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Lake Kislo-Sladkoe is a stratified water body partly isolated from the White Sea. Perennial meromixis in the lake irregularly alternates with mixing events. Taking into account that the protists of Arctic coastal stratified water bodies are understudied, we evaluated for the first time the vertical structure, species richness, and diversity of protists assigned to different taxonomic groups in Lake Kislo-Sladkoe using light, luminescent, and scanning electron microscopy. To test the research hypothesis that a mixing event affects the vertical stratification and species composition of protists in a stratified lake, we compared the protist communities of Lake Kislo-Sladkoe in two extremely different states: strong meromixis vs. full vertical mixing. A total of 97 morphologically distinct phototrophic, heterotrophic, and mixotrophic protists were revealed with the most diverse supertaxa SAR (59), Obazoa (9), and Excavates (14). The hidden diversity of protists (43 species) was a bit less than the active diversity (54 species). A taxonomic list and micrographs of cells for the observed protists are provided. The majority of species revealed are cosmopolitan or widespread in the northern sea waters. The vertical patterns of protist communities were absolutely different in 2018 and 2021. In July 2018, clearly distinct protist communities inhabited different layers of the lake. Bloom of cryptophyte Rhodomonas cf. baltica was detected in chemocline, whereas the maximum density of its grazers was observed in adjacent layers, mainly dinoflagellates Gymnodinium sp. and Scrippsiella trochoidea, as well as a ciliate Prorodon sp. In 2021 due to the recent mixing of lake and seawater, there were no distinct communities in the water column except the superficial 0–1 m layer of fresh water. [ABSTRACT FROM AUTHOR]

In Solotvino, in southwestern Ukraine (Transcarpathia), there is a large group of anthropogenic water reservoirs. Most of these developed in sinkholes formed by the flooding of salt mines and the activation of anthropogenic and karst processes. One reservoir, Solotvino No. 7, was the subject of detailed limnological (hydrographic and hydrochemical) studies. The reservoir has an area of 8493 m2, a maximum depth of 20.5 m, and holds Cl−–Na+ brines. The water in the near-surface layer is hyposaline (3–20 g/L), but periodically becomes mesosaline (20–50 g/L). Hypersaline waters with mineralization > 250 g/L are found below 3 m. The reservoir has three persistent distinct mixolimnion layers that clearly indicate their meromictic type: the surface layer, a chemocline (where the water chemistry changes), and a lower monimolimnion layer. The thermal properties of the reservoir deserve special attention. The water is heated during all seasons at the boundary between the chemocline and monimolimnion; the water temperature is 32 °C in winter and 54 °C in summer, despite the lack of volcanism. The water is heated by a physical phenomenon in the layer where the water density increases, which is a heliothermal process. Also noteworthy is that throughout the year, the oxygen profiles are positive and heterograde, with the water being up to 380% oxygen saturated. [ABSTRACT FROM AUTHOR]

The article presents data on the composition and abundance of algal flora in the coastal stratified Kislo-Sladkoe Lake (the Karelian coast of the White Sea) after its flushing with seawater in autumn. The seasonal dynamics and vertical structure of the phytoplankton were monitored from March 17 to October 3, 2019. We also studied ice flora on March 17 and April 1. The largest carbon biomass of ice algae (0.79 g C/m3) dominated by Nitzschia frigida was recorded in April. The integrated biomass of phytoplankton in the water column varied from 0.01 g C/m2 in March to 5.83 g C/m2 during spring bloom in late May (when it was dominated by Chaetoceros invisibilis) and remained at the level of 0.07 to 0.29 g C/m2 until October; heterotrophic forms replaced autotrophic algae during this period. A regression analysis has shown that the structure of the phytoplankton was influenced by length of daylight, water temperature, dissolved oxygen level, depth, pH, salinity, and illuminance. [ABSTRACT FROM AUTHOR]

