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Environmental physical and chemical factors controlling the abundance and biodiversity of zooplankton in permafrost-affected lakes are poorly known yet they determine the response of aquatic ecosystems to on-going climate change and water warming. Here, we assess the current status of zooplankton communities in lakes of the Bolshezemelskaya Tundra (permafrost peatlands of NE Europe), and provide new information about the composition and structure of zooplankton. The results demonstrate that the structure of zooplankton communities is influenced by the morphometric features of lakes and the degree of lake overgrowth by macrophytes. According to the level of quantitative development of zooplankton, most tundra lakes were of the oligotrophic type with an average wet biomass of up to 1 g/m3. The largest number of species was observed in zooplankton communities of small thaw ponds with an area of up to 0.02 km2 and overgrown with macrophytes. The analysis of factors that influence the formation of the lake zoocenosis demonstrated that the species composition and quantitative characteristics of zooplankton are chiefly controlled by pH and water mineralization. A comparison of the results obtained with the literature data on the lakes of this region collected 60 years ago suggests that the ecosystems of these lakes are in a stable state. Overall, these new insights will improve our knowledge of factors controlling the zooplankton spatial dynamics in unique but quite abundant thermokarst lakes of NE European Tundra, subjected to on-going climate warming.

Small lakes of the boreal zone are highly sensitive indicators of both global environmental change and local anthropogenic pressure. This work presents the results of multidisciplinary study of three small, seasonally stratified lakes of NW Russia, conducted since 2002 to the present time. These lakes were subjected to various degree of impact by rural population and agricultural activity since the beginning of twentieth century but the population and anthropogenic pressure have decreased since 1990s. To monitor the recovering of these lakes from human impact and to relate the evolution of their hydrochemical parameters to short-term weather events and global climatic trend, we measured the lake water temperature and chemical composition (carbon, macro- and micronutrients, trace elements) in summer and winter over full depth of the water column. The pH and concentration of DIC, SO4 and H2S decreased over past decade. Concentrations of Si and nitrite increased over past 15 years, probably linked to progressive replacement of diatoms by cyanobacteria. We also recorded an increase in concentration of Cl and major cations in all lakes, likely linked to increasing connection to groundwater reservoirs. Some trace elements (Fe, Mn, Al, Zn, As, Pb) can serve as suitable indicators of on-going natural processes controlling lake response to external factors. Unlike widely reported global trend of DOC and Fe concentration increase in boreal surface waters of Europe and Northern America, such trend was not discernable in studied Fe-rich humic lakes. We conclude on the necessity of complex, multidisciplinary studies of small humic lakes in the boreal zone, because these “model” lakes can serve as important proxies of on-going environmental changes in highly abundant aquatic ecosystems of poorly characterized regions of NW Russia.

This work is a continuation of research on the quantitative assessment of the level of heavy metal pollution (Fe, Mn, Zn, Cu, Cd, Pb, Ni) of the ecosystem components of the estuary region of the Northern Dvina River. In this article, to assess the levels of contamination with the studied metals of the ecosystem of the estuary region of the Northern Dvina River in the area of the greatest anthropogenic impact (the area of the urban agglomeration Novodvinsk-Arkhangelsk-Severodvinsk), we use the method of comparative analysis with the conditional background area of the object of study, taking for it – the section of the river located above the urban development. Using spatial mapping of the values of the enrichment coefficients for the components of the aquatic ecosystem, areas with an abnormal excess of the metal content in the mollusk tissues, bottom sediments and the bottom water layer were identified. It is established that the area adjacent to the central part of the city and to the industrial zone on the left bank is subject to the greatest anthropogenic load in the water area of the top of the river delta. The most likely sources of contamination with the considered heavy metals (HM) are the ingress of surface wastewater (meltwater and rain storm runoff), untreated municipal wastewater and direct near-atmospheric transport. The analysis of mollusk tissues proved to be a more informative and indicative approach to the assessment of HM pollution in water areas with complex hydrological and hydrochemical gradients of the environment, compared to the bottom layer of water and bottom sediments.

Trace elements in freshwater bivalve shells are widely used for reconstructing long-term changes in the riverine environments. However, Northern Eurasian regions, notably the European Russian North, susceptible to strong environmental impact via both local pollution and climate warming, are poorly studied. This work reports new data on trace elements accumulation by widespread species of freshwater mussels Unio spp. and Anodonta anatina in the Severnaya Dvina and the Onega River Basin, the two largest subarctic river basins in the Northeastern Europe. We revealed that iron and phosphorous accumulation in Unio spp. and Anodonta anatina shells have a strong relationship with a distance from the mouth of the studied river (the Severnaya Dvina). Based on multiparametric statistics comprising chemical composition of shells, water, and sediments, we demonstrated that the accumulation of elements in the shell depends on the environment of the biotope. Differences in the elemental composition of shells between different taxa are associated with ecological preferences of certain species to the substrate. The results set new constraints for the use of freshwater mussels’ shells for monitoring riverine environments and performing paleo-reconstructions.

