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Extensive efforts are underway to reduce phosphorus (P) export from the Lake Erie watershed. On the Canadian side, the Thames River is the largest tributary source of P to Lake Erie’s western basin. However, the role of dams in retaining and modifying riverine P loading to the lake has not been comprehensively evaluated. We assessed whether Fanshawe Reservoir, the largest dam reservoir on the Thames River, acts as a source or sink of P, using year-round discharge and water chemistry data collected in 2018 and 2019. We also determined how in-reservoir processes alter P speciation by comparing the dissolved reactive P to total P ratio (DRP:TP) in upstream and downstream loads. Annually, Fanshawe Reservoir was a net sink for P, retaining 25% (36 tonnes) and 47% (91 tonnes) of TP in 2018 and 2019, respectively. Seasonally, the reservoir oscillated between a source and sink of P. Net P release occurred during the spring of 2018 and the summers of 2018 and 2019, driven by internal P loading and hypolimnetic discharge from the dam. The reservoir did not exert a strong influence on DRP:TP annually, but ratio increases occurred during both summers, concurrent with water column stratification. Our analysis demonstrates that Fanshawe Reservoir is not only an important P sink on the Thames River, but also modulates the timing and speciation of P loads. We therefore propose that the potential of using existing dam reservoirs to attenuate downstream P loads should be more thoroughly explored alongside source based P mitigation strategies.

Wilson Mine is a former vanadium mine site located in the Ouachita Mountains near Hot Springs, Arkansas. The site, which drains via two streams to Lake Catherine, has undergone extensive reclamation to significantly reduce groundwater and surface water contact with mine spoils. One of the streams passes through a former mine pit forming East Wilson Pond, and flux from pit lake sediments can result in elevated metal, that is, zinc (Zn), concentrations in overlying water. To mitigate potential risks, an investigation was conducted to evaluate the efficacy of capping materials for partitioning Zn-contaminated sediments from overlying water in East Wilson Pond. A 28-day laboratory study compared the effectiveness of capping materials including combinations of limestone, bentonite clay, and gravel for mitigating Zn flux, including under reasonable worst-case conditions (pH 5.5) encountered in the hypolimnion. Dissolved Zn was monitored over time in overlying water and in sediment porewaters within untreated controls and within the capping layer of treated systems. The use of limestone and/or bentonite clay improved buffering capacity compared to the noncapped control, and pH declined gradually but only modestly in the overlying water and porewater of all treated systems. Concentrations of Zn in overlying water of the noncapped control increased from approximately 30 to 100 µg/L during the study period, while concentrations in the overlying water and porewater of systems containing capping materials remained low (10-30 µg/L). The results demonstrated the effectiveness of the capping materials for neutralizing pH and reducing Zn flux, and a three-layer cap consisting of limestone (top) + bentonite clay (middle) + gravel (bottom) was determined to be most effective. These results were used to inform the selection of materials for the application of a cap to reduce Zn flux from the pit lake sediments. Environ Toxicol Chem 2022;41:193-200. © 2021 SETAC.

Calcite (Cc) has been tested as a potential phosphorus (P) binding agent to be used for non-invasive P removal from lakes. In this approach P binding agent is placed in the water column and subsequently removed together with the sorbed P. Mechanism of orthophosphate (OP) sorption onto Cc and Cc behaviour were investigated under epilimnetic and hypolimnetic temperature, pH and saturation with respect to Cc (25°C, pH = 9.0, saturation index for Cc (SICc) = 1.4, and 10°C, pH = 7.1, SICc = -0.7, respectively) at an initial OP concentration of ∼ 1 mg PO43− L−1 in the range of Cc dosages of 0.01–20.1 g L−1, resulting in OP:Cc mass ratios of 0.06–85 mg:g. Prevailing OP binding processes were adsorption and precipitation, depending on OP:Cc mass ratio (adsorption predominated at low OP:Cc and precipitation at high OP:Cc). Under epilimnetic conditions, approximately five times lower OP:Cc mass ratio was needed to induce precipitation as compared to hypolimnion, indicating at a greater probability of precipitation in epilimnion, while adsorption was more likely to occur in hypolimnion. In epilimnion, co-precipitation of OP was also involved together with adsorption. Calculated maximum adsorption was 35% higher in epilimnion, whereas obtained precipitation rates (measured as unit sorption) were 87% higher, showing promoting effect of epilimnetic conditions on OP removal. Application of Cc into epilimnion will result in a pH drop which can reduce occurrence of cyanobacteria. Application of Cc into hypolimnion will result in its partial dissolution, while most of the applied Cc mass will be removable after the finished treatment.

The dynamics of total phosphorus (TP) in 18 strategic reservoirs of the high-density reservoir network of the Brazilian semiarid was evaluated during the wet and dry periods for the past 12 years. Seasonal overlying concentrations presented no significant differences for about 90% of the reservoirs (p>0.05). This was attributed to a trade-off between the hydrological/limnological processes occurring in the two seasons. Then, a transient complete-mix mass balance model was applied with particular adaptations for the tropical semiarid reservoirs to estimate the TP load for each season. Because of the relatively well-mixed conditions and high hypolimnetic dissolved oxygen concentrations during the wet season, the wet load was assumed to represent the external TP load. On the other hand, because of the absence of reservoir inflow during the dry season, phosphorus release under anoxic sediment conditions and wind-induced resuspension under shallow water depths, the dry load was assumed to reflect the internal TP load. The maximum external loads were related to peak inflows, notably after drought periods. Consistently, the largest internal loads were obtained during the drought periods, when the reservoirs were shallower and more prone to phosphorus release and resuspension. By comparing the impact of the two input load types, the wet period load was predominant in 72% of the reservoirs. The areal phosphorus loads ranged from 0.66 to 7.29 gP m2 year−1, which were consistent with the literature, despite the very high density of reservoirs. Finally, power-law curves including data for all studied reservoirs were adjusted between the dry period load and volume, dry and wet period loads, wet period load and inflow, and total load and catchment area, resulting in satisfactory R2 (0.84–0.98).

The eutrophication of reservoirs can change the physicochemical parameters of water, thus affecting the migration and transformation of heavy metals. At present, there is insufficient research on the coupling mechanisms between nutrients and heavy metals, especially between heavy metals in suspended particles. In this paper, spatial and temporal distribution characteristics of nutrients dissolved heavy metals, and heavy metals in suspended particles were analyzed in a seasonally stratified reservoir. Combined with the nitrogen and phosphorus biogeochemical process, the coupling mechanisms between heavy metals and nutrients were discussed. The results showed that the Aha Reservoir had temperature and dissolved oxygen stratification in April and July. The reduction and dissolution of Fe and Mn oxide/hydroxide and the resuspension of sediments might result in a simultaneous increase in the concentrations of nutrients, dissolved heavy metals and heavy metals in suspended particles in hypolimnion in July and October. In the presence of dissimilatory iron-reducing bacteria (DRIB), the dissolution of iron-bound phosphorus in sediments and suspended particulate matter (SPM) might lead to the simultaneous release of iron and phosphorus into the water. The dissolution of metal sulfides in the sediments and SPM under the action of dissimilatory nitrate reduction to ammonium (DNRA) bacteria might lead to the simultaneous release of ammonia nitrogen and heavy metals into the water. Due to the coupling between nitrogen and phosphorus and heavy metals, seasonal stratified reservoir may face the risk of periodic simultaneous pollution of eutrophication and heavy metals in summer and autumn. This research provides theoretical support for the treatment of heavy metal and eutrophication combined pollution in karst areas.