Climate warming is causing changes in the physics of deep lakes, such as longer summer stratification, increased water column stability, reduced ice cover, and a shallower depth of winter overturns. An ultimate consequence of warming would be a transition to a different mixing regime. Here we investigate the role of physical, chemical, and biological feedback mechanisms that unfold during a shift in mixing regime, and whether these feedbacks could prompt and stabilise the new regime. Although climate, interannual temperature variation, and lake morphometry are the main determinants of a mixing regime, when climate change causes shifts in mixing regime, internal feedback mechanisms may gain in importance and modify lake ecosystem functioning.We review the role of these feedbacks in three mixing regime shifts: from polymictic to seasonally stratified, from dimictic to monomictic, and from holomictic to oligomictic or meromictic.Polymictic lakes of intermediate depth (c. 3–10 m mean depth) could experience seasonal stratification if a stratification event triggers phytoplankton blooms or dissolved organic matter release, reducing transparency and therefore further heating the surface layer. However, this feedback is only likely to have influence in small and clear lakes, it would be easily disturbed by weather conditions, and the resulting stratified state does not remain stable in the long term, as stratification is lost in winter.The ice‐albedo feedback might cause an accelerated shift from ice‐covered (dimictic) to ice‐free (monomictic) winters in sufficiently deep (mean depth 50 m or more) lakes, where temperature memory is carried over from one winter to the next. Nevertheless, there is an ongoing debate into whether this process can persist during natural weather variations and overcome self‐stabilising mechanisms such as thermal insulation by snow. The majority of studies suggest that a gradual transition from dimictic to monomictic is more likely than an abrupt transition.A shift from a holomictic to a meromictic regime can occur if anoxia is triggered by incomplete mixing and an increase in deep‐water density—through the accumulation of solutes—exceeds a density decrease by hypolimnetic warming. A shift to meromixis would strongly alter the biology of a lake and might be difficult to reverse. If solutes accumulate only minimally in the hypolimnion, an oligomictic regime is formed, in which years with complete and incomplete mixing alternate.Understanding the importance of feedback mechanisms and the role of biogeochemistry when lakes shift in mixing regime could lead to a better understanding of how climate change affects lake ecosystems. [ABSTRACT FROM AUTHOR]

Long-term measurements show that the sedimentation dynamics of okenone (a carotenoid of purple sulfur bacteria) reflects the abundance of purple sulfur bacteria, the hydrogen sulfide content, and the stratification stability in the closed saline meromictic Shira Lake (Southern Siberia, Russia). The profile of fossil okenone in bottom sediments has been compared with the data on the water surface level in the lake over the last 100 years. The peaks of okenone correspond to lake transgressions, and the drops in the okenone content coincide with periods of stable or decreasing water level. Our observations show that fossil okenone may be used as a qualitative proxy of meromictic conditions and as a quantitative proxy of the water level dynamics in closed stratified lakes. Therefore, okenone can be used for qualitative reconstructions of the water balance and climate humidity in arid zones. [ABSTRACT FROM AUTHOR]

Estuarine Pettaquamscutt River is a unique habitat 10 km in length with physical and biogeochemical characteristics analogous to a miniature fjord. Its meromictic upper basins are characterized by a permanent oxic–anoxic vertical gradient in which a well-oxygenated upper layer overlies a deeper, anoxic reservoir, with persistent blooms of phototrophic anoxygenic bacteria (Chromatium, Chlorobium) at the oxic–anoxic transition layer. A diverse assemblage of nanoplanktonic, centric diatoms (Cyclotella caspia, Thalassiosira pseudonana, cf. Cyclotella cryptica) dominated the seasonal phytoplankton cycle in the aerobic layer, similar to comparable meromictic habitats elsewhere. This assemblage of nano-centric diatoms appears to be trait-separated from other species clusters and is potentially useful as a functional group flora with ecophysiology diagnostic of marine estuarine rivers and meromictic habitat niche structure. The most conspicuous phytoplankton feature, however, was the year-round occurrence of the photoautotrophic euglenid Euglenaformis (Euglena) proxima, restricted in its upper water column distribution to the O2–H2S boundary layer where it formed a consortium with the photosynthetic green and purple sulfur bacteria community. The association of E. proxima with meromixis, H2S, and phototrophic anoxygenic bacteria is similar to that reported previously in a brackish Norwegian oyster poll and a brackish loch in Scotland. [ABSTRACT FROM AUTHOR]

