This paper is devoted to the study of the mesothermal temperature maximum layer (MTML) in Lake Baikal, which is observed during the period of winter stratification. On the one hand, this is a rather well-known phenomenon; on the other hand, it is not sufficiently studied, although it has a significant impact on the thermal regime in winter and water dynamics during the periods of formation and breakdown of inverse temperature stratification. Our work presents the results of analyzing the spatial and temporal variability of the main MTML parameters and their dependence on hydrometeorological factors. For this purpose, CTD soundings and mooring data obtained in the western part of the southern basin of Lake Baikal in 2000–2022 were analyzed in comparison to ERA5-Land reanalysis. The MTML parameters have noticeable within-season and interannual spatial and temporal variability. This is obviously related to the influence of the processes of vertical turbulent mixing, internal wave action, and current patterns. The analysis of interannual differences revealed four types of behavior of the maximum MTML temperature during the ice season. The influence of wind conditions on the main MTML parameters (maximum MTML temperature, depth of its occurrence, and depth of the upper MTML boundary) was shown not only in the fall, but also in the summer period, when heat accumulation in the Baikal water column takes place. With the increased wind activity in the late fall, the MTML is formed deeper and has lower maximum temperature values. At lower wind activity in the fall, the MTML is closer to the surface and the values of the maximum MTML temperature are higher. A change in wind activity in the summer leads to the opposite effect. In spite of the essential trends over the study period in the dates of the occurrence of hydrological events, no noticeable trends were registered for the maximum MTML temperature, its depth, and the depth of the upper boundary of the MTML.

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.

In recent decades, lakes have experienced unprecedented ice loss with widespread ramifications for winter ecological processes. The rapid loss of ice, resurgence of winter biology, and proliferation of remote sensing technologies, presents a unique opportunity to integrate disciplines to further understand the broad spatial and temporal patterns in ice loss and its consequences. Here, we summarize ice phenology records for 78 lakes in 12 countries across North America, Europe, and Asia to permit the inclusion and harmonization of in situ ice phenology observations in future interdisciplinary studies. These ice records represent some of the longest climate observations directly collected by people. We highlight the importance of applying the same definition of ice-on and ice-off within a lake across the time-series, regardless of how the ice is observed, to broaden our understanding of ice loss across vast spatial and temporal scales.

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.

Caption: Lightning discharges appear in various colours depending on the scatter of light inside the thundercloud and in the atmosphere.The intracloud lightning discharges in the centre of the thundercloud appear to be white with a bluish tint, and the cloud-to-ground discharge below appears to be orange.The right hand side of the thundercloud exhibits a green tint that is attributed to the unique composition of hydrometeors inside the thundercloud.The photo was taken in the late evening of 10 September 2013, near Tarragona in northeastern Spain.

Continuous monitoring of ice cover belongs to the key tasks of modern climate research, providing up-to-date information on climate change in cold regions. While a strong advance in ice monitoring worldwide has been provided by the recent development of remote sensing methods, quantification of seasonal ice cover is impossible without on-site autonomous measurements of the mass and heat budget. In the present study, we propose an autonomous monitoring system for continuous in situ measuring of vertical temperature distribution in the near-ice air, the ice strata and the under-ice water layer for several months with simultaneous records of solar radiation incoming at the lake surface and passing through the snow and ice covers as well as snow and ice thicknesses. The use of modern miniature analog and digital sensors made it possible to make a compact, energy efficient measurement system with high precision and spatial resolution and characterized by easy deployment and transportation. In particular, the high resolution of the ice thickness probe of 0.05 mm allows to resolve the fine-scale processes occurring in low-flow environments, such as freshwater lakes. Several systems were tested in numerous studies in Lake Baikal and demonstrated a high reliability in deriving the ice heat balance components during ice-covered periods.

Long‐term lake ice phenological records from around the Northern Hemisphere provide unique sensitive indicators of climatic variations, even prior to the existence of physical meteorological measurement stations. Here, we updated ice phenology records for 60 lakes with time‐series ranging from 107–204 years to provide the first re‐assessment of Northern Hemispheric ice trends since 2004 by adding 15 additional years of ice phenology records and 40 lakes to our study. We found that, on average, ice‐on was 11.0 days later, ice‐off was 6.8 days earlier, and ice duration was 17.0 days shorter per century over the entire record for each lake. Trends in ice‐on and ice duration were six times faster in the last 25‐year period (1992–2016) than previous quarter centuries. More extreme events in recent decades, including late ice‐on, early ice‐off, shorter periods of ice cover, or no ice cover at all, contribute to the increasing rate of lake ice loss. Reductions in greenhouse gas emissions could limit increases in air temperature and abate losses in lake ice cover that would subsequently limit ecological, cultural, and socioeconomic consequences, such as increased evaporation rates, warmer water temperatures, degraded water quality, and the formation of toxic algal blooms.

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.

Data on the vertical distribution of dissolved methane in Baikal water column are analyzed. The zone of open lake now shows an increase in the concentrations of dissolved methane compared with 2002–2004. The possible causes of this phenomena are considered. Methane concentration in the aerobic water column of Baikal decreases from the surface toward the bottom. The active layer of the lake, as well as that in the ocean, contains a peak of methane concentration. In the main, deep part of the water column, methane concentration is lower than that equilibrium with the atmosphere because of the activity of aerobic methanotrophic microflora. The standard procedure of the static headspace analysis for determining methane is supplemented by measuring the pressure in a closed system. The concentrations of methane determined by methods of vacuum extraction and static headspace analysis are compared.