In order to better understand the biogeochemical cycle of nitrogen in meromictic lakes, which can serve as a model for past aquatic environments, we measured dissolved concentrations of nitrate, nitrite, ammonium and organic nitrogen in deep (39 m maximal depth) subarctic Lake Svetloe (NW Russia). The lake is a rare type of freshwater meromictic water boy with high concentrations of methane, ferrous iron, manganese and low concentrations of sulfates and sulfides in the monimolimnion. In the oligotrophic mixolimnion, the concentration of mineral forms of nitrogen decreased in summer compared to winter, likely due to phytoplankton bloom. The decomposition of the bulk of organic matter occurs under microaerophilic/anaerobic conditions of the chemocline and accompanied by the accumulation of nitrogen in the form of N-NH4 in the monimonimlion. We revealed a strong relationship between methane and nitrogen cycles in the chemocline and monimolimnion horizons. The nitrate concentrations in Lake Svetloe varied in the range from 9 to 13 μM throughout the water column. This fact is rare for meromictic lakes, where nitrate concentrations up to 13 µM are found in the monimolimnion zone down to the bottom layers. We hypothesize, in accord with available data for other stratified lakes, that under conditions of high concentrations of manganese and ammonium at the boundary of redox conditions and below, anaerobic nitrification with the formation of nitrates occurs. Overall, most of organic matter in Lake Svetloe undergoes biodegradation essentially under microaerophilic/anaerobic conditions of the chemocline and the monimolimnion. Consequently, the manifestation of the biogeochemical nitrogen cycle is expressed in these horizons in the most vivid and complex relationship with other cycles of elements.

In order to assess the current state of the ecosystem of the southeast of the Onega Bay of the White Sea affected by fuel oil spill in 2003, the accumulation of petroleum hydrocarbons was analyzed by the dominant species of aquatic organisms collected on the littoral of the most polluted coast in the areas of Purnem and Lyamts villages. In 2012, samples of aquatic organisms were taken in an area where all the species discussed in this work are represented on a small area: bivalved mollusks, attached molluscs, gastropods, polychaetes. In 2013 and 2018, samples of hydrobionts were additionally selected, in the three-kilometer strip of the coast on either sides of the givin point. In 2012 and 2013, high concentrations of HC in the tissues of bivalves were recorded. In 2018, the concentrations of hydrocarbons in the tissues of the studied hydrobionts were comparable to background values. A non-parametric test of Mann-Whitney showed a significant decrease in НС in mussel tissue from 2013 to 2018, at a significance level of 0.05. Taking into account the low levels of HC in the aqueous medium (less than 1 MPC of fishfarm) and in bottom sediments (from 0.34 to 9.03 mg/kg, the median of 1.41 mg/kg) in 2018, and is comparable with the background contents of hydrocarbons in tissues of aquatic organisms. We can conclude that after 15 years of the fuel oil spill, the condition of the Cape Deep ecosystem in terms of the content of hydrocarbons returns to the baseline state, continuing emissions of oil-sand lumps do not adversely affect the ecosystem. Based on the work done, it can also be concluded that ecotoxocological methods are priority in assessing the prolonged (or delayed) accidental impact of heavy petroleum products on aquatic ecosystems. The conclusion about the presence or absence of a negative impact on the aquatic ecosystem of hydrocarbons, based solely on the analysis of abiotic components, may not be sufficiently informative because it does not take into account the accumulative and deferred effects, especially manifested in the cold Arctic waters.

Permafrost thaw in continental lowlands produces large number of thermokarst (thaw) lakes, which act as a major regulator of carbon (C) storage in sediments and C emission in the atmosphere. Here we studied thaw lakes of the NE European permafrost peatlands - shallow water bodies located within frozen peat bogs and receiving the majority of their water input from lateral (surface) runoff. We also conducted mesocosm experiments via interacting lake waters with frozen peat and dominant ground vegetation - lichen and moss. There was a systematic decrease in concentrations of dissolved C, CO2, nutrients and metals with an increase in lake size, corresponding to temporal evolution of the water body and thermokarst development. We hypothesized that ground vegetation and frozen peat provide the majority of C, nutrients and inorganic solutes in the water column of these lakes, and that microbial processing of terrestrial organic matter controls the pattern of CO2 and nutrient concentrations in thermokarst lakes. Substrate mass-normalized C, nutrient (N, P, K), major and trace metal release was maximal in moss mesocosms. After first 16 h of reaction, the pCO2 increased ten-fold in mesocosms with moss and lichen; this increase was much less pronounced in experiments with permafrost peat. Overall, moss and lichen were the dominant factors controlling the enrichment of the lake water in organic C, nutrients, and trace metals and rising the CO2 concentration. The global significance of obtained results is that the changes in ground vegetation, rather than mere frozen peat thawing, may exert the primary control on C, major and trace element balance in aquatic ecosystems of tundra peatlands under climate warming scenario.