This paper reports a study of oxygen and redox conditions, trophic status, and phytoplankton community in the meromictic Lake Idro (Italy) from 2010 to 2014. The sequence of causes and effects of meromixis are also evaluated by comparing recent research with studies conducted from the late 1960s to the mid-1990s. In the last half century, Lake Idro was steadily meromictic due to solutes which accumulated in its deep waters, along with both dissolved nutrients and chemically reduced substances produced by the anaerobic microbial metabolism. These substances were retained in bottom waters and made unavailable to upper layers until stratification broke. Mixing episodes occurred in 2005-2006 altering stratification, and oxygen and nutrient distribution within the lake. The potential full overturn effects were also evaluated as potential oxygen consumption due to the oxidation of reduced substances to forecast possible oxygen exhaustion and collapse of biological communities. Finally, meromixis is discussed as a potential threat for deep perialpine lakes using Lake Idro as a reference to comparatively evaluate the present status and possible future trends. [ABSTRACT FROM AUTHOR]

Extreme environmental conditions are key factors in the formation of the structure and diversity of microbial communities. In meromictic ecosystems, extreme conditions and a stable stratification of physical, chemical and biological parameters lead to diversity and heterogeneity of microenvironments. Lake Doroninskoe is located in an extreme geographical area and differs from other known meromictic reservoirs of the world by a low level of illumination in the chemocline and a rare type of alkaline water in sedimentary rocks formed by evaporative concentration. To understand the key factors that shape the composition and structure of the microbial community, the macro- and micro-variations in space and time are of great importance. We investigated the short-term dynamics of the structure and diversity of microbial communities of the meromictic soda lake, Lake Doroninskoe, at day and night using highthroughput sequencing and bioinformatics. Metagenomic analysis of 16S rRNA gene amplicons showed that the microbial communities had a high taxonomic diversity both at day and night. Sixteen bacterial and three archaeal phyla were identified. Proteobacteria were dominant and comprised 75% during the day, increasing to 90% at night. Metabolically stable denitrifying bacteria that were able to use a variety of alternative electron acceptors and electron donors were prevalent in Lake Doroninskoe. They belonged to the families Enterobacteriaceae (class Gammaproteobacteria) and Alcaligenaceae (class Betaproteobacteria). Statistically significant differences between day and night microbial communities were found. During the day, the microbial community was the most diverse. We discuss the peculiarities of the underexplored shortterm dynamics of the structure and diversity of the microbial communities of the meromictic soda Lake Doroninskoe, and propose topics for prospective studies. [ABSTRACT FROM AUTHOR]

Recent expansion of anoxia has become a global issue and there is potential for worsening under global warming. At the same time, obtaining proper long-term instrumental oxygen records is difficult, thus reducing the possibility of recording long-term changes in oxygen shifts that can be related with climate or human influence. Varved lake sediments provide the better time frame to study this phenomenon at high resolution. We tracked the oxic/anoxic shifts of the varved Lake Montcortès since 1500 CE, and tried to recognise anthropogenic and climatic influences combining biological and geochemical proxies. Four main scenarios emerged: 1) years with abrupt sediment inputs (A); 2) years with outstanding mixing and oxygenation of the water column (B); 3) years with strong stratification, anoxia, intense sulfur bacterial activity and increased biomass production (C); 4) years with stratification and anoxia, but relatively less biomass production (D). In line with current limnologic trends, high supra-annual variability in the occurrence of oxygenation events was observed. Interestingly, at least 45.3% of the years were mixing years and, like the meromictic ones, were mostly clustered into groups of consecutive years, thus alternating years of monomixis with years of meromixis. Most years of D belong to the period 1500–1820 CE, when human activities were the most intense. Most years of A belonged to the climatic unstable period of 1850–1899 CE. Years of B were irregularly distributed but were best represented in the period 1820–1849 CE. Most years of C belonged to the 20th century. More than 90% of the years with climatic instrumental records belonged to B and C. Current climate warming seems to be taking control over the oxygenation capacity of the lake, especially since the second half of the 20th century. Our results support recent findings related to hypoxia spreading at the global scale. [ABSTRACT FROM AUTHOR]

We analysed the spatial and temporal variability of benthic nitrogen fluxes and denitrification rates in a sub-alpine meromictic lake (Lake Idro, Italy), and compared in-lake nitrogen retention and loss with the net anthropogenic nitrogen inputs to the watershed. We hypothesized a low nitrogen retention and denitrification capacity due to meromixis. This results from nitrate supply from the epilimnion slowing down during stratification and oxygen deficiency inhibiting nitrification and promoting ammonium recycling and its accumulation. We also hypothesized a steep vertical gradient of sedimentary denitrification capacity, decreasing with depth and oxygen deficiency. These are important and understudied issues in inland waters, as climate change and direct anthropic pressures may increase the extent of meromixis. Nearshore sediments had high denitrification rates (87 mg m day) and efficiency (~ 100%), while in the monimolimnion denitrification was negligible. The littoral zone, covering 10% of the lake surface, contributed ~50% of total denitrification, while the monimolimnion, which covered 70% of the sediment surface, contributed to < 13% of total denitrification. The persistent and expanding meromixis of Lake Idro is expected to further decrease its nitrogen removal capacity (31% of the incoming nitrogen load) compared to what has been measured in other temperate lakes. Values up to 60% are generally reported for other such lakes. Results of this study are relevant as the combination of anthropogenic pressures, climate change and meromixis may threaten the nitrogen processing capacity of lakes. [ABSTRACT FROM AUTHOR]

