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.

This article analyzes the features of spatiotemporal variability of alkaline phosphatase activity (APA) in the mixing zone of the Selenga River, the main tributary of Lake Baikal. We characterize the relationship of this indicator with the concentrations of organic and mineral phosphorus in water. This study confirms the possibility of using phosphatase activity to assess ecological condition and trophic state of aquatic ecosystems.

The first data on the diversity and structure of bacterial communities in the Selenga River, its tributaries, and delta waters (Selenga shallows) during the summer high-water period of 2021 were obtained. The chemical indices (alkalinity, mineralization, as well as concentrations of sulfate and chloride ions) in the Russian part of the Selenga River exhibited a tendency to decrease gradually in the downstream direction. As before, the chemical composition of the Selenga River waters was determined by water content and anthropogenic pressures, while the effect of tributary waters was local. At a distance of 1 km from the mouths in the mixing zone of river water and lake water, the chemical composition and taxonomic structure of bacterial communities were close to those observed in the pelagic zone of Lake Baikal. In all analyzed 16S rRNA gene libraries, nine common bacterial phyla were identified, with a predominance of Pseudomonadota (51.71–76.83

This work is devoted to the study of the spatial distribution of horizontal currents in subglacial anticyclonic eddies leading to the formation of ring structures on the spring ice of Lake Baikal. The horizontal fields of geostrophic currents were calculated using the dynamic method, with the main attention paid to the comparison of different approaches to the calculation of water density, including the TEOS-10 software package specially adapted for Baikal conditions. Measured water temperature and conductivity in the area of the ring structure in 2009 were used as input data. And as reference data - series of profiles of velocity and direction of currents in a similar vortex in 2020, measured for the first time. Comparisons showed that the use of outdated methods (such as the Krotova method or the Chen-Millero equation) leads to significant discrepancies with real measurements, while the adapted TEOS-10 package demonstrates the best agreement. In general, the dynamic approach proved to be valid and can be successfully applied to study the flow fields in Lake Baikal. In the course of the conducted studies, a number of characteristic features of the current structure in the investigated subglacial circulations were revealed and described for the first time.

The construction of a dam for the Irkutsk hydroelectric power plant caused the water level in Lake Baikal to increase by 80 cm between 1958 and 1961. This led to a downward migration of the base of the gas hydrate stability zone (BGHSZ) in the sediments of the lake, which was followed by a long transition process to a new state of equilibrium. In the first stage of the transition, new gas hydrates formed at the BGHSZ, which was accompanied and followed by a decrease in pore pressure, a decrease in methane transfer from the BGHSZ to the lake floor (via fast transfer pathways, such as faults or mud volcanoes), a decrease in the intensity of methane release from the lake floor into the water column, and a decrease in the methane concentrations in the water. In the second stage, which covers the last 10–15 years, methane concentrations in the water column have started to increase again, possibly in response to an uptick in methane flux from the lake floor. In this paper, we look at possible explanations. Mathematical modeling of the migration of the BGHSZ allowed us to estimate how long the transition process takes. The modeled transition times are different for different locations in the lake, depending mainly on the sedimentation rate and the gas hydrate content of the sediments. In the near future, Lake Baikal may reach a quasi-stationary state again similar to that before the construction of the dam. This stationary state likely involves much higher methane concentrations in the water column than what is observed today, as well as adverse effects on biota of pulsed expulsions of methane, sourced from the BGHSZ, into the water column by means of e.g. mud-volcano eruptions. Such effects may include events of mass deaths of the endemic deep-water fish, golomyanka, similar to what was reported to have occurred in the 19th and first half of the twentieth century, prior to the construction of the dam. This study also reemphasizes how variations in the dynamics of a natural gas hydrate system may have a profound impact on the water bodies in which they occur and on the ecosystems within these water bodies. It also highlights which effects can be expected in other hydrate-bearing marine basins where climate-induced sea-level rise will impact the dynamics of the hydrate reservoirs.