Arctic sea-ice loss is emblematic of an amplified Arctic water cycle and has critical feedback implications for global climate. Stable isotopes (δ18O, δ2H, d-excess) are valuable tracers for constraining water cycle and climate processes through space and time. Yet, the paucity of well-resolved Arctic isotope data preclude an empirically derived understanding of the hydrologic changes occurring today, in the deep (geologic) past, and in the future. To address this knowledge gap, the Pan-Arctic Precipitation Isotope Network (PAPIN) was established in 2018 to coordinate precipitation sampling at 19 stations across key tundra, subarctic, maritime, and continental climate zones. Here, we present a first assessment of rainfall samples collected in summer 2018 (n = 281) and combine new isotope and meteorological data with sea ice observations, reanalysis data, and model simulations. Data collectively establish a summer Arctic Meteoric Water Line where δ2H = 7.6⋅δ18O–1.8 (r2 = 0.96, p < 0.01). Mean amount-weighted δ18O, δ2H, and d-excess values were −12.3, −93.5, and 4.9‰, respectively, with the lowest summer mean δ18O value observed in northwest Greenland (−19.9‰) and the highest in Iceland (−7.3‰). Southern Alaska recorded the lowest mean d-excess (−8.2%) and northern Russia the highest (9.9‰). We identify a range of δ18O-temperature coefficients from 0.31‰/°C (Alaska) to 0.93‰/°C (Russia). The steepest regression slopes (>0.75‰/°C) were observed at continental sites, while statistically significant temperature relations were generally absent at coastal stations. Model outputs indicate that 68% of the summer precipitating air masses were transported into the Arctic from mid-latitudes and were characterized by relatively high δ18O values. Yet 32% of precipitation events, characterized by lower δ18O and high d-excess values, derived from northerly air masses transported from the Arctic Ocean and/or its marginal seas, highlighting key emergent oceanic moisture sources as sea ice cover declines. Resolving these processes across broader spatial-temporal scales is an ongoing research priority, and will be key to quantifying the past, present, and future feedbacks of an amplified Arctic water cycle on the global climate system. © Copyright © 2021 Mellat, Bailey, Mustonen, Marttila, Klein, Gribanov, Bret-Harte, Chupakov, Divine, Else, Filippov, Hyöky, Jones, Kirpotin, Kroon, Markussen, Nielsen, Olsen, Paavola, Pokrovsky, Prokushkin, Rasch, Raundrup, Suominen, Syvänperä, Vignisson, Zarov and Welker. We gratefully acknowledge all participating PAPIN stations. Tarja T?rm?nen and Aino Erkinaro assisted with sample analysis at the Stable Isotope Laboratory, University of Oulu. Shawn Marriott, Patrick Duke and Polar Knowledge Canada are acknowledged for their support with the Canadian High Arctic Research Station. Sergey Serikov and Aleksandr Sokolov assisted with Russian precipitation sampling. The NOAA Air Resources Laboratory (ARL) is gratefully acknowledged for provision of the HYSPLIT transport model used in this study. NCEP/NCAR data were obtained from the National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory?s (ESRL) Physical Sciences Laboratory database. Funding. The Pan-Arctic Precipitation Isotope Network (PAPIN) received funding from the European Union?s Horizon 2020 Project INTERACT, under Grant Agreement No.730938 (JW PI). An Academy of Finland Grant (316014-JW PI). Support was also provided by a University of the Arctic Research Chairship to JW that funded isotope analyses and provided postdoctoral support for HB and K-RM and postgraduate research support for MM. A Russian Science Foundation Grant (No. 18-11-00024) to KG funded isotope analyses. SK was thankful to Russian Science Foundation (No. 20-67-46018). Russian Foundation for Basic Research (BFBR) supported isotopic analyses conducted by AP (#18-05-60203-Arktika).

Emission of greenhouse gases (GHGs) from inland waters is recognized as highly important and an understudied part of the terrestrial carbon (C) biogeochemical cycle. These emissions are still poorly quantified in subarctic regions that contain vast amounts of surface C in permafrost peatlands. This is especially true in NE European peatlands, located within sporadic to discontinuous permafrost zones which are highly vulnerable to thaw. Initial measurements of C emissions from lentic waters of the Bolshezemelskaya Tundra (BZT; 200,000 km2) demonstrated sizable CO2 and CH4 concentrations and fluxes to the atmosphere in 98 depressions, thaw ponds, and thermokarst lakes ranging from 0.5 × 106 to 5 × 106 m2 in size. CO2 fluxes decreased by an order of magnitude as waterbody size increased by > 3 orders of magnitude while CH4 fluxes showed large variability unrelated to lake size. By using a combination of Landsat‐8 and GeoEye‐1 images, we determined lakes cover 4% of BZT and thus calculated overall C emissions from lentic waters to be 3.8 ± 0.65 Tg C yr−1 (99% C‐CO2, 1% C‐CH4), which is two times higher than the lateral riverine export. Large lakes dominated GHG emissions whereas small thaw ponds had a minor contribution to overall water surface area and GHG emissions. These data suggest that, if permafrost thaw in NE Europe results in disappearance of large thermokarst lakes and formation of new small thaw ponds and depressions, GHG emissions from lentic waters in this region may decrease.

Acidophilic archaea of the archaeal Richmond Mine acidophilic nanoorganisms (ARMAN) group from the uncultured candidate phylum “Candidatus Micrarchaeota” have small genomes and cell sizes and are known to be metabolically dependent and physically associated with their Thermoplasmatales hosts. However, phylogenetically diverse “Ca. Micrarchaeota” are widely distributed in various nonacidic environments, and it remains uncertain because of the lack of complete genomes whether they are also devoted to a partner-dependent lifestyle. Here, we obtained nine metagenome-assembled genomes of “Ca. Micrarchaeota” from the sediments of a meromictic freshwater lake, including a complete, closed 1.2 Mbp genome of “Ca. Micrarchaeota” Sv326, an archaeon phylogenetically distant from the ARMAN lineage. Genome analysis revealed that, contrary to ARMAN “Ca. Micrarchaeota,” the Sv326 archaeon has complete glycolytic pathways and ATP generation mechanisms in substrate phosphorylation reactions, the capacities to utilize some sugars and amino acids as substrates, and pathways for de novo nucleotide biosynthesis but lacked an aerobic respiratory chain. We suppose that Sv326 is a free-living scavenger rather than an obligate parasite/symbiont. Comparative analysis of “Ca. Micrarchaeota” genomes representing different order-level divisions indicated that evolution of the “Ca. Micrarchaeota” from a free-living “Candidatus Diapherotrites”-like ancestor involved losses of important metabolic pathways in different lineages and gains of specific functions in the course of adaptation to a partner-dependent lifestyle and specific environmental conditions. The ARMAN group represents the most pronounced case of genome reduction and gene loss, while the Sv326 lineage appeared to be rather close to the ancestral state of the “Ca. Micrarchaeota” in terms of metabolic potential. IMPORTANCE The recently described superphylum DPANN includes several phyla of uncultivated archaea with small cell sizes, reduced genomes, and limited metabolic capabilities. One of these phyla, “Ca. Micrarchaeota,” comprises an enigmatic group of archaea found in acid mine drainage environments, the archaeal Richmond Mine acidophilic nanoorganisms (ARMAN) group. Analysis of their reduced genomes revealed the absence of key metabolic pathways consistent with their partner-associated lifestyle, and physical associations of ARMAN cells with their hosts were documented. However, “Ca. Micrarchaeota” include several lineages besides the ARMAN group found in nonacidic environments, and none of them have been characterized. Here, we report a complete genome of “Ca. Micrarchaeota” from a non-ARMAN lineage. Analysis of this genome revealed the presence of metabolic capacities lost in ARMAN genomes that could enable a free-living lifestyle. These results expand our understanding of genetic diversity, lifestyle, and evolution of “Ca. Micrarchaeota.”