Assemblages of dinoflagellate cysts in lakebed sediments across the deep, meromictic basin of Sluice Pond (Lynn, Massachusetts) were compared with measurements of water quality and sediment geochemistry and with testate amoeba and tintinnid assemblages. Lakebed sediments below oxygenated waters (DO >2 mg/L) contain diverse testate amoebae, but sparsePeridinium willei-dominated dinoflagellate cyst assemblages. Sediments collected under hypoxic conditions (DO < 2 mg/L) showed higher preservation of organic carbon and nitrogen, and these deep basin sediments are characterised by diverse dinoflagellate cyst assemblages, but sieved microfossil assemblages are typically dominated by the planktonicCodonella crateraand the pseudo-planktonicCucurbitella tricuspis. Strong inverse relationships of cysts ofPeridinium willei(R2= 0.81) andDifflugia oblongatests (R2= 0.7) with water depth are attributed to variations in bottom water oxygen concentrations. Both ecological and taphonomic factors must be considered when reconstructing palaeoenvironments from microfossil assemblages. [ABSTRACT FROM AUTHOR]

Reliable gas measurements from supersaturated deep waters still remain a challenge. However, good information is mandatory to investigate the limnic carbon cycle, assess the endangerment through limnic eruptions and evaluate a potential source of exploitable energy. We addressed these three points in a heavily polluted mine pit lake in Germany. We quantified the ebullition of methane from deep waters and the sediment below. Exposed to continuous percolation of gas bubbles, the deep (monimolimnetic) water had accumulated high concentrations of gas: directly measured gas pressures indicated the proximity to spontaneous ebullition. Consequently, the possibility of a limnic eruption was assessed by initiating a self-sustained flow through a vertical pipe. Despite the high gas pressures, the flow was slow and the endangerment was considered low. A sampling strategy with bags was developed to achieve a reliable measurement of gas content and gas composition in the monimolimnion. As a result, directly measured gas pressures could be confirmed and were nearly exclusively attributed to methane and nitrogen. Contrary to lakes that had shown limnic eruptions, carbon dioxide played a much subordinate role, and hence the driving force for a violent outburst of gases was missing. Nevertheless the amount of dissolved methane was remarkably high. This investigation closes with some estimates of the commercial value of the deposit and limiting conditions for a possible exploitation. [ABSTRACT FROM AUTHOR]

Meromictic soda and saline lakes are unique ecosystems characterized by the stability of physical, chemical and biological parameters, and they are distributed all over the world. Lakes located in regions with average annual negative air temperature are of particular interest because of the presence of two periods with intensive and dynamic processes: the so-called biological summer and the long ice season with the biological spring. Soda Lake Doroninskoe is located in Eastern Transbaikalia (51°14′N, 112°14′E) in the permafrost zone in an extreme continental climate, and is covered by ice for seven months per year. The structure and diversity of the microbial communities throughout the water column of the lake was studied by 16S rRNA gene amplicon metasequencing. Different species with specific functions were found to dominate at different depths. Metabolically flexible bacteria with a capacity to switch between anoxygenic photosynthesis and aerobic chemotrophic metabolism dominate in soda Lake Doroninskoe. [ABSTRACT FROM AUTHOR]