Although hypolimnetic anoxia is a common occurrence in deep lakes and reservoirs, understanding the processes of oxygen depletion in water columns and sediment is challenging. This study investigates oxygen depletion in both the hypolimnion and sediments in a stratified reservoir. The areal hypolimnetic oxygen demand (AHOD) and sediment oxygen demand (SOD) were evaluated to determine key factors of hypolimnetic anoxia. To evaluate the characteristics of the AHOD in the Jinpen Reservoir (JPR), China, 4 years of field observations of dissolved oxygen (DO) and temperature datasets in the water column are analyzed. SOD was calculated as the sum of the sediment oxygen uptake ( $${\text{J}}_{{\text{O}}_{2}}$$ ) and the flux of reduced substances from deeper sediment layers into the overlying water (Fred), where $${\text{J}}_{{\text{O}}_{2}}$$ was obtained from DO profiles across the sediment–water interface (SWI) at a depth of approximately 94 m in the reservoir, and Fred was obtained from sediment cores. The AHOD in the JPR varied from 0.299 to 1.451 g m−2 day−1 during the summer stratification between 2014 and 2017. The SOD, containing $${\text{J}}_{{\text{O}}_{2}}$$ and Fred, decreased from 0.534 to 0.386 g m−2 day−1 during stratification. Within the SOD, $${\text{J}}_{{\text{O}}_{2}}$$ decreased from 0.415 to 0.168 g m−2 day−1, whereas Fred increased from 0.119 to 0.218 g m−2 day−1. The increase in primary productivity (e.g., diatom blooms), inflows during rainfall, and oxygen concentrations can significantly increase AHOD. The relative importance of $${\text{J}}_{{\text{O}}_{2}}$$ at 40–70% of the SOD is high in the JPR; thus, oxic respiration of organic carbon is the dominant pathway of DO consumption.

Decline in total phosphorus (TP) during lake reoligotrophication does not apparently immediately influence carbon assimilation or deep-water oxygen levels. Traditional monitoring and interpretation do not typically consider the amount of organic carbon exported from the productive zone into the hypolimnion as a measure of net ecosystem production. This research investigated the carbon-to-phosphorus ratios of suspended particles in the epilimnion, (C:P)epi, as indicators of changing productivity. We report sestonic C:P ratios, phytoplankton biomass, and hypolimnetic oxygen depletion rates in Lake Hallwil, a lake whose recovery from eutrophic conditions has been documented in 35 years of historic water-monitoring data. This study also interpreted long-term (C:P)epi ratios from reoligotrophication occurring in four other lakes. Lake Hallwil exhibited three distinct phases. (i) The (C:P)epi ratio remained low when TP concentrations did not limit production. (ii) (C:P)epi increased steadily when phytoplankton began optimizing the declining P and biomass remained stable. (iii) Below a critical TP threshold of ∼15 to ∼20 mg P m-3, (C:P)epi remained high and the biomass eventually declined. This analysis showed that the (C:P)epi ratio indicates the reduction of productivity prior to classic indicators such as deep-water oxygen depletion.

Taste and odor episodes caused by off-flavor secondary metabolites, such as 2-methylisoborneol (MIB) and geosmin, pose one of the greatest challenges for drinking water utilities around the world. The prevalence of these compounds is predicted to increase in the future as a function of nutrient enrichment and elevated temperatures of surface drinking water sources. We conducted a manipulative field experiment in a drinking water reservoir to elucidate patterns for two taste and odor compounds, MIB and geosmin, as well as two taxa known to produce these compounds, phytoplankton (more specifically, cyanobacteria) and actinobacteria, across different depths in response to nutrient enrichment with two common dissolved nitrogen forms, organic urea or inorganic nitrate. In general, we found that MIB levels increased by greater than 250% with nutrient enrichment mediated by increased phytoplankton biomass. However, the effect of the fertilization treatments on MIB decreased with depth with a 35% reduction at 7 m versus 1.5 m. In contrast, geosmin levels reached a maximum at the lowest measured depth (7 m), were unaffected by the fertilization treatments, and followed a similar pattern to the abundance of actinobacteria. Thus, our data suggest that the positive response of phytoplankton (e.g., cyanobacteria, such as Oscillatoria species) to the fertilization treatments is likely responsible for increased MIB, while geosmin concentrations may be a function of actinobacteria-mediated decomposition in the hypolimnion in our study system.

Freshwaters are important emitters of carbon dioxide (CO2) and methane (CH4), two potent greenhouse gases (GHGs). While aquatic surface GHG fluxes have been extensively measured, there is much less information about their underlying sources. In lakes and reservoirs, surface GHG can originate from horizontal riverine flow, the hypolimnion, littoral sediments, and water column metabolism. These sources are generally studied separately, leading to a fragmented assessment of their relative role in sustaining CO2 and CH4 surface fluxes. In this study, we quantified sources and sinks of CO2 and CH4 in the epilimnion along a hydrological continuum in a permanently stratified tropical reservoir (Borneo). Results showed that horizontal inputs are an important source of both CO2 and CH4 (>90 % of surface emissions) in the upstream reservoir branches. However, this contribution fades along the hydrological continuum, becoming negligible in the main basin of the reservoir, where CO2 and CH4 are uncoupled and driven by different processes. In the main basin, vertical CO2 inputs and sediment CH4 inputs contributed to on average 60 % and 23 % respectively to the surface fluxes of the corresponding gas. Water column metabolism exhibited wide amplitude and range for both gases, making it a highly variable component, but with a large potential to influence surface GHG budgets in either direction. Overall our results show that sources sustaining surface CO2 and CH4 fluxes vary spatially and between the two gases, with internal metabolism acting as a fluctuating but key modulator. However, this study also highlights challenges and knowledge gaps related to estimating ecosystem-scale CO2 and CH4 metabolism, which hinder aquatic GHG flux predictions.

Lakes and reservoirs globally produce large quantities of methane and carbon dioxide in their sediments, which accumulate in the hypolimnia (bottom waters) during thermally stratified conditions. A key parameter controlling hypolimnetic greenhouse gas concentrations is dissolved oxygen. Land use and climate change have increased hypolimnetic anoxia worldwide in lakes and reservoirs, which is expected to affect their methane and carbon dioxide concentrations. We conducted whole‐ecosystem oxygenation experiments to assess the effects of oxygen concentrations on dissolved hypolimnetic greenhouse gas concentrations in comparison to a reference reservoir and calculated the maximum hypolimnetic global warming potential in both reservoirs over three summers. We observed significantly greater hypolimnetic methane under anoxic conditions but similar carbon dioxide concentrations, leading to greater hypolimnetic global warming potential of anoxic hypolimnia. Our study indicates that the global warming potential of hypolimnetic greenhouse gas concentrations may increase as the prevalence of hypolimnetic anoxia increases due to global change.

The management of black water depends primarily on the knowledge of the dynamics of organic matter (OM), iron (Fe), sulfide (S), and manganese (Mn), at the water-sediment boundary (WSB). However, the mechanistic path of these substances leading to black water remains unsettled. In this study, a 35-day field study was conducted using the thin-film diffusion gradient technology (DGT) and the planar optrode to address the unknown combined effects of Fe, Mn, OM, S, and tannins from Eucalyptus species on Tianbao reservoir.Our results indicated that the hypolimnion was hypoxic due to thermal stratification, which caused the reduction of insoluble Fe and Mn from sediments to bottom water. Correlation analysis (Fe:S (r:0.5-0.9); Mn:S (r:0.2-0.8)) and elevated fluxes (Fe2+, Mn2+, S2-) connoted that these parameters interacted chemically to give black matter. The content of OM, Fe2+, and tannic acid in the benthic region diminished remarkably (p < 0.05) from day 1 (strong stratification) to day 35 (weak stratification), connoting that these parameters also interacted chemically to give black matter. The turbidity (clarity of the water) increased from day 1 to 35 with a significant difference (p < 0.05) recorded on day 14 confirming that black water was formed on this day when the thermal structure of the reservoir was annihilated. Correlation analysis supported the assertion that the variability in oxygen and redox conditions caused changes in Fe, Mn, and OM content at the WSB.The finding from the field research provides useful information to stakeholders on how to improve the quality of freshwater management designs.