Rivers play a key role in the water cycle on the earth via integrating all hydrological channels to return terrestrial precipitations back to the oceans. In addition, rivers, together with groundwater, are powerful transformers of the surface lithosphere, responsible for chemical weathering of rocks and the removal of solute into the ocean. Tracing the dynamics of surface water and groundwater versus atmospheric feeding of rivers presents important issues in Arctic regions due to the ongoing change of the structure of hydrological runoff. In this study, stable water isotopes were used to reveal the temporal dynamics of water sources and to predict their possible change under the conditions of ongoing climate warming of the largest European Arctic river, the Severnaya Dvina, and adjacent groundwater. The isotopic composition of the river waters of the studied region is formed by the mixing of atmospheric precipitation with groundwater. The isotopic depletion in the springtime is mainly due to the recharge of thawed snow waters. A less pronounced effect in the autumn-winter period is provided by the discharge of groundwater into rivers, including the meltwater of the Last Glacial Period. This depletion is partially offset due to discharge of isotopically light thawed snow waters and is linked to evaporation in headwater streams, reservoirs, and wetlands. The isotopic composition of groundwater with low mineralization was formed throughout the Holocene and to a large extent depends on paleoclimatic conditions in the study area. In addition to fresh groundwater, brackish groundwater also takes part in the river's recharge. These brackish waters are associated with ancient and modern marine transgressions on the estuarine site and with the dissolution of Ca sulfate rocks in a karst region located in the middle reaches of the river. According to isotope data, the average annual input of the underground source to the total river flow is 25%. The results of this work will serve as the basis for continuing monitoring of the isotopic composition of river waters with an assessment of hydrological processes and observation of short as well as long-term climatic and anthropogenic impacts.

The article contains the results of the research of the content of a mass fraction of petroleum hydrocarbons in the bottom layer of water as well as in sediment and tissues of bivalves sampled in the estuarial of the Northern Dvina River during 2009–2016 in the seasons of the highest filtration activity of mollusks. Accumulation levels of petroleum hydrocarbons in water, sediments and tissues of bivalves in the Northern Dvina estuary were estimated. Dependence between petroleum hydrocarbons accumulation level in bivalves’ tissues and pollution level of their habitat was studied. Quantitation of petroleum hydrocarbons content in the tissues of bivalves, in the bottom layer of water and sediments was carried out by fluorometric method. It is found that the mass fraction of petroleum hydrocarbons in the tissues of bivalves didn’t exceed the level for relatively clean water in all studied areas of the river. Considering the mass fraction of petroleum hydrocarbons in bottom sediments of the Northern Dvina estuary, it can be attributed mostly to the uncontaminated and slightly contaminated areas, except for local areas of the river within the city limits. In the bottom layer of water in the studied region, the content of the mass fraction of the petroleum hydrocarbons exceeds the state standards in some sampling points. It is concluded that further comprehensive analysis of biotic and abiotic factors is needed to study complex environmental gradients.

Photo-induced degradation of dissolved organic matter (DOM) and organo-mineral colloids is one of the major factor responsible for transformation of DOM and dissolved metals in boreal and subarctic waters. In contrast to fairly good understanding of this process in inland waters of high latitude zone, the transformation of riverine DOM and associated trace element (TE) colloids in the Arctic estuaries remains virtually unknown. We incubated, under sunlight in outdoor pools, quartz reactors filled with mixtures of sterile filtered riverine and estuarine water. The water samples were collected in the estuarine zone of the largest European Arctic river, Severnaya Dvina. After 1 month of exposure to sunlight, ≤ 5% change of dissolved organic carbon (DOC) concentration and specific ultraviolet (254 nm) absorption occurred. This decrease was within the experimental uncertainty and it implies quite high resistance of river dissolved organic matter to photo-degradation in this estuary. Moreover, very low photodegradability of DOM in the freshwater point of the Severnaya Dvina River may require revisiting the current paradigm of the importance of DOC photolysis in large Arctic rivers. A novel finding was that the percentages of overall removal of Fe and some insoluble elements were quite similar across the full range of studied salinities, whereas the apparent rate of metal removal decreased with the increase of salinity. Overall, the salinity weakly impacted the removal of riverine DOC and metals in the estuarine water via photolysis and coagulation under sunlight. As a result, photoreactivity of DOM and dissolved metals in riverine end members corrected for estuarine dilution can be used to approximate the photolytic transformation of riverine material in the Arctic coastal zone.