Saline runoff from application of road salt deicers has the potential to create persistent density gradients in urban freshwater lakes, resulting in significant changes to physical mixing and biogeochemical cycling. In this study, the seasonal influence of road salt influx on the geochemical cycling of a small, urban kettle lake is examined. Water column samples were collected approximately twice per month for fifteen months and analyzed for temperature, pH, conductivity, major ions, nutrients and redox-sensitive solutes. The lake water column was redox-stratified with high concentrations of dissolved Mn(II), Fe(II), ammonium, phosphate and, during some periods, sulfide and methane in the hypolimnion. Bottom water anoxia was persistent throughout the entire sampling period. Concentrations of sodium and chloride, which correlated strongly with conductivity, were over a hundred times greater compared with levels in nearby rural lakes. Conductivity, chloride and calculated density profiles indicated the presence of a pycnocline at ~ 9 m depth that persisted throughout the sampling period. The chemical data demonstrate that road salt input has caused the lake to become meromictic, with a complete lack of fall and spring turnover. [ABSTRACT FROM AUTHOR]

Understanding the physical limnology of pit lakes is essential to planning closure, managing water quality, and using pit lakes to address other water quality problems at mine sites. Pit lakes are generally deep, have relatively small surface areas sheltered by pit walls, and often contain saline water. As a consequence pit lakes are candidates for meromixis, where the salinity stratification is sufficiently strong to inhibit mixing between the surface and deep water. To illustrate a variety of processes that can act to disrupt meromixis, we examine data from six pit lakes at three different sites in northern Canada. In five of the six cases the deep water was disturbed by factors such as groundwater inflow, sludge deposition, wall failure, and water transfers as a result of mine site management. We also discuss a number of factors that enhance meromixis, including salinity, salt exclusion from ice, and runoff. [ABSTRACT FROM AUTHOR]

Lake Szmaragdowe and Czarnogłowy Reservoir are both characterized by atypical origin as both were formed when quarries were flooded. The geological substrates of the two basins are unique, and they are both highly valued as recreational and tourist destinations. These two basins are among the most interesting in northwestern Poland. Studies of their thermal regimes confirmed that water mass mixing in Lake Szmaragdowe is bradymictic. The waters in the Czarnogłowy Reservoir mix entirely to a depth of 20 m, and this layer is also characterized by bradymixia. Deeper waters do not mix. This indicates that mixing in this basin is of the meromictic type. [ABSTRACT FROM AUTHOR]

Highlights: [•] The lake is made up by mine groundwater and surface water and is in hydraulic balance. [•] The different sources of recharge at lake cause the meromixis in spite of little depth. [•] In winter the runoff inflow thickens mixolimnion and the system losses groundwater. [•] In summer the evaporation slims mixolimnion and the lake gains groundwater. [•] A conceptual model explaining its seasonal limnological evolution is advanced. [ABSTRACT FROM AUTHOR]

Density differences are the key parameter for stratification stability. We used data from the iron-meromictic Waldsee, Germany, a lignite mine pit lake, to quantify the contribution of single solutes to water density and analyzed the density gradient. Iron meromictic lakes maintain their density gradient through chemical reactions. Hence, quantifying the contributions of separate solutes is essential for understanding the entire process. Based on solute concentrations and literature values of partial molal volumes, substance specific density contributions were quantitatively evaluated. Then, by direct measurements of the density of IHSS Waskish peat fulvic acid, we quantified the density contribution of dissolved organic carbon (DOC). While several solutes contributed to the density throughout the water column, only those substances that occurred at higher concentrations in the anoxic monimolimnion than in the oxic mixolimnion were crucial to sustaining the density difference between the two layers. In Waldsee, the density difference between monimolimnion and mixolimnion was attributed to dissolved Fe (0.23 g/L, resulting in a 45 % of the density difference due to solutes) and to the carbonate system (HCO, about 0.16 g/L and CO, 0.03 g/L) while Ca and DOC delivered only a small contribution. In summer, total density differences were dominated by temperature differences; during winter, solutes sustained meromixis. Finally, we present a complete list of specific density fractions for basically all of the density-relevant substances in fresh waters. [ABSTRACT FROM AUTHOR]

Abstract: Lake Pavin is a deep meromictic lake. Its water column is divided into two parts: the mixolimnion, which is subjected to mixing during seasonal overturns; and the monimolimnion which remains unmixed. Using high precision and high frequency temperature and conductivity profiles along with continuous temperature measurements, this study reveals the presence of a sublacustrine, intermittent cold spring at the bottom of the mixolimnion at a depth between 50 and 55m. This cold-water input rises in the water column by saline convection. The use of a simple conceptual model, representing turbulent diapycnal diffusivity and convection correlated with the presence of the spring within the water column indicates its role in maintaining the meromixis characteristic of the lake on the intra-annual time scale. The spring also influences seasonal overturns and thus contributes to establish the depth of the mixolimnion–monimolimnion interface on the inter-annual time scale. [Copyright &y& Elsevier]

