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The relevance of studying the structure of food chains in high-latitude lakes characterized by oligotrophy and a significant influx of allochthonous organic matter from adjacent marshes is beyond doubt. In this study, an attempt was made to identify carbon sources in various trophic groups (zooplankton, zoobenthos, fish) and analyze trophic relationships between them, taking into account seasonal dynamics in a small subarctic humified lake located in the north of Karelia (White Sea coast). To achieve this goal, an isotopic analysis of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes was carried out in the tissues of consumers and potential producers in different seasons (summer, autumn, winter and spring) of 2019-2020. The results of the study revealed significant differences in the structure of the lake food web between the open water and ice-covered periods, which is likely due to changes in the level of phytoplankton primary production, sources and rates of allochthonous organic matter input, and a significant energy contribution from heterotrophic organisms associated with methanotrophs. The important role of methane carbon in the formation of organic matter in this reservoir was indicated by low δ¹³C values (<-40 ‰) in consumers such as zooplankton (dominated by Eudiaptomus graciloides) and chaoborids (Chaoborus sp.), apparently actively involved in the transfer of methane carbon within food chains. In summer, a significant contribution of diazotrophic nitrogen fixed by planktonic cyanobacteria acted as an important source of nutrients. Seasonal variations in δ¹⁵N values and trophic position were determined for the common perch (Perca fluviatilis). The obtained data contribute to the understanding of the mechanisms of energy and trophic links formations in humified lake food web with an influence of allochthonous organic matter.

Multimetric indexes (MMI) are an important biomonitoring tool that is widely used to assess the ecological status of aquatic ecosystems worldwide. The effectiveness of MMI lies in their ability to combine both the structural and functional features of communities of organisms, as well as the features of the entire ecosystem. The use of organisms such as macroinvertebrates in the development of MMI is most common in lake biomonitoring. No such work has been carried out in Russia to assess the ecological state of lakes. In this study, a multimetric index based on the composition and structure of macroinvertebrate communities was developed for low-altitude Altai lakes experiencing high recreational and agricultural stress. The studied lakes were grouped into two groups according to the degree of organic pollution: reference and disturbed. Using a step-by-step statistical analysis, five metrics were selected from 55 indicators of macrozoobenthos communities, which were included in the final multimetric index. The selection tests included sensitivity (discrimination), stability (seasonality), redundancy, and correlation with environmental variables (abiotic factors). The new index was compiled by the following metrics: the Pielou Evenness index, the number of mayflies, the occurrence of shredders, the number of caddisflies and gammarids. These indicators were evaluated on a continuous scale and divided into 6 classes of water quality in accordance with the classification adopted in Russia. The developed index demonstrated the ability to distinguish not only dirty and clean lakes, but also moderately polluted reservoirs. The new index can become an informative tool for monitoring and assessing the ecological status of lakes.

In 2023-2024, the composition and quantitative indicators of zoobenthos of Lake Teletskoye were studied for the first time during the period of glaciation in annually freezing areas of the reservoir. A comparison of water quality according to the state of zoobenthos in the same type of communities in the water area with increased anthropogenic load (North-western shallow) and in the water area of the reserve (Kamginsky Bay) is given. The current standards for environmental monitoring of the reservoir have been optimized.