High-altitude Himalayan lakes act as natural storage for environmental evidence related to climate change and environmental factors. A great number of lakes are distributed in the southern slope area of the central Himalayas; however, research concerning the hydrochemical processes of these lakes is still insufficient. Herein, we present a comprehensive study on the water chemistry of the lake waters and the inlet stream waters from Rara Lake in western Nepal based upon samples collected in November 2018. The pH, dissolved oxygen, chlorophyll-a concentration (chl-a), water temperature, electric conductivity (EC) and total dissolved solids (TDS) were measured in situ, and the concentrations of major ions (Ca2+, Mg2+, K+, Na+, Cl−, SO42−, and NO3−) were analyzed in the laboratory. The results revealed that the water in Rara Lake is slightly alkaline, with pH values ranging from 7.6–7.98. The cations, in decreasing order of concentration in the lake water, are Ca2+ >Mg2+ >K+ >Na+ with average concentrations of 20.64 mg·L−1, 11.78 mg·L−1, 1.48 mg·L−1 and 0.72 mg·L−1, respectively; the order and concentrations for the anions is HCO3−>SO42−>Cl−>NO3−, with average concentrations of 122.15 mg·L−1, 2.15 mg·L−1, 0.46 mg·L−1 and 0.55 mg·L−1, respectively. The dominant cation and anion in the lake water are Ca2+ and HCO3−, and they account for 48.14% and 71.8% of the totals, respectively. The range of lake water TDS is from 95 mg·L−1 to 98 mg·L−1, with an average of 96.85 mg·L−1. The high ratio of (Ca2+ + Mg2+) to total cations and the low ratio of (Na+ + K+) to total cations indicate that Rara Lake receives ions from rock weathering, especially from carbonate rocks. Similarly, Gibbs boomerang diagrams and Piper diagrams also support the hydrochemistry of Rara Lake as being dominated by rock-weathering patterns. Likewise, other statistical analysis tools, such as Principal Component Analysis (PCA) and correlation strongly suggest the dominance of weathering of calcium and magnesium bicarbonate rocks as the major source of ions in Rara Lake. However, several traces of anthropogenic inputs into the lake were noticed, and the hypolimnion in the lake appears to be oxygen deficient, which may not be an issue at present but cannot be ignored in the future.

Attempts to restore Lake Lugano, Switzerland and Italy, from eutrophication have produced weak responses in the target variables (primary productivity and hypolimnetic oxygen concentrations), indicating shortcomings in the underlying eutrophication model. An analysis of monitoring data showed that the decrease in phosphorus concentration, although nearly compliant with restoration targets, produced only slight decreases in primary productivity and no change in hypolimnetic oxygen conditions. These target variables were equally or more sensitive to factors external to trophic state, including plankton structure, which influenced primary productivity, and the depth of mixing during turnovers, which influenced hypolimnetic oxygen. To improve the chance of success, the restoration approach should revise the phosphorus concentration target and explicitly account for the influence of external variation, especially mixing depth.

Mercury (Hg) methylation is a microbially mediated process that converts inorganic Hg into bioaccumulative, neurotoxic methylmercury (MeHg). The metabolic activity of methylating organisms is highly dependent on biogeochemical conditions, which subsequently influences MeHg production. However, our understanding of the ecophysiology of methylators in natural ecosystems is still limited. Here, we identified potential locations of MeHg production in the anoxic, sulfidic hypolimnion of a freshwater lake. At these sites, we used shotgun metagenomics to characterize microorganisms with the Hg-methylation gene hgcA. Putative methylators were dominated by hgcA sequences divergent from those in well-studied, confirmed methylators. Using genome-resolved metagenomics, we identified organisms with hgcA (hgcA+) within the Bacteroidetes and the recently described Kiritimatiellaeota phyla. We identified hgcA+ genomes derived from sulfate-reducing bacteria, but these accounted for only 22% of hgcA+ genome coverage. The most abundant hgcA+ genomes were from fermenters, accounting for over half of the hgcA gene coverage. Many of these organisms also mediate hydrolysis of polysaccharides, likely from cyanobacterial blooms. This work highlights the distribution of the Hg-methylation genes across microbial metabolic guilds and indicate that primary degradation of polysaccharides and fermentation may play an important but unrecognized role in MeHg production in the anoxic hypolimnion of freshwater lakes.

Lakes and reservoirs globally are experiencing unprecedented changes in land use and climate, depleting dissolved oxygen (DO) in the bottom waters (hypolimnia) of these ecosystems. Because DO is the most energetically favorable terminal electron acceptor (TEA) for organic carbon mineralization, its availability controls the onset of alternate TEA pathways (for example, denitrification, manganese reduction, iron reduction, sulfate reduction, methanogenesis). Low DO concentrations can trigger organic carbon mineralization via alternate TEA pathways in the water column and sediments, which has important implications for greenhouse gas production [carbon dioxide (CO2) and methane (CH4)]. In this study, we experimentally injected supersaturated DO into the hypolimnion of a eutrophic reservoir and measured concentrations of TEAs and terminal electron products (TEPs) in the experimental reservoir and an upstream reference reservoir. We calculated the electron equivalents yielded from each TEA pathway and estimated the contributions of each TEA pathway to organic carbon processing in both reservoirs. DO additions to the hypolimnion of the experimental reservoir promoted aerobic respiration, suppressing most alternate TEA pathways and resulting in elevated CO2 accumulation. In comparison, organic carbon mineralization in the reference reservoir’s anoxic hypolimnion was dominated by alternate TEA pathways, resulting in both CH4 and CO2 accumulation. Our ecosystem-scale experiments demonstrate that the alternate TEA pathways that succeed aerobic respiration in lakes and reservoirs can be manipulated at the ecosystem scale. Moreover, changes in the DO dynamics of freshwater lakes and reservoirs may result in concomitant changes in the redox reactions in the water column that control organic carbon mineralization and greenhouse gas accumulation.

In many temperate oligotrophic lakes, algal accumulations can form below the mixing zone. However, Deep Chlorophyll Maxima (DCM) have also been found in some eutrophic, tropical lakes and in this paper we aim to identify if they are recurrent features in these kinds of lakes and to recognize the factors that favor their formation. We analyzed 5 years of thermal stratification, water quality, and chlorophyll a concentrations in a tropical eutrophic lake in Central Mexico. Thermal stratification patterns were characteristic of warm monomictic lakes. Full water column deoxygenation during winter mixing was recorded in 3 of the analyzed years, and an increase of ~ 1 °C in the hypolimnion was detected between 2011 and 2015. DCM were detected in 4 out of the 5 studied years, at the top of the hypolimnion when the water column was stratified (spring–summer). This study is the first report of recurrent DCM formation in the northern limit of the Neotropics. It confirms that high light penetration is a necessary condition for DCM. Stratified nutrients with epilimnetic P depletion are also factors favoring DCM formation.

The mixing and stratification present under the ice during winter can have a profound influence on the following summertime hypolimnetic oxygen levels. During winter, plankton rely on updrafts caused by convection to remain in the photic zone in ice-covered lakes, thus there is a crucial link in winter between light levels, under-ice circulation and dissolved oxygen (DO) production. Detailed observations of temperature and oxygen over 3 winters suggest that radiatively driven convection is correlated to oxygen increases in the mixing layer. Both plankton abundance and dissolved oxygen were maximum near the end of the winter before the ice melted. Oxygen became supersaturated by the end of the severe winter of 2015 when the ice cover duration was the longest, whereas DO was slightly below saturation in the warmer winters of 2016 and 2017. After ice-off, the combination of high-frequency measurements through winter and bi-weekly sampling in spring through to summer suggests that decreases in DO started when spring overturn ended and the water column became weakly stratified, which was very close to the timing of when the mean water column temperature first exceeded 4oC. The implication of this work is that the winter oxygen dynamics are important for the hypolimnetic oxygen concentrations when the water column becomes stratified, which in turn sets initial conditions for the degree of any late summer hypoxia.

