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The results of a hydrochemical analysis of the consequences of an accidental fuel release in the Norilsk–Pyasino water system are presented. The pollution of watercourses in the catchment of Lake Pyasino (a nameless (Nadezhdinsky) creek, the Daldykan River, and the Ambarnaya River) with oil products, phenols, easily oxidizable and hard to oxidize organic matter (COD, PO, and BOD5), suspended solids, inorganic salts, and heavy metals at concentrations exceeding the background levels and MPC for fishery water bodies, as well as a temperature rise in waters of the nameless creek near Norilsk Heat and Power Plant 3 (CHPP-3), have been revealed. The contamination of the surface water decreases downstream in ascending order: nameless creek–Daldykan River–Ambarnaya River. The occurrence of oil products, phenols, and organic substances in the surface waters 2 months after the fuel spill is obviously due to their diffusion from the river bottom sediments, which accumulated a considerable quantity of heavy fractions of diesel fuel after the accident. Increased concentrations of Ca, Cu, Zn, Mn, Co, and Ni in the waters of the studied tributaries of Lake Pyasino are not directly related to the accident; they result from the general technogenic pollution of the territory and the increased geochemical background for these elements. Water contamination with oil products and phenols in the studied areas of Lake Pyasino (its central and northern parts) and the Pyasino River has not been detected. However, Pb concentrations exceed the MPC and Cd is recorded in the water, which is probably due to pollutants that accumulated in previous years.

Salinity largely determines the species diversity and structure of the food web in lake ecosystems. Many studies have been focused on the salinity tolerance of various plankton species. There are fewer studies investigating the modifications in the food web interactions under the effect of salinity. An appropriate system for such studies is closely located lakes of different salinity. We took samples during the summer stratification and analyzed the physicochemical and biological characteristics of 20 lakes (salinity 0.1-35 g/l) in southern Siberia in order to provide general description of this system and to test a number of hypotheses about the effect of salinity on the structure of the food web. The analysis revealed two key factors structuring ecosystems of the studied lakes. The species composition of zooplankton was determined by salinity, with a decrease in the number of species and the transformation of the community from a diverse community of zooplankton (salinity up to 3 g/l) to the dominance of large daphnia (3-8 g/l), copepods (> 8 g/l), small cladocerans and rotifers (20-30 g/l), and Artemia (> 30 g/l). The top predator (fish) was eliminated at salinity above 10 g/l. The biomass of zooplankton did not depend on salinity, but significantly increased in lakes with an increased nutrient load. The nutrient load also had a significant effect on the concentration of total phosphorus, turbidity and transparency of water, and the concentration of chlorophyll “a”. Thus, we recorded the structuring effect of the nutrient load (bottom-up control) on the biomass of the trophic levels and the parameters of water transparency, while salinity transformed the trophic chain from the top, leading to the disappearance of fish and a change in the dominant species of zooplankton. At the same time, salinity did not affect the biomass of the trophic levels. In order to detect top-down effects in the ecosystems of saline lakes, it is necessary to perform a comparative analysis of the seasonal dynamics of ecosystems of lakes with different salinity

In recent decades, freshwater ecosystems have regularly experienced blooms of green algae (green tides), causing environmental and economic damage. Blooms of green alga Spirogyra sp. occur in the ‘Abakanskaya’ anabranch of the Yenisei River in summer months. The anabranch is influenced by several anthropogenic factors: construction of a dam in the upper reaches; heated water discharge from a thermal power plant; fish farming. The aim of this study was to assess the impact of anthropogenic factors on the ecosystem of the Abakanskaya anabranch and to identify the main factors favoring Spirogyra sp. blooms. Samples were taken in May-September 2018-2019 at four sites: 1 – upstream of the dam (control), 2 – downstream of the dam, 3 – near the point of heated water discharge, 4 – downstream of the fish farm. Physical and chemical parameters, biomass and species composition of phytoplankton, phytoperiphyton, zooplankton, zoobenthos, and higher water plants, gross primary production (GPP), nitrogen content in biomass of periphyton, higher plants, and zoobenthos were studied. The main factor that caused changes in the community was that there was no water flow in the anabranch because of the low capacity of the dam drainage pipes. While upstream of the dam most ecosystem features were within the limits typical for this part of the Yenisei River, downstream of the dam, the system seemed to change from lotic to lentic: percentage of motile forms of phytoplankton, biomass of zooplankton, percentage of true zooplankton species, taxon number and taxonomic diversity of zooplankton and zoobenthos became higher; phytoperiphyton primary production rose (because of Spirogyra sp. bloom); higher water plants thrived. The other investigated factors, namely, thermal, nutrient, and organic pollution evidently produced combined effect on the ecosystem because of the absence of water flow, and their specific effects were not obvious