Ring structures on Baikal ice were found in satellite images in the period when ice cover thickness decreased (April). Analysis of satellite data on lake surface temperature in summer showed that local zones with lower temperature of water surface can sometimes be seen during the navigation period. According to field studies of ring structures on lake ice, they form and develop under the effect of currents which contribute to a decrease in ice thickness. The available data show that the generation of anticyclonic currents can be caused by a local rise of deep water. Similar phenomena can be seen in summer. Data of satellite temperature measurements show local drops of lake surface temperature in summer caused by upwelling. This is directly related with the rise of cold water from a deep zone. Depending on the temperature stratification (direct or reverse), upwelling contributes to the generation of either cyclonic or anticyclonic currents, respectively. The different directions of currents are due to the difference between temperature stratification in winter and summer. The formation of ring structures on ice and local drops of surface water temperature in summer are due to local water rises (upwelling). The upwelling can be caused by convection. The decrease in the density of the underlying water can be due to (a) temperature increase; (b) a decrease in salinity; (c) an increase in the concentration of methane dissolved in water in bottom zone. In addition, the upward flows can be due to ascending gas hydrates.

This study revises the δ18O and δ2H status of Lake Baikal. The mean values of δ18O and δ2H varied from −15.9 to −15.5‰ and from −123.2 to 122.2‰, respectively, for the past 30 yr. The isotopic composition of the lake remained more ‘‘light” compared to the regional precipitation and rivers inflows. The isotopic composition of the lake has begun to change since ca.1920 after the Little Ice Age; however, Lake Baikal still has not reached the isotopically steady state in the present. The calculated composition of the steady-state should be −12.3‰ for δ18O and −103.6‰ for δ2H. If regional climate parameters do not change dramatically, Lake Baikal will reach these values in ca. 226 yr. Based on isotopic fingerprints of the upper (0 to 150 m) and near-bottom layers (ca. 150 m from the bottom floor), the renewal in the southern and central basins of Lake Baikal has occurred recently compared to the northern Baikal basin, and the size of the mixing-cell of downwelling is close to 30 km.

A high abundance of planktonic microalgae is typically thought to be related to their ‘bloom’, that is, to active population growth. Diatom blooms in the photic zone of Lake Baikal generally occur during hydrological spring (April–June); when the summer arrives and the surface water temperature increases, diatoms are replaced by other microalgae. In July 2019, we found a concentration of the diatom Fragilaria radians at a station in South Baikal that was extremely high for that season. This species generally blooms in spring, but in spring (May) of 2019, this alga was nearly absent from the phytoplankton population. Microscopic analysis of the sample taken in July 2019 revealed that the cells were in a dormant stage. The species composition of microalgae in phytoplankton samples from May 2018 and July 2019 was similar. According to the temperature profile analysis, a summer upwelling event from a depth of ca. 100 m occurred in 2019. We hypothesised that this event caused the resuspension of microalgae, including Fragilaria radians, which were deposited on the slopes of the lake in 2018. Hence, the high abundance is not always a ‘bloom’ or an active growth.

Microorganisms exhibit seasonal succession governed by physicochemical factors and interspecies interactions, yet drivers of this process in different environments remain to be determined. We used high-throughput sequencing of 16S rRNA and 18S rRNA genes to study seasonal dynamics of bacterial and microeukaryotic communities at pelagic site of Lake Baikal from spring (under-ice, mixing) to autumn (direct stratification). The microbial community was subdivided into distinctive coherent clusters of operational taxonomic units (OTUs). Individual OTUs were consistently replaced during different seasonal events. The coherent clusters change their contribution to the microbial community depending on season. Changes of temperature, concentrations of silicon, and nitrates are the key factors affected the structure of microbial communities. Functional prediction revealed that some bacterial or eukaryotic taxa that switched with seasons had similar functional properties, which demonstrate their functional redundancy. We have also detected specific functional properties in different coherent clusters of bacteria or microeukaryotes, which can indicate their ability to adapt to seasonal changes of environment. Our results revealed a relationship between seasonal succession, coherency, and functional features of freshwater bacteria and microeukaryotes.

Preliminary results of joint expeditions with the Laboratory of Hydrophysics of the Limnological Institute of the Siberian branch of RAS to study eddies, responsible for the appearance of rings on ice of Lake Baikal are presented. We conducted summer and winter campaigns in two areas of the Lake: near the Cape Nizhnee Izgolovye, and in the southern part of Baikal near Kultuk settlement.