Meromictic lakes are a unique model for the study of the anaerobic processes of organic matter degradation, such as methanogenesis and sulphate reduction. As a result of activity of sulfate-reducing bacteria hydrogen sulfide accumulates in monimolimnion of these lakes, whereas chemocline is the area of sharp vertical gradients of physical and chemical conditions and as a consequence is the ecological niche for different planktonic microorganisms. This paper describes some characteristics of meromictic freshwater Lake Svetloe, located in the north of the Arkhangelsk region, for which iron-manganese type of meromixia was defined. The distribution of the studied parameters corresponds to the distribution of the specific for meromictic lakes water layers formed in the presence of stable stratification. In the transition layer (chemocline) a sharp decrease in the oxygen content (almost to the analytical zero) is accompanied by an equally sharp increase in the value of the water conductivity and the emergence of dissolved hydrogen sulfide in the water. In the anaerobic zone the conductivity values remain high and accumulation of nutrients, iron, manganese, and hydrogen sulphide (average 30 mg/l) occurs. Besides, the significant decrease in the content of dissolved sulphate caused by its consumption by sulfate-reducing bacteria was noted in this layer. The number of sulphate-reducing bacteria in the water column during the time of observation varied in the range of 10 – 106 cells/ml with the highest content in the chemocline zone, where the maximum amounts of hydrogen sulphide (up to 130 mg/l) were also found

The relationship between dissolved organic matter (DOM) and heterotrophic bacterioplankton controls the ecological status of freshwater ecosystems and the fate of metals in aquatic environments. To improve our understanding of physicochemical and biological processes controlling the DOC and related trace elements in the Arctic estuary, we conducted experiments of the biodegradation and physicochemical coagulation of DOM and associated trace elements. For the experiments, water from the estuarine zone of the Severnaya Dvina River in Russia, the largest pristine European Arctic river, was mixed with sterile river water from the freshwater zone, sampled during spring flood in June and summer base flow in August. We revealed a lack of transformation of allochthonous organic matter in spring and measurable degradation of more autochthonous DOM in summer. According to their behavior during incubation experiments, several groups of trace metals were distinguished, comprising (1) nonconservative Fe, Mn, A...

Assessing multi-annual riverine export fluxes of carbon, nutrients and metals from the continent to the Arctic Ocean is crucial for constraining the current status and foreseeing future changes in riverine export induced by climate change. Over 3 consecutive years (2012–2014), we measured daily to weekly concentrations of C, N, P, all major and 45 trace elements (TE) in filtered ( The spring flood (May and June) provided 50% of overall water and DOC, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Sb, and Pb flux and 60% of low-soluble TE (Al, Y, Nb, REEs, Ti, Zr, Hf and Th). The nutrients (K, Rb, Si, P, N) exhibited around 40% share of annual export during spring flood, whereas the export of soluble mobile elements (DIC, Cl−, SO42−, Li, B, Na, Mg, Ca, Sr, Mo and U) mainly occurred during winter (35–40%) with only 20–25% of annual flow provided by spring flood. The 3-year mean export fluxes of dissolved components were 30 to 50% lower than (Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cu, Cd, and Pb) or comparable to (±30% for all other elements including DOC and for DIC) previous estimates of this river from 2007 to 2008 and the historic data of major components from 1930 to 1950. In particular, there was no trend in mean monthly seasonal and annual concentrations and export fluxes of DOC, DIC, SO4, Cl since the beginning of chemical analysis between 1929 and 1930 and in Ca, Mg, Si, and Fe since the 1950's through the duration of this study (2012–2014). The lack of seasonal and annual increases in discharge, carbon, and element concentrations in the permafrost-free Severnaya Dvina River for >120 years of discharge and over 60 years of chemical composition measurements is at odds with the reported short-term trend (based on measurements that began in 1970s) showing that element export fluxes increased in other Arctic rivers due to on-going climate warming and permafrost thaw. Novelty statement Historical and contemporary measurements of element fluxes in the largest European Arctic river did not reveal sizeable changes in discharge and DOC, DIC, Ca, Mg, Fe, Si, SO4, Cl concentrations and fluxes over past >60 years.

Midlatitude anthropogenic mercury (Hg) emissions and discharge reach the Arctic Ocean (AO) by atmospheric and oceanic transport. Recent studies suggest that Arctic river Hg inputs have been a potentially overlooked source of Hg to the AO. Observations on Hg in Eurasian rivers, which represent 80% of freshwater inputs to the AO, are quasi-inexistent, however, putting firm understanding of the Arctic Hg cycle on hold. Here, we present comprehensive seasonal observations on dissolved Hg (DHg) and particulate Hg (PHg) concentrations and fluxes for two large Eurasian rivers, the Yenisei and the Severnaya Dvina. We find large DHg and PHg fluxes during the spring flood, followed by a second pulse during the fall flood. We observe well-defined water vs. Hg runoff relationships for Eurasian and North American Hg fluxes to the AO and for Canadian Hg fluxes into the larger Hudson Bay area. Extrapolation to pan-Arctic rivers and watersheds gives a total Hg river flux to the AO of 44 ± 4 Mg per year (1σ), in agreement with the recent model-based estimates of 16 to 46 Mg per year and Hg/dissolved organic carbon (DOC) observation-based estimate of 50 Mg per year. The river Hg budget, together with recent observations on tundra Hg uptake and AO Hg dynamics, provide a consistent view of the Arctic Hg cycle in which continental ecosystems traffic anthropogenic Hg emissions to the AO via rivers, and the AO exports Hg to the atmosphere, to the Atlantic Ocean, and to AO marine sediments.