Kallis, Jahn, Leo Bodensteiner, and Anthony Gabriel, 2010. Hydrological Controls and Freshening in Meromictic Soap Lake, Washington, 1939-2002. Journal of the American Water Resources Association (JAWRA) 46(4): 744-756. DOI: 10.1111/j.1752-1688.2010.00446.x The chemically stratified layer of naturally formed meromictic lakes exhibits unusual and often extreme physical and chemical conditions that have resulted in the evolution of uniquely adapted species. The Columbia Basin Irrigation Project appears to have had a marked effect on the hydrology of Soap Lake, a meromictic lake in the Grand Coulee of central Washington. The relation of hydrology to salinity was assessed by analyzing water budgets before and after the introduction of the irrigation project. Before irrigation, water gains were balanced by losses; after irrigation began groundwater gains approximately doubled. To manage lake levels and reduce groundwater influx, wells were installed to intercept groundwater. Although the hydrological cycle has been restored to pre-irrigation conditions, the meromictic character of the lake continues to change. Interception wells remove 10 to 16 Mm3 of groundwater annually, but influx continues based on change in the monimolimnion. From 1958 to 2003 the chemocline descended 1.1 m and the volume of the monimolimnion from 698,000 m3 to 114,000 m3. Annual loss of volume is occurring at a rate of 1.9% since 1958. Although groundwater interception wells are maintaining the volume of the entire lake, the recession of the chemocline indicates that conditions that have maintained meromixis at Soap Lake are currently not in equilibrium. [ABSTRACT FROM AUTHOR]

Physicochemical and microbiological examinations were conducted on the meromictic Lake Starodworskie located in Olsztyn. The temperature profile revealed a characteristic, small increase in temperature at a depth between 13 and 14 m, and permanent anaerobic conditions in the deeper water layers. Accumulation of dissolved mineral substances enhanced the thermal stability of the lake waters; electrolytic conductivity values in the water deeper than 15 m oscillated within the range of 533 - 568 μS cm-1. [ABSTRACT FROM AUTHOR]

Many general mass-balance models that simulate processes in one or two water layers have been successfully constructed, tested and used to predict effects from remediating lake pollution and other environmental disturbances. However, these models are poorly suited for meromictic lakes which consist of yet another water layer. In order to determine a cross-systems based algorithm for the depth of the boundary between the two lowest layers ( Dcrit2; in m), data from 24 three-layer lakes were analysed, and this depth could be predicted from the maximum depth and the lake surface area. The resulting model was tested with good results against independent data from 6 lakes which were not used for model development. Furthermore, Dcrit2 was predicted at a considerably lower depth than the theoretical wave base (a previously defined functional separator between the two top layers) in 110 out of 113 meromictic lakes. This indicates that the equation for Dcrit2 estimated in this study may be used for developing general mass-balance models for a large number of lakes which contain three stable water layers. [ABSTRACT FROM AUTHOR]

Summary: Groundwater–lake interaction in conjunction with surface mixing and vertical exchange were quantitatively examined for the monimolimnion in a small stratified mining lake. Although the lake is shallow (∼4.7m), it shows a strong meromixis. The permanent chemocline, which is connected with stable density stratification, separates the mixolimnion from the anoxic monimolimnion at a variable depth of only 0.6–1.9m. We conducted a tracer experiment, injecting ∼3μmol of anthropogenic gas SF6 into the monimolimnion. With monthly measured vertical concentration profiles extended over 2 years we detected and quantified the groundwater coupling. An exchange time close to once per year was estimated for the monimolimnion by balancing the tracer. From CTD casts and thermistor chain data we extracted additional information about mixing. For the water and tracer balances, we included the erosion of the monimolimnion surface by strong daily convective mixing in the mixolimnion during the warm period. SF6 background measurements before spiking indicate that diffusive transport through the chemocline density step must be very low, i.e., close to the molecular level. The observed seasonal variability of the chemocline depth can be considered as the result of a virtually steady inflow of groundwater, which supports the growth of the monimolimnion, and strong seasonal mixing in the mixolimnion, which acts against the stratification and erodes the monimolimnion. Over longer time scales, the chemocline is to be found at the level where erosive processes balance the recharging of the monimolimnion from groundwater. [Copyright &y& Elsevier]