Results of two scientific cruises carried out in July 2018 and September 2019 were used for an analysis of variability in spectral bio-optical properties of Lake Baikal. Chlorophyll a concentration, spectral light absorption coefficient of phytoplankton, non-algal particles and colored dissolved organic matter varied in ~order of magnitude between stations. Vertical distribution of these bio-optical parameters depended on water column stability in euphotic layer. The chlorophyll a profiles were characterized by maximum near the bottom of euphotic zone. In the layer of deep chlorophyll a maximum the phytoplankton absorption spectra had a specific feature – a local maximum at 550-570 nm. It was associated with relative increasing of phycoerythrin containing species in phytoplankton community. The absorption budget at 440 nm showed that the CDOM was the main optically active component, which provided ~ 50 % contribution to total absorption on averaged over Lake Baikal surface.

The complex investigations of hydrophysical, biological and spectral bio-optical characteristics have been carried out in Lake Baikal during two scientific cruises in July 2018 and September 2019. The depth-dependent variability of investigated parameters has been analysed.

Phytoplankton and bacterioplankton play a key role in carbon cycling of aquatic ecosystems. In this study, we found that co-occurrence patterns between different types of phytoplankton, bacterioplankton, and environmental parameters in Lake Baikal during spring were different over the course of three consecutive years. The composition of phytoplankton and bacterial communities was investigated using microscopy and 16S rRNA gene pyrosequencing, respectively. Non-metric multidimensional scaling (NMDS) revealed a relationship between the structure of phytoplankton and bacterial communities and temperature, location, and sampling year. Associations of bacteria with diatoms, green microalgae, chrysophyte, and cryptophyte were identified using microscopy. Cluster analysis revealed similar correlation patterns between phytoplankton abundance, number of attached bacteria, ratio of bacteria per phytoplankton cell and environmental parameters. Positive and negative correlations between different species of phytoplankton, heterotrophic bacteria and environmental parameters may indicate mutualistic or competitive relationships between microorganisms and their preferences to the environment.

Changes in the ionic component of the under-ice mineralization in the southern basin of Lake Baikal in 2001–2016 are analyzed. Data on the under-ice layer mineralization variations derived from water conductivity measurements using the SBE-25 high-precision CTD sonde are compared with the results of calculation of salt separation during the formation and growth of the ice cover. The studies demonstrated that the mineralization increase is affected not only by salinization during ice crystallization but also by the water inflow from the central basin of the lake and from the Selenga River (more than a half of its runoff propagates to Southern Baikal in winter).

Lake Baikal is facing several environmental stressors, including climate change and nearshore eutrophication. To assess recent ecological changes in Lake Baikal and provide a baseline for future comparisons, we sampled spring plankton communities from the pelagic zone of the lake in 2016 and compared these data with unpublished and published historical information going back to 1990. In 2016, one pelagic long-term monitoring station was sampled in early spring (March) during ice cover and 21 long-term monitoring stations located throughout the lake were sampled in late spring (May-June). We measured water chemistry parameters at most stations and the abundance, taxonomic composition and biomass of bacteria, ciliates and phytoplankton at several locations in different areas of the lake. Biotic parameters from 2016 were compared with historical data, showing significant changes in the spring pelagic microbial community since the 1990s. We show increased quantities of small species, mixotrophic ciliates, and the appearance (or increasing number) of small coloured and colourless flagellates. We also show substantially decreased densities of formerly dominant heavily silicified diatoms such as Aulacoseira spp. Since 2007, Synedra acus subsp. radians, a smaller and weakly silicified diatom, has dominated the spring plankton of the lake. These results suggest that Lake Baikal's pelagic plankton community may be changing, with climate likely playing a dominant role in these changes.

This paper provides a novel report of methane hydrates rising from bottom sediments to the surface of Lake Baikal, validated by photo and video records. The ascent of hydrates in the water column was confirmed by hydroacoustic data showing rising objects with velocities significantly exceeding the typical speeds (18–25 cm s−1) of gas bubbles. Mathematical modelling along with velocity and depth estimates of the presumed methane hydrates coincided with values observed from echograms. Modelling results also showed that a methane hydrate fragment with initial radius of 2.5 cm or greater could reach the surface of Lake Baikal given summer water column temperature conditions. Results further show that while methane bubbles released from the deep sedimentary reservoir would dissolve in the Lake Baikal water column, transport in hydrate form is not only viable but may represent a previously overlooked source of surface methane with subsequent emissions to the atmosphere. Methane hydrates captured within the ice cover may also cause the formation of unique ice structures and morphologies observed around Lake Baikal. Sampling of these ice structures detected methane content that exceeded concentrations measured in surrounding ice and from the atmosphere demonstrating a link with the methane transport processes described here.

