after heat shock exposure as a long-term compensatory response to prevent telomere shortening. However, we observed heat-induced telomere shortening in non-adapted hybrids, which can be explained by a decrease in mitochondrial membrane stability and a gradual increase in energy demand, leading to a decrease in protective telomerase activity. The pre-adapted groups of hump-snout whitefish and hybrids showed a long-term or delayed response of telomerase activity to heat shock, which served as a therapeutic mechanism against telomere shortening. We concluded that the telomerase and telomere responses to thermal stress demonstrate plasticity of tolerance limits and greater stability in hump-snout whitefish compared with hybrids.

In the last decade, the mass mortality of endemic sponges has been observed in the coastal zone of Lake Baikal. This paper presents the results of observations of the Baikal sponge fauna within the transects established throughout Baikal in 2015. The types of damage of Baikal sponges have been characterized based on visual descriptions. Between 2015 and 2020, a downward trend has been revealed for the average projective cover of healthy sponges of various body forms on the Baikal bottom. For encrusting, branched, and globular forms, the decrease in this index is 3.6, 2, and 4 times, respectively. The total area occupied by healthy and damaged sponges of various forms have reduced more than twice (from 238 to 110 m(2)).

Hydroelectric dams create new ecosystems such as reservoirs. Several hydroelectric dams forming shallow reservoirs were built on the Angara River flowing out of Lake Baikal. The first of them in downstream Angara is Irkutsk Reservoir, with several shallow bays. Since silica-scaled chrysophytes are effective bioindicators for aquatic ecosystems, this paper aimed to determine their distribution, taxonomic structure and species richness in South Baikal and Irkutsk Reservoir, which have different environmental parameters. Thirty-one species were found using scanning and transmission electron microscopy. Only seven of them inhabited South Baikal in June 2023 at 3.66–4.51 °C and pH 7.80–8.24, with Chrysosphaerella baicalensis, Spiniferomonas trioralis f. cuspidata and Mallomonas alpina being prevalent. Only one species (M. alpina) was dominant in Irkutsk Reservoir at all stations at a water temperature of 5.33–11.55 °C and pH 8.10–8.52, alongside three other abundant species, Synura cf. glabra, Mallomonas acaroides and M. crassisquama. The maximum number of species (23) was found in a shallow bay of the reservoir at maximal values of temperature (11.5 °C) and pH (8.57) and minimal values of phosphate and nitrate concentrations during the study. The enrichment of Irkutsk Reservoir in species of silica-scaled chrysophytes was due both to cosmopolitan widespread and polyzonal species as well as to rare boreal, arctic–boreal, and unknown, possibly new species.

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.

Проанализированы многолетние ряды наблюдений потоков углекислого газа и метана в системе «вода-атмосфера» в прибрежной зоне озера Байкал. По результатам многолетних измерений показано, что суммарный сток углекислого газа из атмосферы на водную поверхность составляет 5,9 гСО2 м-2 год-1, а выход метана из воды оценивается в 100мгСН4 м-2 год-1. Для сравнительной оценки радиационного эффекта предположим, что молекула метана поглощает ИК излучение в 20 раз эффективней по сравнению с молекулой СО2. Показано, что суммарный сток углекислого газа из атмосферы в течение года достоверно снижает радиационный эффект, обусловленный выходом метана.

Long-term observation series of carbon dioxide and methane fluxes in the water-atmosphere system in the coastal zone of Lake Baikal are analyzed. It is shown that the total sink of carbon dioxide from the atmosphere to the water surface is 5.9 gСО2 m-2 year-1, while the methane emission from water is estimated as 80 mgСН4 m-2 year-1. For comparative estimation of the radiative forcing, it is taken that the contribution of the atmospheric methane molecule to global warming is 27 times greater than that of the СО2 molecule. Then, the coastal area of Lake Baikal contributes to global warming due to the methane emission 8-20% more than to the cooling due to absorption of carbon dioxide by the water.