This paper is the first assessment of the balance of biogenic elements and organic carbon, the main components of the trophic state of the Lake Baikal ecosystem. Based on long-term studies (2011–2020), we calculated the influxes of different forms of phosphorus, nitrogen, silica, and organic carbon via rivers, precipitation, gaseous impurities, and aerosols in the lake. Studies between 2011 and 2020 were carried out during a period of low river water level (2011–2017) and a period of increasing river flow to a level equal to long-term averages (2018–2020), revealing patterns in the influx of these elements into the lake with the river runoff during different water levels. Tributaries are the main suppliers of elements to the lake, although atmospheric deposition of mineral forms of nitrogen and phosphorus has increased in recent years to 56% and 58%, respectively, of their total annual input at the end of the past century. For the first time, organic forms of phosphorus and nitrogen were taken into account in chemical balance calculations. These external inputs were compared with their internal load in Lake Baikal waters and outflow from the lake with the Angara River. Statistical analysis revealed that the range of intra- and interannual fluctuations in the concentrations of nitrogen and phosphorus in the water column of the lake remains unchanged. Mass balances for biogenic elements and organic carbon demonstrate that in the modern period, as previously, most of the elements remain in the lake.

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.

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.

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.

Long-term observation series of the methane content in the near-water atmosphere and in water in the coastal zone of Lake Baikal are analyzed. The measurements were carried out in the Baikal Atmospheric-Limnological Observatory (BALO SB RAS, which is a part of the Resource Sharing Center “Atmosfera”). A steady increase of the methane content in the atmosphere by about 12 ppb per year is observed. The average diurnal methane content in March 2021 (the last measurement cycle) was 2.012 ppm. For the period of ling-term observations, the measured partial methane pressure in the surface water always exceeded that in the atmosphere. This fact indicates that this part of Lake Baikal is a source of methane into the atmosphere. For the coastal zone, the average value of methane flux from water was 380μgСН4 m-2 day1 . The observed increased methane concentration in the near-bottom layer indicates the predominance of bottom methanogenesis. Our data show that the increment rate of dissolved methane in surface water of Lake Baikal is 0.015 μgCH4/l per year.

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.

Currently, with an increase in technological progress, anthropogenic impact on the environment is increasingly apparent. Since surface waters are the most vulnerable part of the natural environment, a detailed study of water pollution is one of the urgent tasks. This article presents the results of the instrumental measurements and the assessment of the pollution of the surface waters in the littoral zone of the Barguzin and Chivyrkuy bays of Lake Baikal. To determine the degree of water pollution, we compared hydrochemical indicators with the standards of the maximum permissible concentrations. We revealed that in different parts of the Chivyrkuy Bay, water quality varies in a wide range from slightly polluted to very polluted and, in some cases, highly polluted. The waters in the Barguzin Bay can mainly be classified as slightly polluted, and only in some areas – as moderately polluted or highly polluted. According to the comprehensive water pollution index, the waters in the Chivyrkuy and Barguzin bays is assessed as conditionally polluted and conditionally pure, respectively.

Within the framework of many-year studies of the carbon cycle in the water-atmosphere system and in order to determine high-priority measures for conservation of the unique ecosystem of Lake Baikal, the specialized combined expedition was carried out in August 2019 in Barguzin and Chivyrkuy Bays of Lake Baikal. With the unique onboard instrumentation system, we have measured the gas content in the surface water and the near-water atmosphere, as well as concentrations of biogenic elements and organic matter at the sampling stations. The spatial distribution of the increased methane concentrations in the surface water of Barguzin Bay has been analyzed to determine the main direction of the Barguzin River’s water inflow into the bay. The observational data were compared with the results of scenario calculations by a large-scale model of currents in the lake. This model gives us some understanding of a general nature of water mass circulation in the bay. For modelling small-scale manifestations in the near shore zones, it is necessary to use higher resolution models for nested regions.

Starting from 2006, we conduct combined spring expeditions over the water area of Lake Baikal. These expeditions include measurements of fluorescent characteristics at different water levels from the surface down to the bottom. Direct methods for determination of biomass and chlorophyll concentration in algae are quite laborious and do not allow in situ analysis of the results. Fluorescent methods, which are fast enough, are generally indirect and should be specially justified to provide the quantitative information about biomass or chlorophyll concentration. In this study, the results of synchronous measurements are used to examine the feasibility of empirical calibration and quantitative reconstruction of the chlorophyll content from measured fluorescent characteristics.