Changing hydrothermal regime together with dendrophages outbreaks, are the main factor that determining the vitality, growth dynamics and changes in the ranges of conifers. The causes of mass dieback of Abies sibirica Ledeb in the taiga of the Eastern Sayan Mountains (the Krasnoyarsk Stolby National Park) and fir trees vitality within the area of fir dieback were studied. The mortality fir was preceded by a chronic decrease in the trees growth index (GI) associated with water stress which was observed since warming start in the 1970s. The unprecedented dieback of the fir forest was triggered by the abnormal drought in 2012, which led to an abrupt increase in water stress and a strong decrease of the GI of trees. The extreme deterioration of the fir forests vitality stimulated the outbreak of the bark beetle Polygraphus proximus Blandford. The trees dieback, the maximum of which was observed in 2013-2018, resulted in the mortality of up to 75 % of fir-dominant forests. The reaction of fir to P. proximus attacks included the formation of a “physic-chemical” barrier in the tree rings, consisting of closed rows of traumatic resin ducts and xylem lignification. Currently, the number of viable fir regeneration (3-10 thousand/ha) is potentially sufficient for the fir restoration in most zone of fir mortality. However, the predicted worsening of hydrothermal regime in fir habitat in synergy with P. proximus outbreaks suggested future cycles of fir mortality and partial restoration in the 21st century. The considered unprecedented fir forests mortality is within the phenomenon of conifer mesophytes health status worsening in the southern parts of their ra range.

The diversity of ecosystems and soil cover in the floodplain-delta part of the Uldza River were studied for the first time. Marshy, meadow-marshy, meadow and steppe-meadow ecosystems are the most common. The small area of the territory under the study is characterized by a variety of soils - from poorly developed stratified alluvial to highly-humused alluvial dark-humus and dark-humus (quasi)gley soils. They are supplemented by highly waterlogged and subaqueous soils of swamps and swampy depressions, where alluvial processes proper are less pronounced, and accumulation of humus, peat, formation of gley or quasi-gley horizons prevail. The main and additional (cryo-, eolian- and halogenesis) soil-forming factors for different types of soils are characterized. In the area of detailed soil survey, alluvial dark-humus (meadow) and light-humus (turf) carbonate soils prevailed, which are loamy in the upper horizons and sandy-sandy loam in the underlying ones. They are characterized by strongly alkaline pH values and carbonate saturation, insignificant humus content, except for the organogenic horizon (AU) of the alluvial dark humus (meadow) soil. The absorption capacity is at an average level of provision, but significantly decreases in sandy-sandy loam sediments. The exchangeable cations are dominated by Ca²⁺ and Mg²⁺, but the proportion of Na⁺ increases with depth. Alluvial soils are slightly saline, and the salinity chemistry is predominantly magnesium-sodium according to the cation ratio, and chloride-soda, taking into account anions. Geochemical features of the soils were established to be the accumulation of Ca, Sr, As and dispersion of iron group elements. A weak degree of chemical weathering (CIA), low biological activity, high heterogeneity of sediments (HM) were revealed according to geochemical coefficients. Soils under the study should be preserved as protected natural objects, as the basis for the diversity and functioning of original protected ecosystems. The obtained materials will be necessary for monitoring of possible functional and dynamic changes in the ecological status of floodplain-delta landscapes both under global and cyclic climate changes.

Peat soils (Drainic Hemic Histosols) of a drained mesotrophic swamp of the South taiga subzone of Western Siberia occupied by pine forests (Pinus sylvestris L.) were studied. The most favorable conditions for the development of soil catalase activity were observed in the regime of moderate hydro-reclamation. By this level, the oxidase activity of weakly and intensively drained soils was 71 % and did not differ statistically from each other. A reliable relationship by parabola-type between seasonal catalase activity and time factor has been established. Parabolic trends of weakly and intensively drained soils show: Catalase activity accelerated weekly on average by 1.04 and 1.27 absolute values with weekly average deceleration by 0.12 and 0.14 units, respectively. In soils of moderate drainage, the average weekly decrease was 1.31 with a weekly average acceleration of 0.076 units. Analytical indicators of the time series made it possible to identify intra-seasonal periods of catalase activity, determine the direction and intensity of changes, and differentiate the soil profile by 0-5, 5-10, and 10-30 cm in terms of the intensity of processes. An area of 5-10 cm, equivalent to the concept of median (growth coefficients increase up the profile, decrease down by about one and a half or two times), corresponds to one approximation or another to the indicators of seasonal enzymatic activity of the soil profile as a whole (0-30 cm) per season. A highly significant advantage of the parabola type has been established: a positive regression relationship between the catalase activity of soils and bulk moisture >17…<71 %, pH 3.6-4.4, redox potential >600 mV, negative - with a temperature from 2 to 17 °C. According to the canonical analysis, the interrelated effect of the discussed indicators in soils of weak drainage was 82 %, moderate - 40 % and deep - 57 %. The coefficients of the factor structure postulated bulk moisture as the dominant environmental parameter for regulating the enzymatic activity of peat soils.