ABSTRACT We present a metagenomic study of Lake Baikal (East Siberia). Two samples obtained from the water column under the ice cover (5 and 20 m deep) in March 2016 have been deep sequenced and the reads assembled to generate metagenome-assembled genomes (MAGs) that are representative of the microbes living in this special environment. Compared with freshwater bodies studied around the world, Lake Baikal had an unusually high fraction of Verrucomicrobia . Other groups, such as Actinobacteria and Proteobacteria , were in proportions similar to those found in other lakes. The genomes (and probably cells) tended to be small, presumably reflecting the extremely oligotrophic and cold prevalent conditions. Baikal microbes are novel lineages recruiting very little from other water bodies and are distantly related to other freshwater microbes. Despite their novelty, they showed the closest relationship to genomes discovered by similar approaches from other freshwater lakes and reservoirs. Some of them were particularly similar to MAGs from the Baltic Sea, which, although it is brackish, connected to the ocean, and much more eutrophic, has similar climatological conditions. Many of the microbes contained rhodopsin genes, indicating that, in spite of the decreased light penetration allowed by the thick ice/snow cover, photoheterotrophy could be widespread in the water column, either because enough light penetrates or because the microbes are already adapted to the summer ice-less conditions. We have found a freshwater SAR11 subtype I/II representative showing striking synteny with Pelagibacter ubique strains, as well as a phage infecting the widespread freshwater bacterium Polynucleobacter . IMPORTANCE Despite the increasing number of metagenomic studies on different freshwater bodies, there is still a missing component in oligotrophic cold lakes suffering from long seasonal frozen cycles. Here, we describe microbial genomes from metagenomic assemblies that appear in the upper water column of Lake Baikal, the largest and deepest freshwater body on Earth. This lake is frozen from January to May, which generates conditions that include an inverted temperature gradient (colder up), decrease in light penetration due to ice, and, especially, snow cover, and oligotrophic conditions more similar to the open-ocean and high-altitude lakes than to other freshwater or brackish systems. As could be expected, most reconstructed genomes are novel lineages distantly related to others in cold environments, like the Baltic Sea and other freshwater lakes. Among them, there was a broad set of streamlined microbes with small genomes/intergenic spacers, including a new nonmarine Pelagibacter -like (subtype I/II) genome.

The preliminary results of complex bio-optical investigations carried out at Lake Baikal in July 2018 showed high variability in the light absorption properties of all optically active components, as well as their relation with hydrophysical characteristics. Vertical distribution of chlorophyll a was characterized by the presence of a maximum near the bottom of the euphotic zone. In this deep chlorophyll maximum layer, there were specifc features of phytoplankton light absorption spectra reflecting the abundance of phycobilin-containing blue-green algae in the phytoplankton community.

Abstract—The aquatic surface microlayer at the interface between the atmosphere and the hydrosphere occupies 70% of the Earth’s surface, covering all water bodies. The surface microlayer is about 50 μm thick. The special microbial community formed here is referred to as neuston. The total bacterial abundance in the surface microlayer (SML) and underlying waters (UW) of Lake Baikal was studied by epifluorescence microscopy. The physicochemical features of the surface microlayer of Lake Baikal have been established for the first time. SML was sampled throughout all of Lake Baikal in May–June of 2013–2016 and in August of 2013, 2015, and 2016. SML samples were taken from a boat with Garret’s metal mesh screen (26.5 cm in diameter) mostly in calm weather. The average values of total bacterial abundance in SML varied from year to year within the range of (0.93–1.49) × 106 cells/mL in May–June and (1.73–2.24) × 106 cells/mL in August; in the UW (at a depth of about 15–20 cm), these values were 0.79–0.89 × 106 cells/mL in May–June and 1.15–1.4 × 106 cells/mL in August. There was a significant difference and direct relationship between the total abundances of neuston and plankton bacteria of Lake Baikal in the summer period. The differences in chemical composition between the surface microlayer and the subsurface water layer were revealed in all the seasons under study. The surface microlayer was enriched in $${\text{PO}}_{4}^{{3 - }}$$, total organic carbon, and suspended particulate matter compared to the underlying water layers. There was a direct relationship between bacterial numbers in the surface microlayer and the concentration of suspended particulate matter.