The Hells Canyon Complex (HCC) along the Snake River (Idaho-Oregon border, U.S.A.) encompasses three successive reservoirs that seasonally stratify, creating anoxic conditions in the hypolimnion that promote methylmercury (MeHg) production. This study quantified seasonal dynamics and interannual variability in mercury concentrations (inorganic divalent mercury (IHg) and MeHg) and loads at four reservoir inflow and outflow locations through the HCC (2014-2017). We observed (1) that the HCC is a net sink for both IHg and MeHg, (2) interannual variability in IHg and MeHg loads largely reflecting streamflow conditions, and (3) seasonal variability in particulate IHg loading at the inflow (greatest from February to April) and MeHg export from the outflow (greatest from September to December) of the HCC. Seasonal export of MeHg was evidenced by increases in monthly mean concentrations of unfiltered MeHg (approximately 2-fold) and the percentage of total mercury (THg) as MeHg (≥4-fold) coincident with reservoir destratification. Despite evidence of seasonal export of MeHg from the HCC, annual loads indicate a 42% decrease in unfiltered MeHg from HCC inflow to outflow. Results from this study improve the understanding of seasonal variability in mercury transport through and transformation within a reservoir complex.

The aim of the study was to assess the efficiency of mineral aggregates in orthophosphate (OP) removal from hypolimentic water (HW) withdrawn from eutrophic lakes. Four P reactive materials were tested in sorption experiments: light weight expanded aggregate (LECA), zeolite (ZL), dolomite (DM) and limestone (LS). The two materials proved to be most effective in OP binding were LS and LECA. For these agents we conducted further investigations to reveal the effect of variable pH and temperature in the range commonly found in HW withdrawn from eutrophic lakes (6.0–8.0 and 5–15°, respectively). Kinetic studies under varying OP concentrations (0.1–2.0 mg PO43− L−1) in synthetic solutions and HW were involved to check influence of hypolimnion chemistry on OP fixation. LS was the most efficient in OP removal, followed by LECA, dolomite and zeolite. OP sorption by LS and LECA increased with decreasing pH and increasing temperature and was faster and higher in HW as compared to standard solutions. The results indicate that OP fixation onto these materials would be most efficient in case of lakes with acidic and relatively warm bottom layer. Hypolimnion warming, which is often observed in restored lakes, will further enhance OP removal. Use of LS mixed with LECA was recommended for a potential treatment of HW withdrawn from eutrophic lakes. OP retention before hypolimnion transport downstream can mitigate side effects of hypolimnion removal by preventing eutrophication of receiving water bodies which can be regarded as substantial improvement of this lakes` restoration method.

Lakes and reservoirs contribute substantially to atmospheric concentrations of the potent greenhouse gas methane. Lake sediments produce large amounts of methane, which accumulate in the oxygen-depleted bottom waters of stratified lakes. Climate change and eutrophication may increase the number of lakes with methane storage in the future. Whether stored methane escapes to the atmosphere during annual lake overturn is a matter of controversy and depends critically on the response of the methanotroph assemblage. Here we show, by combining 16S rRNA gene and pmoA mRNA amplicon sequencing, qPCR, CARD-FISH and potential methane-oxidation rate measurements, that the methanotroph assemblage in a mixing lake underwent both a substantial bloom and ecological succession. As a result, methane oxidation kept pace with the methane supplied from methane-rich bottom water and most methane was oxidized. This aspect of freshwater methanotroph ecology represents an effective mechanism limiting methane transfer from lakes to the atmosphere., Mayr et al. show that the methanotroph assemblage grows fast enough to limit methane outgassing to the atmosphere during lake overturn when methane from the bottom water is transferred to the surface layer in lake Rotsee. The study suggests that an expanding and changing methanotroph assemblage is responsible for the increasing methane oxidation capacity during autumn overturn.

The byproducts of anaerobic biogeochemical processes, many mediated at the sediment–water interface, can degrade water quality in the bottom of reservoirs. Using both experimental sediment–water chambers and field monitoring, this study assessed nutrient and metals cycling in the profundal zone of hypereutrophic Hodges Reservoir, San Diego (maximum depth = 29.1 m; maximum surface area = 6.0 km2). A focus of the study was methylmercury (MeHg), a toxic form of mercury that bioaccumulates in aquatic food webs. Results confirmed that oxic conditions repressed release of redox-sensitive compounds (e.g., MeHg, ammonia, phosphate, manganese) from profundal sediment, though high sediment oxygen demand complicated experimental efforts to maintain a well-oxygenated sediment–water interface. In both experimental chambers and in the reservoir, patterns of MeHg cycling correlated with manganese cycling, suggesting that moderately low redox conditions that stimulate reductive dissolution of manganese also enhance net Hg methylation. Sediment MeHg flux was tightly coupled with sulfate uptake under moderately reduced conditions, reinforcing the link between sulfate-reducing bacteria and Hg methylation. Results highlight the fact that hypolimnetic oxygenation in Hodges Reservoir using pure oxygen gas, which is planned for spring 2020, must maintain high oxygen concentrations at the profundal sediment–water interface. In addition, results indicate that the presence of manganese in surface water can be used as an indicator of oxygenation’s effectiveness in lowering rates of internal nutrient and MeHg loading.

Gonyostomum semen is a bloom-forming freshwater raphidophyte that is currently on the increase, which concerns water managers and ecologists alike. Much indicates that the recent success of G. semen is linked to its diel vertical migration (DVM), which helps to overcome the spatial separation of optimal light conditions for photosynthesis at the surface of a lake and the high concentration of phosphate in the hypolimnion. I here present data from a field study conducted in Lake Lundebyvannet (Norway) in 2017-2019 that are consistent with the idea that the DVM of G. semen also allows for a hypolimnetic uptake of ammonium. As expected, microbial mineralization of organic matter in a low-oxygen environment led to an accumulation of ammonium in the hypolimnion as long as G. semen was absent. In contrast, a decreasing or constantly lower concentration of hypolimnetic ammonium was found in presence of a migrating G. semen population. In summer of 2019, a short break in the DVM of G. semen coincided with a rapid accumulation of hypolimnetic ammonium, which was equally rapidly decimated when G. semen resumed its DVM. Taken together, these data support the idea that G. semen can exploit the hypolimnetic pool of ammonium, which may be one reason for the recent success of the species and its significant impact on the structure of the aquatic food web.

Effects of hypolimnetic aeration (pumping of epilimnetic water into the hypolimnion) on the quantity of settling material in eutrophied Lake Vesijärvi, Finland were studied by comparing spatially comprehensive gross sedimentation rates as dry and organic matter prior to aeration activity and during two aerated years. Possible changes in the organic matter (as loss on ignition, LOI), carbon (C) and nitrogen (N) contents and changes in the C/N ratio of the settling material and surface sediment were quantified. Thermal stratification broke up earlier due to aeration and was followed by sedimentation peaks. The absolute amount of dry and organic matter as well as C and N settling to the lake bottom were significantly higher in the aerated years. Increased sedimentation rates were especially pronounced in the deep zones indicating enhanced sediment focusing. Increased sedimentation of C and N reflected higher primary production during the aerated years, which most likely was associated with increased temperature and turbulence and the subsequent regeneration and recycling of nutrients in the water body. Aeration seemed to slightly enhance degradation, but contrary to its ultimate aim, it failed to decrease the phosphorus content of the water column and deposits of organic material in the deep zones of the lake.