Traditionally, the abundance, cell size distribution and species identification of algae are determined by microscopic counts. In recent years, various rapid methods have been developed for routine algal studies. However, each of these methods has its drawbacks. It is important for aquatic ecologists to understand the advantages, disadvantages, and limitations of these methods. We compared the sensitivity of three rapid methods (multichannel fluorimeter FluoroProbe, imaging flow cytometer FlowCam, and CASY particle counter) to changes in cell abundance of three algae species (Chlorella vulgaris Beyerinck, Arthrospira platensis Gomont, and Nostoc sp.). We also assessed the ability of rapid methods to estimate the cell abundance of different species in the mixed samples. All instruments showed high sensitivity to changes in the cell abundance of different algae species and a mixture of these species. Any one of these methods, once calibrated, can be reliably used to estimate the abundance of a single-species/laboratory culture of microalgae. At the same time, FlowCam, without preliminary calibration, recorded the cell abundance closest to microscopic counts. When analysing a mixture of three microalgae differing in their cell sizes and spectral characteristics, FluoroProbe showed the highest accuracy in assessing the proportions of species in the mixture and FlowCam – in assessing their abundance. To study mixtures of algae and/or natural phytoplankton communities, it is advisable to use jointly a flow cytometer and a multichannel fluorimeter. The images of algae saved by the flow cytometer, if necessary, can be used to identify them, with a certain accuracy, to the species. Information on cells size and spectral characteristics obtained by two methods will be detailed enough to perform such common tasks as studying trophic interactions between phyto- and zooplankton or creating warning systems to inform of unwanted blooms of phytoplankton and their individual groups (for example, cyanobacteria)

Climate change and other anthropogenic stressors have led to long-term changes in the thermal structure, including surface temperatures, deepwater temperatures, and vertical thermal gradients, in many lakes around the world. Though many studies highlight warming of surface water temperatures in lakes worldwide, less is known about long-term trends in full vertical thermal structure and deepwater temperatures, which have been changing less consistently in both direction and magnitude. Here, we present a globally-expansive data set of summertime in-situ vertical temperature profiles from 153 lakes, with one time series beginning as early as 1894. We also compiled lake geographic, morphometric, and water quality variables that can influence vertical thermal structure through a variety of potential mechanisms in these lakes. These long-term time series of vertical temperature profiles and corresponding lake characteristics serve as valuable data to help understand changes and drivers of lake thermal structure in a time of rapid global and ecological change., Measurement(s) temperature of water • temperature profile Technology Type(s) digital curation Factor Type(s) lake location • temporal interval Sample Characteristic - Environment lake • reservoir Sample Characteristic - Location global Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.14619009

Midges (Chironomidae, Diptera) are dominant amphibiotic insects in zoobenthos of the large saline Lake Shira located in the arid landscape of South Siberia. During two vegetation seasons we measured the dynamics of chironomid emergence from littoral parts of four stations in the lake. During one of the vegetation seasons we also measured chironomid emergence from profundal parts (depth of 6 m) of the same stations using original floating traps. Both seasonal dynamics and daily means of emergence values significantly varied among the studied stations and depths and were related to the bottom sediment type. The annual export of wet biomass (calculated as the sum of daily values for a vegetation season) was equal to 0.61 g per square meter of water surface and included proportional contributions of the areas with various bottom sediment types. Based on the values of benthic secondary production, we estimated additional annual export of chironomid biomass from the profundal part of the lake of approximately 1 g of wet weight · m−2. Besides, the total export of polyunsaturated fatty acids (PUFA) calculated using the previous data on PUFA content in the studied chironomid taxa (~ 2 mg · m−2 year−1) appeared to be one of the lowest when compared to estimations for chironomids and some other amphibiotic taxa from similar steppe landscapes