Experimental modeling has been used to analyze the influence of drilling fluid composition and temperature on the destabilization of intrapermafrost gas hydrates. The need for this research is linked to the drilling of oil and gas wells in the Arctic permafrost zone, which, in addition to ice, may contain gas hydrate formations, as well as horizons of saline rocks and cryopegs. Since drilling fluids are an integral part of the well drilling process, studying their interaction with the host ice- and hydrate-containing rocks is essential for preventing various emergency situations associated with the dissociation of pore gas hydrates and the melting of ice inclusions. Experiments have been performed on frozen, artificially hydrate-saturated sand samples exposed to drilling fluids of varying compositions, including those containing contaminated cryopegs. The experimental modeling results suggest that the composition of drilling fluids and their temperature can have a significant impact on the destabilization of intrapermafrost gas hydrate formations, especially those under self-preservation conditions. Contamination of drilling fluids during drilling operations with cryopegs leads to a significant intensification of the decomposition of pore hydrates in the frozen rock strata due to active salt transfer processes.

This paper presents an analysis of the relationship between snow cover density and snow depth across the Arctic Zone of the Russian Federation over the modern climatic period (1991-2020). The study is based on snow survey data from various regions of the Russian Arctic. Due to substantial climatic, topographic, and meteorological differences, a single statistically significant correlation between snow density and depth could not be established for the entire Arctic Zone of the Russian Federation. Instead, regional empirical relationships were developed. The results are compared with formulas commonly used in engineering practice and with foreign empirical models. The study highlights the significant influence of factors, such as wind speed, air temperature, snow metamorphism type, and snow stratigraphy on snow density formation. The obtained dependences can be used to estimate snow water equivalent, model ground thermal regimes, and calculate snow loads in regions where snow density data are scarce or unavailable.

Based on the analysis of monthly solar radiation at 5-degree latitudinal zones in the Arctic, the characteristics of changes in their solar climate in the 21st century have been determined. The tendencies of long-term changes in the annual irradiation patterns of 5-degree latitudinal zones in the entire Arctic region (65-90° N) are positive for the period from March to June and negative from July to October. In addition, in the latitudinal range of 70-80°, positive changes in irradiance are observed in February and November; and in the latitudinal range of 65-70°, also in January and December. In the interannual variability of the monthly irradiation intensity, the maximum relative values are observed in March (0.047 %) and October (0.045 %) in the latitudinal zone of 85-90°, as well as in January (0.044 %) and December (0.044 %) in the latitudinal zone of 65-70°. The meridional gradient of insolation (MGI: summer, winter, and annual) in the current century increases in all latitudinal zones of the Arctic. This tendency becomes less pronounced with increasing geographic latitude. Winter MGI exceeds summer MGI by 2.279-2.782 times, and this excess increases northward. In the 21st century, insolation seasonality (IS) of the Arctic solar climate has been weakening, especially in the high latitudes.

On December 25, 2025, at the age of 66, Sergey Vladimirovich Alexeev, Corresponding Member of the Russian Academy of Natural Sciences, Doctor of Geological and Mineralogical Sciences, chief researcher at the Institute of the Earth’s Crust of the Siberian Branch of the Russian Academy of Sciences, a recognized leader in the field of permafrost hydrogeology, passed away. Sergey Alexeev will remain in the memory of the scientific community as an outstanding researcher of the frozen zone of the lithosphere, the author of fundamental works on cryohydrogeology, a talented science manager and teacher.