Temperature–oxygen profiles, collected biweekly to monthly for ∼40 years, were used to calculate end-of-summer volume-weighted hypolimnetic oxygen (VWHO) concentrations in six small lakes located in south-central Ontario, Canada. Coherent decreases in thermocline depth and increases in hypolimnetic volume, mean hypolimnetic dissolved oxygen (DO) concentration, and VWHO were observed in five of the six study lakes. All lakes underwent an abrupt increase in VWHO and mean hypolimnetic DO after 2010. In four of the six study lakes, the highest hypolimnetic DO concentrations were observed in years where chlorophyll a concentrations were low, whereas at five of the six study lakes the highest hypolimnetic volumes were observed when dissolved organic carbon concentrations were relatively high. Warmer spring or winter air temperatures were associated with higher hypolimnetic DO concentrations at two sites, and longer ice-free periods were associated with smaller hypolimnetic volumes at two sites. These results suggest that the recent VWHO increases in the studied south-central Ontario lakes may be a function of multiple drivers that include changes in primary production, lake water transparency, and regional climatic factors.

—The total abundance of bacteria, the number of saprophytic, oligotrophic, and sulfate-reducing microorganisms, the shapes of bacterial cells, the primary and bacterial production, the destruction of organic matter, and the rate of sulfate reduction were studied in small Karelian lakes during the stagnation period. It has been shown that the uneven vertical distribution of light, temperature, oxygen, and hydrogen sulfide creates ecological niches for the development of many various groups of microorganisms. In most lakes, the minimum abundance of microorganisms was found in the surface water layer, while its maximum was in the near-bottom layers. The trophic status of a water body has a great influence on the formation and function of microbial communities. Autochthonic organic matter in the closed forest lakes under study (the local term is “lamba”) was formed due to phytoplankton photosynthesis, but bacterial production exceeded primary production in the hypolimnion of eutrophic lakes. The content of hydrogen sulfide, the abundance of sulfate-reducing bacteria, and the rate of sulfate reduction were maximal in the bottom sediments of eutrophic lambas.

—Studies of epilimnion, metalimnion, and hypolimnion are of great importance for understanding the vertical structure and functioning of aquatic ecosystems. The present article reports new facts related to the stratification of freshwater ecosystems. In particular, the concentration of dissolved oxygen, temperature, and hydrobiological parameters at depths from 0 to 30 m of Glubokoe Lake (Moscow oblast) were determined. The metalimnion (thermocline) which is characterized by a high vertical temperature gradient was found to accumulate a large amount of detritus and contain increased amounts of bacteria and algae cells (phytoplankton). The increased content of organic matter and aquatic organisms is responsible for intense degradation processes which lead to depletion of oxygen.

Nitrogen and phosphorus are key elements in controlling eutrophication in the aquatic system. Water and sediment samples were collected from Hongfeng Lake, a seasonally stratified reservoir in southwest China, in winter and summer. Diffusion fluxes of NH4+, NO3−, and labile P in summer using diffusive gradients in thin films technology were 3.4, −37.2, and 0.9 mg m−2 day−1, respectively, based on Fick’s first law. The diffusion flux of labile P was 2.05 mg m−2 day−1 in winter. The contributions fraction of the labile P diffusion flux from sediment to the overlying water were higher in winter than those in summer, because of the relatively lower external input, concentrations and higher diffusion fluxes in winter. After the external input decreased, all of the three diffusion fluxes were lower than the previous record. To understand the influence effect of hydrodynamics, environmental fluid dynamics code modeling was used to simulate the flow and temperature field in winter and summer. Modeling results showed that velocity in summer was higher than that in winter due to concentrated rainfall within the catchment. Moreover, the velocity and temperature in the euphotic zone were higher than that of the hypolimnion in summer. Less variation of velocity and temperature in vertical profile in winter than that in summer was observed, which may be attributable to the high specific heat capacity and the low heat conductivity of water. There was no significant correlation among velocity, hydrochemistry, nitrogen, and phosphorus concentrations. Hydrodynamics, solar radiation, and water depth affect the position of the thermocline, which was consequently to water temperature, hydrochemistry, dissolved nitrogen, and phosphorus concentration. Correlation analysis suggested that the higher bottom velocity and total bed shear may accelerate labile P, NH4+, and NO3− diffusion fluxes. These results provide evidence and suggestions for preventing and controlling reservoir eutrophication and water safety management.

Methane (CH4) is an important component of the carbon (C) cycling in lakes. CH4 production enables carbon in sediments to be either reintroduced to the food web via CH4 oxidation or emitted as a greenhouse gas making lakes one of the largest natural sources of atmospheric CH4. Large stable carbon isotopic fractionation during CH4 oxidation makes changes in 13C:12C ratio (δ13C) a powerful and widely used tool to determine the extent to which lake CH4 is oxidized, rather than emitted. This relies on correct δ13C values of original CH4 sources, the variability of which has rarely been investigated systematically in lakes. In this study, we measured δ13C in CH4 bubbles in littoral sediments and in CH4 dissolved in the anoxic hypolimnion of six boreal lakes with different characteristics. The results indicate that δ13C of CH4 sources is consistently higher (less 13C depletion) in littoral sediments than in deep waters across boreal and subarctic lakes. Variability in organic matter substrates across depths is a potential explanation. In one of the studied lakes available data from nearby soils showed correspondence between δ13C-CH4 in groundwater and deep lake water, and input from the catchment of CH4via groundwater exceeded atmospheric CH4 emissions tenfold over a period of 1 month. It indicates that lateral hydrological transport of CH4 can explain the observed δ13C-CH4 patterns and be important for lake CH4 cycling. Our results have important consequences for modelling and process assessments relative to lake CH4 using δ13C, including for CH4 oxidation, which is a key regulator of lake CH4 emissions.

Meromictic lakes are unique aquatic ecosystems that occur extremely rarely. The phenomenon of meromixis can result from both natural and anthropogenic factors. The aim of this study was to analyse thermal and chemical stratification in a small, deep (6 ha, H max = 24.5 m) lake. The evaluated lake had a typical summer thermal profile with a shallow epilimnion, a sharp thermocline, and a distinct monimolimnion layer in the hypolimnion, which was also maintained during circulation. The lake had a clinograde oxygen profile, with an oxygen deficit in the metalimnion and permanent anoxic conditions in the deeper layers, including during circulation. A redox zone was identified during summer stagnation. The monimolimnion formed a thermally isolated layer at a depth of around 15 m, and the chemocline was situated above the monimolimnion. In the chemocline, the EC gradient ranged from 61 to 77 μS·cm−1 per meter of depth in the summer and from 90 to 130 μS·cm−1 per meter of depth during circulation. EC was significantly correlated with Ca2+ concentration (r2 = 0.549). Chemical stratification, particularly with regard to organic matter distribution, was observed in the chemocline. The monimolimnion severely limited nutrient internal loading.

Methane (CH4) and carbon dioxide emissions from lakes are relevant for assessing the greenhouse gas output of wetlands. However, only few standardized datasets describe concentrations of these gases in lakes across different geographical regions. We studied concentrations and stable carbon isotopic composition (δ13C) of CH4 and dissolved inorganic carbon (DIC) in 32 small lakes from Finland, Sweden, Germany, the Netherlands, and Switzerland in late summer. Higher concentrations and δ13C values of DIC were observed in calcareous lakes than in lakes on non-calcareous areas. In stratified lakes, δ13C values of DIC were generally lower in the hypolimnion due to the degradation of organic matter (OM). Unexpectedly, increased δ13C values of DIC were registered above the sediment in several lakes. This may reflect carbonate dissolution in calcareous lakes or methanogenesis in deepwater layers or in the sediments. Surface water CH4 concentrations were generally higher in western and central European lakes than in Fennoscandian lakes, possibly due to higher CH4 production in the littoral sediments and lateral transport, whereas CH4 concentrations in the hypolimnion did not differ significantly between the regions. The δ13C values of CH4 in the sediment suggest that δ13C values of biogenic CH4 are not necessarily linked to δ13C values of sedimentary OM but may be strongly influenced by OM quality and methanogenic pathway. Our study suggests that CH4 and DIC cycling in small lakes differ between geographical regions and that this should be taken into account when regional studies on greenhouse gas emissions are upscaled to inter-regional scales.