In deep stratified lakes, the processes of growth and mortality of zooplankton populations result in uneven vertical distributions of living and dead organisms in a water column. The carcasses in the water are removed by sinking, degradation due to microbial decomposition and detritivory, etc. In the case of the epilimnion maximum of zooplankton, provided that the degradation prevails over the sinking, the downward flux of carcasses exponentially decays with depth. This vertical profile of dead organisms, demonstrating the decline in meta- and hypoliminon, can be described by the numerical model presented in this paper. The model approximation of the field data makes it possible to determine non-predator mortality rate m and degradation rate D in relative terms (m/v and D/v, v—sinking velocity) or absolute values (with defined v). For the case of the copepod population of Arctodiaptomus salinus in Lake Shira, the calculated m and D (medians of 0.13 and 0.26 day–1, respectively) were in a good agreement with the literature data. This method also gives the advantage of using the depth-dependent sinking velocity v.

Fish are recognized as the main source of physiologically important omega-3 long-chain polyunsaturated fatty acids, namely, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), for human nutrition. However, muscle tissue contents of these fatty acids in diverse fish species, i.e., their nutritive value for humans, varied within two orders of magnitude. We reviewed contents of EPA and DHA, measured by similar methods using an internal standard during chromatography as mg per g of wet mass in 172 fish species belonging to 16 orders, to evaluate probable variations in phylogenetic and ecological drivers. EPA + DHA content varied from 25.6 mg g−1 of wet mass (Sardinops sagax) to 0.12 mg g−1 (Gymnura spp.). Multidimensional redundancy analysis revealed that among phylogenetic, ecomorphological and abiotic environmental factors, the highest proportion of variation contribution belonged to the shared contribution of sets of phylogenetic and ecomorphological factors. Specifically, the highest values of EPA + DHA content were characteristic of fish belonging to the orders Clupeiformes or Salmoniformes, were pelagic fast swimmers, ate zooplankton and inhabited marine waters or migrated from fresh to marine waters (anadromous migrations). High EPA and DHA content in muscle tissues of the above species appeared to be a metabolic adaptation for fast continuous swimming. In contrast to common beliefs, our meta-analysis did not support the significant influence of higher trophic levels (piscivory) and cold environments (homeoviscous adaptation) on EPA and DHA content in fish. However, many causes of high and low levels of physiologically important fatty acids in certain fish species remained unexplained and require evaluation in future studies.

Saline Lake Shira (Southern Siberia, Russia) was meromictic through the observation period 2002–2015. During the under-ice periods of 2015 and 2016, complete mixing of the water column was recorded for the first time, and hydrogen sulphide temporarily disappeared from the water column of the lake; i.e. in those years the lake turned to holomixis. In the summer of 2015, a sharp increase in chlorophyll a, organic carbon, zooplankton, and phytoflagellates was observed in the lake, which was probably due to the release of nutrients from the monimolimnion. Purple sulfur bacteria completely disappeared from the lake after the first mixing in 2015, and did not reappear despite the restoration of meromixis in 2017. Thus, it was demonstrated that purple sulfur bacteria are sensitive to the weakening of the stratification of Lake Shira. Based on the data of the seasonal monitoring of temperature and salinity profiles over the period 2002–2017, it was presumed that the main cause of deep mixing in 2015 was the weakening of the salinity gradient due to strong wind impact and early ice retreat in the spring of 2014. In addition, it was shown that in previous years a significant contribution to the maintenance of meromixis was made by an additional influx of fresh water, which caused a rise in the lake level in the period 2002–2007. Thus, we identified a relationship between the stratification regime of the lake and the change in its level, which provides valuable information both for the forecast of water quality and for reconstruction of the Holocene climate humidity in this region of Southern Siberia from the sediment cores of Lake Shira.