The article presents the first results of field tests of the automated station developed by the researchers from Limnological Institute SB RAS for measuring hydrophysical, hydrochemical and meteorological parameters in water bodies with the real-time data transmission to a remote Internet server. We show daily and hourly variations of the main hydrophysical and hydrochemical parameters in the littoral zone and near the pier in the Bolshiye Koty settlement. A comparative analysis of the obtained data with the results of parallel chemical analyses of the daily samples revealed their good agreement.

In 2015−2019, we studied the concentrations of mineral forms of nitrogen (NO3-, NO2- and NH4 +) in the waters of the Krestovka River and the streams Kamenushka, Bolshaya Cheremshanka and Malaya Cheremshanka as well as in the snow cover of their basins and precipitation of the Listvyanka settlement (southwest coast of Lake Baikal). Towards Lake Baikal the concentrations of the investigated compounds increase as a result of water pollution with domestic wastewater from the Listvyanka settlement. The highest concentrations of nitrates and nitrites are found in the estuary of the Malaya Cheremshanka, and those of ammonium – in the Kamenushka. Nitrate concentrations in the watercourses beyond the Listvyanka settlement have increased compared to the 1950s, which is due to climate change and air pollution. During the study period, the average annual concentrations of nitrates in precipitation and snow cover increased. The maximum concentration of nitrogen compounds in the snow cover is confined to lower parts of these streams. Thus, the snow cover of the Bolshaya Cheremshanka showed the highest nitrate concentrations. We have revealed that nitrate concentration in precipitation increases in the cold season due to air pollution with nitrogen oxides of anthropogenic origin. We indicate that nitrate concentrations in the waters entering Listvennichny Bay are an order of magnitude higher than in Lake Baikal, which can have a negative effect on the Baikal ecosystem.

Shipborne and ground-based enroute measurements along the southern and eastern coasts of Lake Baikal from Kultuk village to Turka village were conducted in August 15-21, 2018. This period was chosen, because in this period the recreation load on the Baikal coast is the highest and the summer peak in development of the water biota is observed. The route included ten stations with a priori different anthropogenic load. At each station, the content of carbon dioxide and methane in the atmosphere and dissolved in water, as well as their fluxes in the water-atmosphere system, were measured. Here, samples of surface and bottom water were taken, in which the pH value, the concentration of dissolved oxygen, bicarbonate, nitrate, phosphate and fluorescence characteristics were determined. It is shown that the minimum concentrations of nutrients were recorded at the Boyarskii village. The strongest water pollution was found near the coast of Novyi Enkhaluk village. Particularly this part of the coast is subjected to the highest anthropogenic impact from tourist complexes located along this coast line.

The results of long-term routine measurements of the concentration of major ions in the water of the Lake Baikal pelagic zone, the Angara River source, and main tributaries of the lake are analyzed. The average values of ion concentration in the water column of the lake and in the Angara River obtained for the recent decade are presented and compared with the data of previous studies. Long-term trends in the concentration of ions in the main Baikal tributaries are shown. Under low-water conditions, the inflow of ions to Lake Baikal through tributaries decreases despite an increase in their concentration in river water. Changes in the chemical composition of water in the tributaries did not affect the ionic composition of water in the Lake Baikal pelagic zone and in the Angara River source.

We report long-term comprehensive measurements of CO2 in the atmosphere, surface water and the major nutrients obtained for the period 2004-2018. The research was conducted at the Baikal Atmospheric and Limnological Observatory (BALO), which is located in the coastal zone of South Baikal (coordinates 51 degrees 54' N, 105 degrees 05' E). The seasonal variation of these characteristics was analyzed. For our site, during the open water period (May-December) we assessed average flux of CO2, which amounted to -155 mmol m(-2) y(-1) (sink from the atmosphere to the water surface). Carbon dioxide content in the near-water air of Lake Baikal rises with a rate of 2.46 ppm per year, which is in good agreement with the global trend. The common effect of many factors (often multi-directional), such as variability of weather conditions, hydrological processes and productivity cycles of aquatic plankton in the Baikal waters, cause strong variations in the water characteristics, which we observed in different seasons and years. Under these conditions of strong inter-annual and seasonal variability of all characteristics, trend calculations using our long-term observations did not allow us to reveal reliable tendencies of changes in surface water CO2 concentrations. (C) 2019 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.