Lake Sevan is a meso-eutrophic water body, which was severely impacted by anthropogenic level decrease, pollution and eutrophication during the last century. Starting in the 1970s, these processes resulted in the formation of an oxygen-depleted hypolimnion during summer-autumn stratification of the lake. In this work, we demonstrate for the first time that eutrophication of the lake leads not only to the full depletion of oxygen and nitrate in the hypolimnion but as well to the presence of sulfate-reducing microorganisms and toxic hydrogen sulfide. Concentrations of hydrogen sulfide in the hypolimnion of Major and Minor Sevan in October were as high as 9 and 39 μM, respectively. In October 2019, 66 % of lake's bottom was covered by sulfidic waters, while the fraction of sulfidic water volume reached 19 %. Values of δ34S for hypolimnetic sulfide are lower by only 7-12 ‰ compared to epilimnetic sulfate, while δ33S values of sulfide are similar to the δ33S values of sulfate. These isotopic fingerprints are not consistent with microbial sulfate reduction as the sole source of hydrogen sulfide in the hypolimnion. We attribute the formation of a sulfidic deep-water layer to a combination of microbial sulfate reduction in the water column and diffusion of hydrogen sulfide from the sediments.

Most reservoirs in subtropical areas experience periodic variations in the thermal structure of their water columns, with times of strong thermal stratification being succeeded by periods of mixing, over the course of the year. Understanding of the transport and transformation of dissolved inorganic carbon over such thermal cycles in artificial reservoirs remains poor. To address this problem, this study examined the spatiotemporal behavior of dissolved inorganic carbon (DIC), the partial pressure of CO2 (pCO2), carbon isotope ratios (δ13CDIC), and CO2 emission (FCO2), from 2014 to 2018 in a subtropical, groundwater-fed reservoir in southern China. It was found that CO2 emissions during mixing periods are much higher than in thermally stratified periods (particularly during transition from stratified to mixing) as a result of upwelling and release of dissolved CO2 (CO2aq) accumulated in the hypolimnion. CO2 emission fluxes at the water-gas interface accounted for only a small proportion of the DIC in the reservoir. The relationships between of DIC and δ13CDIC displayed two distinct modes, due to spatial differences in water depths and to strong thermal stratification during warmer seasons: (1) DIC concentrations increase and δ13CDIC values decrease from epilimnion to hypolimnion, and (2) δ13CDIC values decrease with increasing DIC concentrations but δ13CDIC is progressively enriched near the bottom during periods of thermal stratification. In addition, this study found three distinct processes of DIC accumulation and consumption in the reservoir: (1) DIC accumulated in the hypolimnion during thermal stratification periods, due to carbon retention but (2) DIC was substantially consumed in the epilimnion during such periods, and (3) average DIC concentrations and pCO2 increased significantly from upstream to downstream along the reservoir, while average δ13CDIC values became lighter. These results highlight that carbon behavior in groundwater‑fed reservoirs is often controlled by a combination of biogeochemical processes and seasonal variations in thermal structure. Sampling and monitoring strategies should consider these factors in order to accurately estimate carbon budgets in reservoirs, lakes or ponds.

Anthropogenic eutrophication caused by excess loading of nutrients, especially phosphorus (P), from catchments is a major cause of lake water quality degradation. The release of P from bed sediments to the water column, termed internal loading, can exceed catchment P load in eutrophic lakes, especially those that stratify during warm summer periods. Managing internal P loading is challenging, and although a range of approaches have been implemented, long-term success is often limited, requiring lake-specific solutions. Here, we assess the manipulation of lake residence time to inhibit internal loading in Elterwater, a shallow stratifying lake in the English Lake District, UK. Since 2016, additional inflowing water has been diverted into the inner basin of Elterwater to reduce its water residence time, with the intention of limiting the length of the stratified period and reducing internal loading. Combining eight years of field data in a Before-After-Control-Impact study with process-based hydrodynamic modelling enabled the quantification of the residence time intervention effects on stratification length, water column stability, and concentrations of chlorophyll a and P. Annual water residence time was reduced during the study period by around 40% (4.9 days). Despite this change, the lake continued to stratify and developed hypolimnetic anoxia. As a result, there was little significant change in phosphorus (as total or soluble reactive phosphorus) or chlorophyll a concentrations. Summer stratification length was 2 days shorter and 7% less stable with the intervention. Our results suggest that the change to water residence time in Elterwater was insufficient to induce large enough physical changes to improve water quality. However, the minor physical changes suggest the management measure had some impact and that larger changes in water residence time may have the potential to induce reductions in internal loading. Future assessments of management requirements should combine multi-year observations and physical lake modelling to provide improved understanding of the intervention effect size required to alter the physical structure of the lake, leading to increased hypolimnetic oxygen and reduced potential for internal loading.

HPLC-ICPMS was used to analyze the spatiotemporal variation of As species in different sections and tributaries of the Aha Reservoir over four seasons, and the migration and transformation mechanisms were clarified by combined analysis of hydrochemical parameters and microbial composition. The results showed that the internal release of As from the reservoir sediments is mainly due to the reduction of iron oxide and the release of adsorbed As(V). The average proportion of As(III) increased from 27.2% in autumn to 46.5% in summer, 68.9% in winter, and up to 70.8% in spring. In spring and summer, the high concentration of As(III) and organic arsenic in the epilimnion under phosphorus restriction was caused by the reductive metabolism of phytoplankton after intake of As(V). The arsenic species in the metalimnion were mainly affected by the oxidation-reduction potential (ORP). In summer and autumn, As-oxidizing bacteria used As(III) as an electron donor, and nitrate played an important role as an electron acceptor, maintaining the dominance of As(V) in the hypolimnion. However, in winter and spring, temperature-controlled ORP was the main process, which was dominated by As(III). In conclusion, As species show annual cycles in different layers of seasonally thermal stratified reservoirs. It provides a systematic mechanism of As species transformation in reservoirs, especially the effect of biological transformation mechanism.

Lake Alboraj, located in southeast Spain, was declared natural Microreserve and included into European Natura-2000 Network due to its contribution to environmental heritage. Unfortunately, the ecological status of the lake has changed dramatically, mainly due to the lowering of water table caused by groundwater abstractions for irrigation. It is a permanent small karstic lake whose surface has reduced in the last decades to nearly the third part of its historical water level. The water column shows a marked seasonal oxycline, that splits an aerobic upper layer (epilimnion) from an anaerobic layer below (hypolimnion). Sequencing 16S rRNA gene amplicons and applying chemical tools at epilimnion, hypolimnion and sediment, showed a clear gradient in the bacterial community structure, which support the co-existence of assimilatory and dissimilatory microbial mediated reactions. Results allows to infer that microbial stratification could provide various physical and chemical environments at different depths in the water column related to biogeochemical reactions providing N-S-C- recycling processes.