We studied biogeochemical characteristics, including organic carbon and nitrogen contents, fatty acid (FA) composition, stable isotope ratios, and primary production in conjunction with species composition of bacterioplankton, using next generation sequencing, in the Yenisei River along a distance ∼1800 km. Basing on FA composition of particulate organic matter (POM) and on other indicators of sources of POM, the river was subdivided into four sections. The upper section 1, situated in mountain region, was the net source of high-quality autochthonous organic matter, produced primarily by diatoms and partly consumed by specialized bacteria species. Section 2 in plain taiga was net sink of high quality allochthonous and autochthonous organic matter, produced by cyanobacteria and green algae. Section 3 was net sink of organic matter, primarily allochthonous, consumed by the specialized species of bacteria. The lowest section 4, situated in tundra, was primarily the conduit of recalcitrant terrestrial organic matter, but also the net source of autochthonous organic matter, produced by diatoms. Biogeochemical traits of sections of the Yenisei River evidently shaped dominant species composition of bacterioplankton of these sections. Regarding the biogeochemical and microbiological data, we concluded that the Yenisei River ecosystem complexly combines features of river mosaic, river continuum, and “neutral pipe.”

Lakes Shira and Shunet (South Siberia, Russia) are saline ectogenic meromictic lakes. This meromixis is annually sustained by ice formation in winter. In spring due to melting of ice the second halocline is formed in the near-surface layers of mixolimnion that prevents the lakes from full circulation. During our observations (1998–2014) the mixolimnion in both the lakes was monomictic and undergone full circulation in autumn. Purple and green sulphur bacteria are abundant in the chemocline ; the phytoplankton diversity is relatively low; the major constituents of the zooplankton community are several genera of ciliates, calanoid copepod Arctodiaptomus salinus, rotifers Brachionus plicatilis and Hexarthra sp.; Paleolimnological study of the content of organic matter in sediments and the concentration of okenone demonstrated that Lake Shira was holomictic about 90 years ago when the water level was low. It became strongly meromictic just after the water level increase in the 1940s.

An integrated study of the middle and lower Yenisei River was performed in the summer of 2012. It involved monitoring the key elements and ecological processes associated with the carbon cycle of the river ecosystem (the study area is more than 1800 km long). Measurements of the production and destruction processes have shown the failure of the “neutral pipe” hypothesis claiming that any river is a simple drain of carbon from terrestrial ecosystems to the ocean. The Yenisei River is not a purely heterotrophic ecosystem. It also has autotrophic areas, where the primary production of planktonic photosynthesis is higher than respiration (above the Angara River and near the Bolshaya and Malaya Heta rivers). According to the data, the respiration rate of the plankton community in the river depends mostly on the water temperature and the content of inorganic phosphorous that can restrict the amount of organic matter consumed by bacterioplankton.

Depths of thermocline and the redox zone, concentrations of dissolved and suspended carbon, and nitrogen and phosphorus in seston were measured in the pelagial of the saline meromictic Lake Shira (southern Siberia) in different years (2007–2011); the relationship of the values for those parameters with air-temperature variations was assessed. Positive correlations between both air temperatures in the previous year and the depth of the redox zone in winter and air temperature in April and the thermocline depth in summer were revealed. In the mixolimnion, the ratio of total nitrogen to total phosphorus almost always exceeded the Redfield ratio (16: 1); seston deficiency both in nitrogen and phosphorus was monitored in different seasons and at various depths. The amount of seston in the mixolimnion in summer almost doubled the amount of seston in winter and was directly related with the depth of the thermocline. In the monimolimnion, seston was rich in nitrogen and phosphorus. The amount of seston in the monimolimnion varied in different years and depended both on the air temperature in the previous year and the size of the zone.

The data set obtained in 19 experiments aimed at studying the growth of Daphnia of the longispina group on natural seston of the Bugach water reservoir (Krasnoyarsk) was used to analyze the relationship between the parameters of somatic and generative growth of the studied animals and the amount and quality of food. Depending on the quality of the seston, two models of the development of Daphnia were proposed. It was demonstrated that, despite the existing positive correlation between somatic and generative growth, a noticeable fraction of the variations of the specific rate of generative production is determined by external factors of the environment, such as the relative content of N and α-linolenic acid (N: C and ALA: C) in seston.

Asynchronous vertical migrations of calanoid copepods Arctodiaptomus salinus were studied in two stratified lakes in the south of Siberia using the method of two-section enclosures. It was found that the presence of a pronounced thermocline and a depth maximum of phytoplankton (Lake Shira) contributes to the appearance of intensive individual migrations of copepods between areas of the epi- and hypolimnion.