Lake Baikal, lying in a rift zone in southeastern Siberia, is the world's oldest, deepest, and most voluminous lake that began to form over 30 million years ago. Cited as the "most outstanding example of a freshwater ecosystem" and designated a World Heritage Site in 1996 due to its high level of endemicity, the lake and its ecosystem have become increasingly threatened by both climate change and anthropogenic disturbance. Here, we present a record of nutrient cycling in the lake, derived from the silicon isotope composition of diatoms, which dominate aquatic primary productivity. Using historical records from the region, we assess the extent to which natural and anthropogenic factors have altered biogeochemical cycling in the lake over the last 2,000 y. We show that rates of nutrient supply from deep waters to the photic zone have dramatically increased since the mid-19th century in response to changing wind dynamics, reduced ice cover, and their associated impact on limnological processes in the lake. With stressors linked to untreated sewage and catchment development also now impacting the near-shore region of Lake Baikal, the resilience of the lake's highly endemic ecosystem to ongoing and future disturbance is increasingly uncertain.

This study discusses the areas with decreased ice thickness detected by satellite images in the late March-early April before the destruction of ice. We compared the areas of jet-like currents near Kadilny Cape with ring structures on the ice. The data on the measurements of the ice thickness and currents in these areas indicate that a decrease in the ice thickness is due to the intensification of under-ice currents. Also we estimated heat fluxes at the ice-water interface for these areas.

Seasonal ice cover on lakes and polar seas creates seasonally developing boundary layer at the ice base with specific features: fixed temperature at the solid boundary and stable density stratification beneath. Turbulent transport in the boundary layer determines the ice growth and melting conditions at the ice–water interface, especially in large lakes and marginal seas, where large-scale water circulation can produce highly variable mixing conditions. Since the boundary mixing under ice is difficult to measure, existing models of ice cover dynamics usually neglect or parameterize it in a very simplistic form. We present the first detailed observations on mixing under ice of Lake Baikal, obtained with the help of advanced acoustic methods. The dissipation rate of the turbulent kinetic energy (TKE) was derived from correlations (structure functions) of current velocities within the boundary layer. The range of the dissipation rate variability covered 2 orders of magnitude, demonstrating strongly turbulent conditions. Intensity of mixing was closely connected to the mean speeds of the large-scale under-ice currents. Mixing developed on the background of stable density (temperature) stratification, which affected the vertical structure of the boundary layer. To account for stratification effects, we propose a model of the turbulent energy budget based on the length scale incorporating the dissipation rate and the buoyancy frequency (Dougherty–Ozmidov scaling). The model agrees well with the observations and yields a scaling relationship for the ice–water heat flux as a function of the shear velocity squared. The ice–water heat fluxes in the field were the largest among all reported in lakes (up to 40 W m−2) and scaled well against the proposed relationship. The ultimate finding is that of a strong dependence of the water–ice heat flux on the shear velocity under ice. The result suggests large errors in the heat flux estimations when the traditional “bulk” approach is applied to stratified boundary layers. It also implies that under-ice currents may have much stronger effect on the ice melt than estimated by traditional models.

Surveys to locate and monitor bubble gas seeps in Lake Baikal have been conducted mainly using the active echolocation methods since the 2000s. This paper compares the active acoustic method with a newly developed optical and passive acoustic methods using the example of the shallow-water gas seep Istok in the Selenga shoal. The optical method is based on processing of underwater video records of ascending bubbles while the passive acoustic uses spectral analysis of gas jet noise recorded using a hydrophone. Optical and passive acoustic methods enables estimation of the bubble size distribution function, which is necessary for the calculation of the bubble gas flux. The latter can be evaluated from active echolocation or optical data. By applying these methods, their possibilities and limitations are revealed as well as benefits of their combined use and ways to improve their accuracy in future.

Seasonal ice cover on lakes and polar seas creates seasonally developing boundary layer at the ice base with specific features: fixed temperature at the solid boundary and stable density stratification beneath. Turbulent transport in the boundary layer determines the ice growth and melting conditions at the ice-water interface, especially in large lakes and marginal seas, where large-scale water circulation can produce highly variable mixing conditions. Since the boundary mixing under ice is difficult to measure, existing models of ice cover dynamics usually neglect or parameterize it in a very simplistic form. We propose a model of the turbulent energy budget in the stably stratified boundary layer under ice, based on the length scale incorporating the dissipation rate and the buoyancy frequency (Dougherty-Ozmidov scaling). The model was verified on fine-scale measurements in Lake Baikal and demonstrated a good agreement with data. The measured ice-water heat fluxes in were among the largest reported in lakes (up to 40 W m−2) and scaled well against the proposed relationship. The model yields a scaling relationship for the ice-water heat flux as a function of the shear velocity squared that suggests the traditional bulk parameterizations may significantly underestimate the ice-water heat flux, especially at strong under-ice current velocities. The ultimate result consists in a strong dependence of the water-ice heat flux on the shear velocity under ice.

We have developed a modified method of headspace analysis of water samples with the control of temperature and total pressure in the gaseous phase of a closed heterogeneous system followed by a gas chromatographic measurement of methane concentration with the methane content in the lake water of < 500 nl СН4/L. Here, we describe the background concentrations and vertical distribution of dissolved methane in the water column of Lake Baikal, which were obtained for several years.

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).