Absence of dissolved oxygen (anoxia) in the hypolimnion of lakes can eliminate habitat for sensitive species and may induce the release of sediment-bound phosphorus. Lake anoxia generally results from decomposition of organic matter, which is exacerbated by high nutrient loads. Total phosphorus (TP) in lakes is regulated by static aspects of the lake's watershed, but lake TP can be readily increased by human activities. In some low-nutrient lakes, basin morphometry may induce naturally occurring anoxia. The occurrence of natural anoxia is especially important to consider in lake water quality assessments that compare observed conditions to expected reference conditions. To investigate the occurrence of natural vs. anthropogenically influenced anoxia, we constructed a logistic regression model to calculate the probability of low-nutrient lakes (TP

In eutrophic lakes and reservoirs, reduced mixing during stratified conditions limits oxygen (O2) supply to the hypolimnion (that is, bottom waters). In the absence of an O2 supply, microbial decomposers consume alternative electron acceptors, generally in order of their thermodynamic favorability, releasing soluble, reduced manganese (Mn), iron (Fe) and methane (CH4) to the water column with implications for reservoir water quality and greenhouse gas dynamics. Still, there are very few studies that quantify intra- and inter-annual controls on lake and reservoir redox chemistry, especially in managed systems. To address this knowledge gap, we examined redox-sensitive water column chemistry before and during four summer’s end water-level drawdown events (~ 2 m in magnitude) in a eutrophic reservoir. We observed lower dissolved Fe and CH4 concentrations in years with higher hypolimnion O2 and NO3−, suggesting that water column oxidant availability controls the extent of the redox cascade. During drawdowns, dissolved CH4, Mn and Fe concentrations increased in the hypolimnion (on average by 50%, 40% and 175%, respectively) concomitant with order of magnitude increases in methane bubbling rates (that is, ebullition). To our knowledge, this is the first in situ evidence for enhanced flushing of sediment pore water into a reservoir during water-level drawdowns. Furthermore, the mass of CH4, Mn and Fe released varied as a function of summertime redox conditions. Thus, the timing of reservoir water-level drawdowns may determine the degree to which reduced solutes enter the water column during drawdown, with longer pre-drawdown periods of hypolimnion hypoxia leading to higher solute fluxes.

Dickson Lake, located in Algonquin Provincial Park, Ontario, is a remote, oligotrophic lake, where cyanobacterial blooms of the genus Dolichospermum (Ralfs ex Bornet & Flahault) P. Wacklin, L. Hoffmann and J. Komarek were reported for the first time in the fall of 2014, and subsequently in the late spring of 2015. To investigate the potential environmental triggers of these bloom events, we assessed long-term trends in water quality using a multi-proxy paleolimnological approach, examining sedimentary diatoms, chironomids, cladocerans, spectrally inferred chlorophyll a, and cyanobacterial akinetes preserved in a 210Pb-dated sediment core. Assemblage changes were modest in all biological proxies. A subtle increase in the abundance of warm-water chironomid taxa (Topt > 15 °C) commences in the year ~ 2000, with further increases in the most recent years of the sediment record (~ 2013–2015). End-of-summer volume-weighted hypolimnetic oxygen concentrations (CI-VWHO), inferred from chironomid remains, reveal a decline in oxygen concentrations over the last two decades coincident with the highest levels of sedimentary chlorophyll a and cyanobacterial akinetes in the sediment record. These paleolimnological findings corroborate observed reports of the onset of cyanobacterial blooms in Dickson Lake in late 2014 and are consistent with increasingly favourable bloom-forming conditions over the past few decades that are related to warmer air temperatures, sharp declines in wind speed, and a lengthening of the ice-free season by 2 weeks since 1975. It is plausible that late ice-out and a quick onset to stratification in 2014 may have resulted in incomplete spring mixing, early onset of hypolimnetic anoxia, and increased internal nutrient loading, that, occurring during a period when climate conditions were particularly ideal for cyanobacterial proliferation, may have fueled the unprecedented algal blooms in this remote lake. Collectively, the factors causing algal blooms in remote lakes such as Dickson Lake are not yet fully understood, and it is worrisome that with continued warming the triggering conditions may become a more common feature of Algonquin Park and other minimally impacted Boreal Shield lakes in the coming years.

The article contains the results of the ecological and hygienic diagnostics of conditions of lands of areas of different sizes and varying degrees of urbanization, located in different agro-climatic conditions of European Russia (the southern Astrakhan region, he city of Moscow). Assessments of the state of territories use a new approach to rationing of Hg according to quantitative ratios of the content of its thermoforms in soils, grounds and alluvium (in litho substrate). The method of diagnostics is based on known properties of Hg forms: their geochemical activity (migration mobility) and toxicity decline along with the increment of the Hg temperature threshold of the release in the process of continuous warming up of the litho substrate sample (up to 1100°C). Thermoforms are not tied to specific minerals or chemicals and are conditionally designated as FR (Free: 500°C) Hg forms. Among forms, also conditionally according to complexes of low, medium and high temperature variations(FR + CL, FS + CS and SU + IZ, respectively, there are selected groups of «mobile», «sustainable» and «inert» forms. For technogenic Hg there is characterized the predominance in the sample of «mobile» and to lesser extent degree - «stable» thermoforms. The more intensive this predominance is, the higher is environmental hazard. The natural accumulation is diagnosed by the presence and prevalence of the sum of moderate - and high-temperature inert forms of Hg. According to the ratios of Hg content, its forms and groups, as well as the density of correlationships between them, 6 indices of the conditions of soils and alluvium were developed by ourselves.Main coefficients out of indices on informativeness of results of diagnostics are the following factors: Endogenous input, Geochemical activity and Hg mobility. With regard to the combination of variability in indices, the total amount of Hg, the genesis and intensity of its accumulation (natural or anthropogenic), there were also developed ranking scales of the ecological and hygienic condition of the land areas and surface water bodies. As a result, on the base of such approach to the regulation of Hg the information value of assessments of mercury contamination of land significantly increased. At the objects of assessments there were identified following areas: foci of the manifestation of Hg hypolimnetic emanations on the landscape surface - dispersion halos in the areas of the Earth crust fracture (the dome of the Astrakhan gas condensate field); environmentally dangerous (sometimes even at a low total content of Hg) parts of cities, towns, their districts, as well as beds of watercourses and water reservoirs bottom (the Volga river valley and delta, the Lefortovo quarter in Moscow). There was also diagnosed the threshold of the background distribution of gross Hg content detected = 0.2-0.3 mg/kg. Its magnitude is consistent with domestic and abroad safe levels of Hg accumulation in soils = 0.3-0.4 mg/kg. The approach provides the differentiation of the environmental hazard lands. Technology of the approach can be used in the development of normative Document for the diagnosis of environmental and sanitary condition of territories in the system of Classifications of hazard of waste, in decontamination of territories, water areas and land zoning according to the criteria of natural or anthropogenic Hg accumulation.

Atmospheric methane (CH4) concentrations have more than doubled in the past ~ 250 yr, although the sources of this potent greenhouse gas remain poorly constrained. Freshwaters contribute ~ 20% of natural CH4 emissions, about half attributed to ebullition. Estimates remain uncertain as ebullition is stochastic, making measurements difficult, time consuming, and costly with current methods (e.g., floating chambers, funnel gas traps, and hydroacoustics). We present a novel approach to quantify basin‐wide hypolimnetic CH4 fluxes at the sediment level based on measurements of bubble gas content and modeling of dissolved pore‐water gases. We show that the relative ebullition flux pathway can be resolved by knowledge of only bubble gas content. As sediment CH4 production, diffusion, and ebullition are interrelated, the addition of a second observation allows closing the entire sediment CH4 balance. Such measurements could include bubble formation depth, sediment diffusive fluxes, ebullition, sediment CH4 production, or the hypolimnetic CH4 mass balance. The measurement of bubble gas content is particularly useful for identifying local ebullitive hotspots and integrating spatial heterogeneity of CH4 fluxes. Our results further revealed the crucial effect of water column depth, production rates, and hypolimnetic dissolved CH4 concentrations on sediment CH4 dynamics. Although we apply the model to cohesive sediments in an anoxic hypolimnion, the model can be applied to shallow, oxic settings by altering the CH4 production rate curve to account for oxidation. Utilizing our approach will provide a deeper understanding of in‐lake CH4 budgets, and thus improve CH4 emission estimates from inland freshwaters at the regional and global scales.