The article contains the results of the research of the content of a mass fraction of petroleum hydrocarbons in the bottom layer of water as well as in sediment and tissues of bivalves sampled in the estuarial of the Northern Dvina River during 2009–2016 in the seasons of the highest filtration activity of mollusks. Accumulation levels of petroleum hydrocarbons in water, sediments and tissues of bivalves in the Northern Dvina estuary were estimated. Dependence between petroleum hydrocarbons accumulation level in bivalves’ tissues and pollution level of their habitat was studied. Quantitation of petroleum hydrocarbons content in the tissues of bivalves, in the bottom layer of water and sediments was carried out by fluorometric method. It is found that the mass fraction of petroleum hydrocarbons in the tissues of bivalves didn’t exceed the level for relatively clean water in all studied areas of the river. Considering the mass fraction of petroleum hydrocarbons in bottom sediments of the Northern Dvina estuary, it can be attributed mostly to the uncontaminated and slightly contaminated areas, except for local areas of the river within the city limits. In the bottom layer of water in the studied region, the content of the mass fraction of the petroleum hydrocarbons exceeds the state standards in some sampling points. It is concluded that further comprehensive analysis of biotic and abiotic factors is needed to study complex environmental gradients.

The relationship between dissolved organic matter (DOM) and heterotrophic bacterioplankton controls the ecological status of freshwater ecosystems and the fate of metals in aquatic environments. To improve our understanding of physicochemical and biological processes controlling the DOC and related trace elements in the Arctic estuary, we conducted experiments of the biodegradation and physicochemical coagulation of DOM and associated trace elements. For the experiments, water from the estuarine zone of the Severnaya Dvina River in Russia, the largest pristine European Arctic river, was mixed with sterile river water from the freshwater zone, sampled during spring flood in June and summer base flow in August. We revealed a lack of transformation of allochthonous organic matter in spring and measurable degradation of more autochthonous DOM in summer. According to their behavior during incubation experiments, several groups of trace metals were distinguished, comprising (1) nonconservative Fe, Mn, Al, trivalent (Ga, Y, rare earth elements) and tetravalent (Ti, Zr, Hf, Th) hydrolysates, subjected to significant removal from solution, especially at low salinities (0.5–2.5‰); and (2) divalent metals such as Ba, Cd, and Pb, subjected to desorption from suspended particulate matter. The trace elements present in the form of oxyanions and neutral molecules (B, Si, As, Sb, Mo) and alkalis (Rb) exhibited highly conservative behavior with negligible removal from the freshwater–seawater mixtures. The spring flood, providing the largest contribution to the annual riverine flux to the ocean, demonstrated high stability of freshwater dissolved organic carbon (DOC) and divalent metals (Cu, Zn, Ni, Cd) to both physicochemical coagulation and biodegradation. This may imply a less significant transformation of the riverine DOC and metal micronutrients delivery to the Arctic Ocean compared to a traditional estuarine removal scheme.

The estuarine behavior of organic carbon (OC) and trace elements (TE) was studied for the largest European sub-Arctic river, which is the Severnaya Dvina; this river has a deltaic estuary covered in ice during several hydrological seasons: summer (July 2010, 2012) and winter (March 2009) baseflow, and the November-December 2011 ice-free period. Colloidal forms of OC and TE were assessed for three pore size cutoffs (1, 10, and 50 kDa) using an in situ dialysis procedure. Conventionally dissolved (< 0.22 mu m) fractions demonstrated clear conservative behavior for Li, B, Na, Mg, K, Ca, Sr, Mo, Rb, Cs, and U during the mixing of freshwater with the White Sea; a significant (up to a factor of 10) concentration increase occurs with increases in salinity. Si and OC also displayed conservative behavior but with a pronounced decrease in concentration seawards. Rather conservative behavior, but with much smaller changes in concentration (variation within +/- 30%) over a full range of salinities, was observed for Ti, Ni, Cr, As, Co, Cu, Ga, Y, and heavy REE. Strong non-conservative behavior with coagulation/removal at low salinities (< 5 parts per thousand) was exhibited by Fe, Al, Zr, Hf, and light REE. Finally, certain divalent metals exhibited non-conservative behavior with a concentration gain at low (similar to 2-5 parts per thousand, Ba, Mn) or intermediate (similar to 10-15 parts per thousand, Ba, Zn, Pb, Cd) salinities, which is most likely linked to TE desorption from suspended matter or sediment outflux. The most important result of this study is the elucidation of the behavior of the 'truly' dissolved low molecular weight LMW < 1 kDa fraction containing Fe, OC, and a number of insoluble elements. The concentration of the LMW fraction either remains constant or increases its relative contribution to the overall dissolved (< 0.22 mu m) pool as the salinity increases. Similarly, the relative proportion of colloidal (1 kDa-0.22 mu m) pool for the OC and insoluble TE bound to ferric colloids systematically decreased seaward, with the largest decrease occurring at low (< 5 parts per thousand) salinities. Overall, the observed decrease in the colloidal fraction may be related to the coagulation of organo-ferric colloids at the beginning of the mixing zone and therefore the replacement of the HMW1 kDa-0.22 mu m portion by the LMW < 1 kDa fraction. These patterns are highly reproducible across different sampling seasons, suggesting significant enrichment of the mixing zone by the most labile (and potentially bioavailable) fraction of the OC, Fe and insoluble TE. The size fractionation of the colloidal material during estuarine mixing reflects a number of inorganic and biological processes, the relative contribution of which to element speciation varies depending on the hydrological stage and time of year. In particular, LMW < 1 kDa ligand production in the surface horizons of the mixing zone may be linked to heterotrophic mineralization of allochthonous DOM and/or photodestruction. Given the relatively low concentration of particulate versus dissolved load of most trace elements, desorption from the river suspended material was less pronounced than in other rivers in the world. As a result, the majority of dissolved components exhibited either conservative (OC and related elements such as divalent metals) or non-conservative, coagulation-controlled (Fe, Al, and insoluble TE associated with organo-ferric colloids) behavior. The climate warming at high latitudes is likely to intensify the production of LMW < 1 kDa organic ligands and the associated TE; therefore, the delivery of potentially bioavailable trace metal micronutrients from the land to the ocean may increase.