During two vegetation seasons (2004–2005), we compared feeding spectra of Arctodiaptomus salinus (Calanoida, Copepoda) populations inhabiting two neighboring salt lakes, Shira and Shunet, Khakasia, Russia, using fatty acid (FA) trophic markers. Sestonic FA composition in two lakes moderately differed, whereas levels of diatom FA markers were higher in Lake Shunet and of Cyanobacteria and green algae markers in Lake Shira. In general, markers in storage lipids—triacylglycerols (TAG) of A. salinus—reflected the differences in sestonic composition of the two lakes. Nevertheless, TAG fraction was also enriched by FA trophic markers of the minor components of seston, which were selectively ingested by the animals. In Lake Shira, A. salinus had significantly higher concentrations of bacterial FA markers in TAG. In Lake Shunet, TAG of A. salinus contained significantly higher relative amounts of 18:4ω3, 18:5ω3 and C22 polyunsaturated fatty acids (PUFA), which indicated marked contribution of cryptophytes or (and) flagellates into the diet. Laboratory experiments showed feeding on Cryptomonas and sulfur purple bacteria in Lake Shunet and ciliates and colonial picoplankton in both lakes, and generally confirmed the differences in FA trophic markers in A. salinus between the lakes. The two populations of A. salinus markedly differed in levels of essential long-chain PUFA, eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids, although the levels of these FA in seston were quite similar between the two lakes. The higher levels of the essential ω3 PUFA in A. salinus in Lake Shunet may be an adaptive response of the animals to a vertical stratification of physico-chemical conditions and significantly higher salinity levels at the boundary of adjacent bottom layer in this lake.

A feature of meromictic lakes is that several physicochemical and biological gradients affect the vertical distribution of different organisms. The vertical stratification of physical, chemical and biological components in saline, fishless meromictic lakes Shira and Shunet (Siberia, Russia) is quite different mainly because both mean depth and maximum depth of lakes differ as well as their salinity levels differ. The chemocline of the Lake Shira, as in many meromictic lakes, is inhabited by bacterial community consisting of purple sulphur and heterotrophic bacteria. As the depth of the chemocline is variable, the bacterial community does not attain high densities. The mixolimnion in Lake Shira, which is thermally stratified in summer, also creates different habitat for various species. The distribution of phytoplankton is non-uniform with its biomass peak in the metalimnion. The distribution of zooplankton is also heterogeneous with rotifers and juvenile copepods inhabiting the warmer epilimnion and older copepods found in the cold but oxic hypolimnion. The amphipod Gammarus lacustris which can be assigned to the higher trophic link in the fishless lake’s ecosystem, such as Lake Shira, is also distributed non-uniformly, with its peak density generally observed in the thermocline region. The chemocline in Lake Shunet is located at the depth of 5 m, and unlike in Lake Shira, due to a sharp salinity gradient between the mixolimnion and monimolimnion, this depth is very stable. The mixolimnion in Lake Shunet is relatively shallow and the chemocline is inhabited by (1) an extremely dense bacterial community; (2) a population of Cryptomonas sp.; and (3) ciliate community comprising several species. As the mixolimnion of Lake Shunet is not thermally stratified for long period, the phytoplankton and zooplankton populations are not vertically stratified. The gammarids, however, tend to concentrate in a narrow layer located 1–2 m above the chemocline. We believe that in addition to vertical inhomogeneities of both physicochemical parameters, biological and physical factors also play a role in maintaining these inhomogeneities. We conclude that the stratified distributions of the major food web components will have several implications for ecosystem structure and dynamics. Trophic interactions as well as mass and energy flows can be significantly impacted by such heterogeneous distributions. Species spatially separated even by relatively short distances, say a few centimetres will not directly compete. Importantly, we demonstrate that not only bacteria, phytoflagellates and ciliate tend to concentrate in thin layers but also larger-sized species such Gammarus (amphipods) can also under certain environmental conditions have stratified distribution with maxima in relatively thin layer. As the vertical structure of the lake ecosystem is rather complex in such stratified lakes as ours, the strategy of research, including sampling techniques, should consider potentially variable and non-homogeneous distributions.