The aim of the study was to determine the seasonal and spatial variability of phosphorus release from bottom sediments of two lakes in Poland (Wielkopolska Region): Lusowskie Lake and Prawe Lake. The comparison of results from these two lakes could have been reliably made, because they were characterized by a similar trophic state (meso-eutrophic), were located in the same climatic zone, and were examined at the same time but differed in terms of morphometry (area, depth, and shape) as well as land use of catchment area. These dimictic lakes were characterized by hypolimnetic oxygen deficits in summer. The internal loading of phosphorus was analysed on the basis of ex situ experiments with intact cores of bottom sediments sampled at two stations, situated in the profundal (the deepest place in the lake) and in the littoral (2.0 m depth). The results indicated that a higher phosphorus load was released from bottom sediments in the profundal zone than in the littoral zone of both lakes. Phosphorus release from the profundal zone was twofold higher in Prawe Lake than in Lusowskie Lake. It reached 2.6 mg P m−2 d−1 in spring and winter in the larger and deeper Lusowskie Lake, compared to 6.04 mg P m−2 d−1 in autumn in the smaller and shallower Prawe Lake. In the littoral zone, phosphorus release reached 1.67 mg P m−2 d−1 and 0.16 mg P m−2 d−1 in both lakes, respectively. Differences in phosphorus internal loading between the lakes were due to differences in the stability of thermal stratification, time of water mixing and oxygen concentration in the depth profile of the lake.

Dissolved oxygen is a key player in water quality. Stratified water bodies show distinct vertical patterns of oxygen concentration, which can originate from physical, chemical or biological processes. We observed a pronounced metalimnetic oxygen minimum in the low-nutrient Rappbode Reservoir, Germany. Contrary to the situation in the hypolimnion, measurements of lateral gradients excluded the sediment contact zone from the major sources of oxygen depletion for the metalimnetic oxygen minimum. Instead, the minimum was the result of locally enhanced oxygen consumption in the open water body. A follow-up monitoring included multiple chlorophyll a fluorescence sensors with high temporal and vertical resolution to detect and document the evolution of phytoplankton. While chlorophyll fluorescence sensors with multiple channels detected a mass development of the phycoerythrin-rich cyanobacterium Planktothrix rubescens in the metalimnion, this species was overlooked by the commonly used single-channel chlorophyll sensor. The survey indicated that the waning P. rubescens fluorescence was responsible for the oxygen minimum in the metalimnion. We hypothesize that pelagic processes, i.e., either oxygen use through decomposition of dead organic material originating from P. rubescens or P. rubescens extending its respiration beyond its photosynthetic activity, induced the metalimnetic oxygen minimum. The deeper understanding of the oxygen dynamics is mandatory for optimizing reservoir management.

The results of long-term research on the concentrations and coexisting manganese species in different-type water bodies of the Ukraine (rivers, reservoirs, estuaries, small lakes, and ponds) are summarized. High manganese concentrations are characteristic, as a rule, of the hypolimnion zone. The ratio of particulate and dissolved manganese species depends on the type of the water body and their transformation processes in this specific water body. The concentrations of dissolved manganese in reservoirs, lakes, and estuaries much increase in conditions of oxygen deficiency and anaerobic deep water. The weight fraction of labile forms is large, which is characteristic of manganese. In water bodies rich in humic substances a large fraction of dissolved manganese is incorporated in complex compounds. In small lake and pond systems, carbohydrates play an essential role in binding of Mn(II).

Lake surveys conducted in 1987–88 and 2018–19 show changes in the chemical and optical properties of Adirondack lakes recovering from acidification. Among chronically acidified lakes (1980s pH

Waterbody methane emissions per area are negatively correlated with the size of the emitting waterbody. Thus, ponds, defined here as having an area smaller than 8 · 104m2, contribute out of proportion to the aquatic methane budget compared to the total area they cover and compared to other waterbodies. However, methane concentrations in and methane emissions from ponds show more spatial variability than larger waterbodies. We need to better understand this variability to improve upscaling estimates of freshwater methane emissions. In this regard, the Arctic permafrost landscape is an important region, which, besides carbon-rich soils, features a high pond density and is exposed to above-average climatic warming. We studied 41 polygonal-tundra ponds in the Lena River Delta, northeast Siberia. We collected water samples at different locations and depths in each pond and determined methane concentrations using gas chromatography. Additionally, we collected information on the key properties of the ponds to identify drivers of surface water methane concentrations. The ponds can be categorized into three geomorphological types with distinct differences in drivers of methane concentrations: polygonal-center ponds, ice-wedge ponds and larger merged polygonal ponds. All ponds are supersaturated in methane, but ice-wedge ponds exhibit the highest surface water concentrations. We find that ice-wedge ponds feature a strong stratification due to consistently low bottom temperatures. This causes surface concentrations to mainly depend on wind speed and on the amount of methane that has accumulated in the hypolimnion. In polygonal-center ponds, high methane surface concentrations are mostly determined by a small water depth. Apart from the influence of water depth on mixing speed, water depth controls the overgrown fraction, the fraction of the pond covered by vascular plants. The plants provide labile substrate to the methane-producing microbes. This link can also be seen in merged polygonal ponds, which furthermore show the strongest dependence on area as well as an anticorrelation to energy input indicating that stratification influences the surface water methane concentrations in larger ponds. Overall, our findings underpin the strong variability of methane concentrations in ponds. No single driver could explain a significant part of the variability over all pond types suggesting that more complex upscaling methods such as process-based modeling are needed.

Reservoirs are an important type of drinking water source for megacities, while lots of reservoirs are threatened by odor problems during certain seasons. The influencing factors of odor compounds in reservoirs are still unclear. During August 2019, a nationwide survey investigating the distribution of odor compounds in reservoirs used as drinking water sources was conducted on seven reservoirs. 2-methylisoborneol (2-MIB) and geosmin were detected in almost every reservoir, and some odor compound concentrations even exceeded the odor threshold concentration. The average concentration of 2-MIB was 2.68 ng/L, and geosmin was 3.63 ng/L. The average chlorophyll a concentration was 8.25 μg/L. The dominant genera of phytoplankton in these reservoirs belonged to cyanobacteria and diatom. Statistical analysis showed that odor compound concentration was significantly related to the chlorophyll a concentration and indicated that the odor compounds mainly came from phytoplankton. The concentration of odor compounds in the euphotic zone was significantly related to phytoplankton species and biomass. Therefore, the odor compound concentrations in the subsurface chlorophyll maxima layer was generally higher than in the surface layer. However, the odor compounds in the hypolimnion layer were related to the density current. This research suggests that both phytoplankton proliferation events and heavy storm events are important risk factors increasing odor compounds in reservoirs. Control of algal bloom, in-situ profile monitoring system and depth-adjustable pumping system will greatly reduce the risk of odor problems in reservoirs using as water supplies for large cities.

As global reserves of phosphorus (P) become scarce, recycling of P will be key to sustainable food production in future. The hypolimnetic withdrawal and purification circuit (HWPC) is a novel method that aims to remove and capture P accumulated in the near-bottom water of eutrophic lakes. Similar to the basic principle of wastewater treatment, the lake water is treated for the precipitation of P and other elements, and the formed particles are collected in a filtering unit while the purified water flows back into the lake. The method has been tested in a pilot project at Lake Kymijärvi, southern Finland.In the current study, we observed the efficiency of three different water treatments in the HWPC in terms of P precipitation: 1) water aeration; 2) aeration + Ca(OH)2 addition; 3) aeration + tannin-based biopolymer addition. Moreover, we studied the chemical composition of the precipitate formed in each treatment to understand its potential for P recycling. The aim of the study was to provide a better understanding to further develop and apply techniques to recover and recycle P from eutrophic lakes.