In the conditions of climate warming, the thawing of permafrost can provoke the formation of new thermokarst lakes and subsidence, which facilitates the removal of organic matter from thawed peat into natural waters. Hydrochemical studies of surface waters of the Bolshezemelskaya tundra have demonstrated the exponential dependence of the physicochemical parameters on the size of the water body (peat subsidence, thaw ponds, small lakes, thermokarst lakes). The hydrochemical features of thermokarst lakes of the Bolshezemelskaya tundra are determined by high content of DOC, surface runoff and the thickness of peat deposits. Measurements of concentrations and fluxes of methane showed that all studied water bodies of the Bolshezemelskaya tundra are oversaturated with CH4, and depressions, subsidence, and small water bodies (2) are characterized by the highest concentrations of DOCs. The contribution of these small reservoirs to the total coverage of the surface of the Bolshezemelskaya tundra area is significant, and their consideration can greatly change the assessment of methane fluxes from the arctic tundra.

The colloidal distribution and size fractionation of organic carbon (OC), major elements and trace elements (TE) were studied in a seasonally stratified, organic-rich boreal lake, Lake Svyatoe, located in the European subarctic zone (NW Russia, Arkhangelsk region). This study took place over the course of 4 years in both winter and summer periods using an in situ dialysis technique (1 kDa, 10 kDa and 50 kDa) and traditional frontal filtration and ultrafiltration (5, 0.22 and 0.025 μm). We observed a systematic difference in dissolved elements and colloidal fractions between summer and winter periods with the highest proportion of organic and organo-ferric colloids (1 kDa-0.22 μm) observed during winter periods. The anomalously hot summer of 2010 in European Russia produced surface water temperatures of approximately 30°C, which were 10° above the usual summer temperatures and brought about crucial changes in element speciation and size fractionation. In August 2010, the concentration of dissolved organic carbon (DOC) decreased by more than 30% compared to normal period, while the relative proportion of organic colloids decreased from 70-80% to only 20-30% over the full depth of the water column. Similarly, the proportion of colloidal Fe decreased from 90-98% in most summers and winters to approximately 60-70% in August 2010. During this hot summer, measurable and significant (30% compared to other periods) decreases in the colloidal fractions of Ca, Mg, Sr, Ba, Al, Ti, Ni, As, V, Co, Y, all rare earth elements (REEs), Zr, Hf, Th and U were also observed. In addition, dissolved (0.22 μm) TE concentrations decreased by a factor of 2 to 6 compared to previously investigated periods. The three processes most likely responsible for such a crucial change in element biogeochemistry with elevated water temperature are 1) massive phytoplankton bloom, 2) enhanced mineralization (respiration) of allochthonous dissolved organic matter by heterotrophic aerobic bacterioplankton and 3) photo-degradation of DOM and photo-chemical liberation of organic-bound TE. While the first process may have caused significant decreases in the total dissolved concentration of micronutrients (a factor of 2 to 5 for Cr, Mn, Fe, Ni, Cu, Zn and Cd and a factor of100 for Co), the second and third factors could have brought about the decrease of allochthonous DOC concentration as well as the concentration and proportion of organic and organo-mineral colloidal forms of non-essential low-soluble trace elements present in the form of organic colloids (Al, Y, Ti, Zr, Hf, Th, Pb, all REEs). It can be hypothesized that climate warming in high latitudes capable of significantly raising surface water temperatures will produce a decrease in the colloidal fraction of most trace elements and, as a result, an increase in the most labile low molecular weight LMW(1 kDa) fraction.

Background/Objectives: This paper presents a fast real-time multi-object tracking algorithm adopted for maritime vessels tracking in UAV videos. Methods/Statistical Analysis: This research applies extended multi-object tracking-bydetection framework in highly dynamic UAV-captured maritime environment. Propagation is performed using particle filter whose particle weights are updated using modified observation model that incorporates a term based upon trackers affinity. The latter is also used for trackers grouping, that handles detector inaccuracies, and preventing identity switches by means of special propagation mode that is enabled when targets approach each other and start to overlap. Findings: Our research has shown that state-of-art multi-tracking algorithm is applicable to maritime vessels real-time tracking in UAV videos, provided the use of weak and fast online classifiers, which weakness is compensated by algorithm features based on trackers affinity matrix. Improvements/Applications: This paper presents an approach to maritime vessels tracking that allows to use fast, but less accurate, simple detectors and classifiers, enabling real-time processing on board of small-sized UAVs, while keeping decent precision and accuracy.