The spatial distribution of the partial pressures of carbon dioxide and methane in the surface water and near-water atmosphere over the Lake Baikal area (shipborne expeditions in late May-early June of 2013 and 2016–2018) is analyzed. It is found that in the zones, where the process of formation of spring homothermy begins to develop, the CO2 concentration increases and the CH4 content decreases in the surface water. The analysis of water samples at hydrological stations has shown that the vertical distribution of biogenic elements and fluorescent characteristics at these parts of the pelagic zone in the upper 200-m water layer alternates considerably.

This study continues the thorough investigation of the impact of physical, hydrological, chemical, and biological processes on the СО2 gas exchange in the water-atmosphere system at Lake Baikal. The results of measurements of the СО2 flux direction, biogenic elements, and fluorescent characteristics over the entire Baikal territory in the spring periods of 2016 (from 25.05 to 06.06.) and 2017 (25.05 to 05.06) are compared. Fluorescent characteristics were measured with a flow-through fluorimeter, whose operation is based on the principle of pulse-amplitude modulation. It is shown that most significant year-to-year differences in the analyzed characteristics are observed in the Middle Baikal region. In 2016, the surface temperature in this zone approaches 40°C, and probably the vertical exchange process manifests itself here.

This work is about the history and the main stages of the hydrochemical studies in Lake Baikal for the last 90 years. The frst studies of Baikal water chemistry, including its deep-water zone, had been carried out at the late 1920s under the guidance of G. Yu. Vereshchagin. At the same time, the guide that was used during many years by chemical hydrologists for water chemistry analyses not only on Baikal had been developed. The long-term hydrochemical research of 1950s-1960s gave data on background concentrations of chemical components both in the lake and in its tributaries. The nowadays data evidence that the content of major ions in the deep-water zone of Lake Baikal is constant. It is found that the temporal and space changes of nutrients concentrations in the pelagic zone of the lake depend mainly on phytoplankton growth, whereas the current increase of nutrients concentrations in the littoral is mainly induced by human activities and the development of tourism. It is found that the Selenga River, the largest tributary of the lake, undergoes the highest anthropogenic load. The pollutants, which enter the river in Mongolia, affect the content of major ions, nutrients and organic matter on the territory of Russia up to the mouth; more pollutants are added by Russian local wastewater sources. Amid the ongoing low water levels and increased anthropogenic load, the annual average concentrations of mineral phosphorous in the Selenga River show a trend to decline, whilst the content of phosphorous organic compounds increases. The efux of mineral and total phosphorous by the Selenga is governed by changes of the water runoff. Changes in chemical composition of low-mineralized rivers of South Baikal (Khara-Murin and Pereemnaya) whose water catchment areas were affected for many years by emission of the Baikal Pulp and Paper Mill and by transfer of pollutants from the industrial complexes of Pribaikalye are registered.

The aquatic surface microlayer is located at the atmosphere–hydrosphere boundary and occupies 70% of Earth's surface, covering all water bodies. The depth of the surface microlayer is about 50 μm. A special microbial community called neuston is formed there. The total bacterial abundance in the surface microlayer (SML) and underlying waters (UW) of LakeBaikalwas studied using epifluorescence microscopy. Physicochemical features of the surface microlayer of LakeBaikalwere revealed for the first time. The SML was sampled throughout LakeBaikalin May–June of 2013 to 2016 and in August of 2013, 2015 and 2016. SML samples were taken from a boat, mainly during calm weather, using Garrett’s metal mesh screen (a diameter of 26.5 cm). The average values of total bacterial abundance in the SML varied through years within a range of (0.93–1.49)×10 6 cells/mL in May–June and (1.73–2.24)×10 6 cells/mL in August; in the UW, at a depth about 15–20 cm, there were (0.79 – 0.89)×10 6 cells/mL in May– June and (1.15–1.4)×10 6 cells/mL in August. Significant differences and a direct relationship between the total bacterial abundance in the surface microlayer and underlying waters ofLakeBaikal in the summer period have been shown. Differences between the chemical composition of the surface microlayer and the subsurface water layer in all studied seasons were revealed. The surface microlayer was enriched with PO43-, total organic carbon and suspended particulate matter as compared with the underlying waters. A direct relationship was found between the numbers of bacteria in the surface microlayer and the suspended particulate matter concentration.