A marked vertical trend of increasing temperature and dissolved metal concentrations is observed in the monimolimnia of some meromictic pit lakes of the Iberian Pyrite Belt (IPB) in SW Spain. Temperature differences between the chemocline and the pit lake bottom can be as high as 15°C (e.g. Herrerías), and the respective concentration of some metals (e.g. Fe) and metalloids (e.g. As) can increase by several orders of magnitude (e.g. Cueva de la Mora). The redox conditions also change drastically from the upper and oxygenated mixolimnion (strongly oxidizing) to the lower and anoxic monimolimnion (moderately reducing). Processes such as the inflow of metal–sulphate laden ground water from flooded shafts and galleries, and other factors such as the pit geometry or the relative depth of the lakes, must be considered to account for the observed stratification pattern. The vertical profiles of physico-chemical parameters and water chemistry obtained in Cueva de la Mora and other meromictic pit lakes of the IPB are also compatible with a reactive bottom in which several geochemical and microbial reactions (including reductive dissolution of Fe3+ minerals, bacterial reduction of Fe3+ and SO4 2− in pore waters within the sediments, and decomposition of organic matter) could be taking place. [ABSTRACT FROM AUTHOR]

Chemical characteristics of permanent stratification in cave pools (meromixis) may provide insight into the geochemical origin and evolution of cave pool waters. The objective of this study was to test the hypothesis that some pools in Lechuguilla Cave may be subject to ectogenic meromixis, where permanent chemical stratification is induced by input of relatively saline or fresh water from an external source. However, because organic C concentrations in Lechuguilla waters are low (typically < 1 mg L-1), biogenic meromixis resulting in O2(g)-depleted subsurface waters is not expected. Four pools at various depths below ground surface (0 m) were studied: (1) Lake Chandalar (-221 m), (2) Lake of the Blue Giants (LOBG) (- 277 m), (3) Lake Margaret (- 319 m), and (4) Lake of the White Roses (LOWR) (- 439 m). Water column profiles of temperature, pH, dissolved O2(g), and electrical conductivity (EC) were collected down to a maximum depth of 13.1 m using a multi-parameter sonde. Opposite pH gradients were observed at Lake Chandalar and LOBG, where pH at the surface (0.3 m) varied by ±0.20 units compared to the subsurface (> 0.9 m), and are probably the result of localized and transient atmospheric CO2(g) concentrations. At LOBG, an EC increase of 93 μS cm-1 at the 0.9-m depth suggests meromictic conditions which are ectogenic, possibly due to surface inflow of fresh water as drips or seepage into a preexisting layer of higher salinity. [ABSTRACT FROM AUTHOR]

Groundwater discharge into an open pit lignite mining lake was investigated using radon-222 as a naturally occurring environmental tracer. The chosen study site was a meromictic lake, i.e., a water body that is divided horizontally into two separate layers - the upper mixolimnion (with seasonal mixing) and the lower monimolimnion (without seasonal mixing). For the estimation of groundwater discharge rates into the lake, a simple box model including all radon sinks and sources related to each layer was applied. Two field investigations were performed. During the October campaign, the total groundwater discharge into the lake was found to be 18.9 and 0.7 m3 d-1 for the mixolimnion and monimolimnion, respectively. During the December campaign, the groundwater discharge into the mixolimnion was 15.0 m3 d-1, whereas no discharge at all was observed into the monimolimnion. Based on the given water volumes, the residence time of lake water was 5.3 years for the monimolimnion and varies between 0.9 and 1.1 years for the mixolimnion. The investigation confirmed radon to be a useful environmental tracer for groundwater and surface water interactions in meromictic lake environments. [ABSTRACT FROM AUTHOR]