The estuarine behavior of organic carbon (OC) and trace elements (TE) was studied for the largest European sub-Arctic river, which is the Severnaya Dvina; this river is a deltaic estuary covered in ice during several hydrological seasons: summer (July 2010, 2012) and winter (March 2009) baseflow, and the November–December 2011 ice-free period. Colloidal forms of OC and TE were assessed using three pore size cutoff (1, 10, and 50 kDa) using an in-situ dialysis procedure. Conventionally dissolved (< 0.22 μm) fractions demonstrated clear conservative behavior for Li, B, Na, Mg, K, Ca, Sr, Mo, Rb, Cs, and U during the mixing of freshwater with the White Sea; a significant (up to a factor of 10) concentration increase occurs with increases in salinity. Si and OC also displayed conservative behavior but with a pronounced decrease of concentration seawards. Rather conservative behavior, but with much smaller changes in concentration (variation within ±30%) over a full range of salinities, was observed for Ti, Ni, Cr, As, Co, Cu, Ga, Y, and heavy REE. Strong non-conservative behavior with coagulation/removal at low salinities (< 5‰) was exhibited by Fe, Al, Zr, Hf, and light REE. Finally, certain divalent metals exhibited non-conservative behavior with a concentration gain at low (~2–5‰, Ba, Mn) or intermediate (~10–15‰, Ba, Zn, Pb, Cd) salinities, which is most likely linked to TE desorption from suspended matter or sediment outflux. The most important result of this study is the elucidation of the behavior of the "truly" dissolved low molecular weight LMW< 1 kDa fraction containing Fe, OC, and a number of insoluble elements. The concentration of the LMW fraction either remains constant or increases its relative contribution to the overall dissolved (< 0.22 μm) pool as the salinity increases. Similarly, the relative proportion of colloidal (1 kDa–0.22 μm) pool for the OC and insoluble TE bound to ferric colloids systematically decreased seaward, with the largest decrease occurring at low (< 5‰) salinities. Overall, the observed decrease of the colloidal fraction may be related to the coagulation of organo-ferric colloids at the beginning of the mixing zone and therefore the replacement of the HMW1 kDa–0.22 μm portion by the LMW< 1 kDa fraction. These patterns are highly reproducible across different sampling seasons, suggesting significant enrichment of the mixing zone by the most labile (and potentially bioavailable) fraction of the OC, Fe and insoluble TE. The size fractionation of the colloidal material during estuarine mixing reflects a number of inorganic and biological processes, the relative contribution of which to element speciation varies depending on the hydrological stage and time of year. In particular, LMW< 1 kDa ligand production in the surface horizons of the mixing zone may be linked to heterotrophic mineralization of allochthonous DOM and/or photodestruction. Given the relatively low concentration of particulate vs. dissolved load of most trace elements, desorption from the river suspended material was less pronounced than in other rivers in the world. As a result, the majority of dissolved components exhibited either a conservative (OC and related elements such as divalent metals) or non-conservative, coagulation-controlled (Fe, Al, and insoluble TE associated with organo-ferric colloids) behavior. The climate warming in high latitudes is likely to intensify the production of LMW< 1 kDa organic ligands and the associated TE; therefore, the delivery of potentially bioavailable trace metal micronutrients from the land to the ocean may increase.

Small aquatic ecosystems of the boreal zone are known to be most sensitive indicators of on-going environmental change as well as local anthropogenic pressure, while being highly vulnerable to external impacts. Compared to rather detailed knowledge of the evolution of large and small lakes in Scandinavia and Canada, and large lakes in Eurasia, highly abundant small boreal lakes of northwest Russia have received very little attention, although they may become important centers of attraction of growing rural population in the near future. Here we present the results of a multidisciplinary, multi-annual study of a small boreal humic lake of NW Russia. A shallow (3 m) and a deep (16 m) site of this lake were regularly sampled for a range of chemical and biological parameters. Average multi-daily, summer-time values of the epilimnion (upper oxygenated) layer of the lake provided indications of possible trends in temperature, nutrients, and bacterio-plankton concentration that revealed the local pollution impact in the shallow zone and overall environmental trend in the deep sampling point of the lake. Organic phosphorus, nitrate, and lead were found to be most efficient tracers of local anthropogenic pollution, especially visible in the surface layer of the shallow site of the lake. Cycling of trace elements between the epilimnion and hypolimnion is tightly linked to dissolved organic matter speciation and size fractionation due to the dominance of organic and organo-ferric colloids. The capacity of lake self-purification depends on the ratio of primary productivity to mineralization of organic matter. This ratio remained >1 both during winter and summer periods, which suggests a high potential of lake recovery from the input of allochthonous dissolved organic matter and local anthropogenic pollution.

The colloidal distribution and size fractionation of organic carbon and trace elements were studied in a seasonally stratified, organic-rich boreal lake, Lake Maselga, located in the European subarctic zone (NW Russia, Arkhangelsk region). This study took place over the course of 5 years in winter (glacial) and summer periods and during the spring and autumn overturn. A newly developed in situ dialysis technique (1, 10, and 50 kDa) and traditional frontal filtration and ultrafiltration (20, 10, 5, 0.22, and 0.025 μm) were used to assess element concentrations at different depths. No significant changes in element concentrations occurred during filtration through sub-colloidal pore-size membranes (20–0.22 μm), suggesting a negligible amount of particulate Fe, OC, and associated trace metals. Large colloids (0.025–0.22 μm) were found to be the main carriers of poorly soluble elements (Fe, Al, Ti, Zr, REEs, Th, and U) during the summer and winter stratification. There was also a clear change in the vertical pattern of the percentage of colloidal Al, Ti, V, Cr, Fe, and Ni during different seasons, and the greatest proportion of colloidal forms was observed during the spring and autumn overturn. This pattern is most likely linked to the dominance of soil (allochthonous) organic carbon, which complexes with trace metals during these periods. During the summer seasons, autochthonous production of small exometabolites or photodegradation increases the concentration of the low-molecular weight fractions (