1. Dissolved organic carbon (DOC) concentration was determined for a range of lakes of varying conductivity (30–4000 μS cm−1) in the low Arctic of SW Greenland. DOC concentration range from <1 to >100 mg C L−1, occasionally approaching 200 mg C L−1 in meromictic, oligosaline lakes. DOC concentration was strongly related to [log10] conductivity and total nitrogen. 2. Peak DOC concentrations (>80 mg L−1) occur in lakes located approximately 50 km from the present ice sheet margin, a zone of low effective precipitation; evaporative concentration is the first-order control on DOC concentration. Lakes at the coast and closer to the ice margin had lower DOC concentrations (<20 mg C L−1). Local factors, notably the presence or absence of an outflow and catchment morphometry, resulted in considerable variability in concentration (20–100 mg C L−1) within the area of maximum concentration around 51°W. 3. Despite their high DOC concentration, these lakes are essentially colourless. Dissolved organic matter (DOM) absorption ( a375) was low in most lakes (<10 m−1) with maximum values (approximately 20 m−1) occurring in one humic-stained lake in the area. Absorption values corrected for DOC concentration ( ) were very low (<0.6 m2 g−1 C) for all lakes apart from those at the coast, perhaps reflecting greater allochthonous inputs at these sites. 4. S, the spectral slope coefficient, ranged from 16 to 27 μm−1 and was weakly correlated with DOC concentration. Both a375 and S showed similar distribution patterns along the sampling gradient as did DOC, with maximum values at approximately 51°W. High and low S may indicate fresher, more rapidly flushed, systems with less degraded DOM or greater inputs from their catchments. 5. The lakes closer to the head of the fjord with higher conductivity, had low (<0.2 m2 g−1 C) and high S (>21 μm−1) and this may reflect increasingly longer lake water residence times, greater DOM age and photochemical degradation. [ABSTRACT FROM AUTHOR]

We analyzed the relationship between chrysophyte cyst assemblages in surface sediment samples and limnological and geographical variables for 70 lakes located along Søndre Strømfjord in southwest Greenland. Over 247 stomatocysts were identified and of these, 153 were sufficiently abundant for use in statistical analyses. Eight stomatocysts were considered to be new and are described formally. Canonical correspondence analysis indicated that conductivity was the dominant variable explaining cyst distribution, reflecting the large conductivity gradient in lake water chemistry in this area. High conductivity lakes had distinctive cyst assemblages with lower diversity than low alkalinity lakes, where assemblages were similar to alpine soft-water lakes elsewhere. The high conductivity lakes, however, had similar cysts to other saline lakes elsewhere in the arctic. Additionally, pH, calcium, maximum depth, longitude, sulfate, total phosphorus, and altitude all explained significant amounts of variability of cyst assemblages. Longitude was the only geographical variable that explained cyst variability independently of other variables (i.e. had a unique effect), which suggests that the climatic gradient from the coast to the head of the fjord has a structuring effect on cyst assemblages. Conductivity (weighted-averaging partial least squares, r2=0.917; root mean square error=0.142; r2jack=0.861, root mean square error of prediction=0.191) and pH inference models (weighted averaging, r2=0.924; root mean square error=0.158; r2jack=0.826, root mean square error of prediction=0.240) were developed. For the pH model, high conductivity lakes (>800 μS20·cm−1) were removed. Both models are statistically robust and could be applied to lakes in west Greenland to reconstruct conductivity and/or pH. Such paleolimnological reconstructions provide the means of acquiring long-term data for use in the evaluation of, for example, regional paleoclimatic models. [ABSTRACT FROM AUTHOR]

Abstract: Comparative sequence analysis of almost complete 16S rRNA genes of members of the Desulfobacteriaceae retrieved from two gene clone libraries of uncultured bacteria of the chemocline of Lake Cadagno, Switzerland, resulted in the molecular identification of nine sequences, with a tight cluster of five sequences that represented at least three different populations of bacteria with homology values of 95% and 93% to their closest cultured relatives Desulfomonile tiedjei and Desulfomonile limimaris, respectively. In situ hybridization with probes DsmA455 targeting two subpopulations and DsmB455 targeting one subpopulation, detected bacteria with a peculiar morphology previously described as “morphotype R”. The individual probes detected about the same number of cells in all samples and together added up to represent all cells of “morphotype R” suggesting that the basic ecophysiological requirements of the subpopulations might be similar. In the monimolimnion, “morphotype R” cells accounted for up to 29% of all Bacteria and entirely represented the Desulfobacteriaceae, the most prominent sulfate-reducing bacteria. In the sediment, “morphotype R” was similarly prominent in the upper cm only where it represented all Desulfobacteriaceae and up to 50% of all Bacteria. Numbers and importance within the Desulfobacteriaceae and Bacteria declined significantly with depth in sediments suggesting potential effects of changing environmental conditions on the fate of “morphotype R”. [Copyright &y& Elsevier]

Abstract: Since 1994 seasonal variations of temperature, salinity and vertical distribution of dissolved oxygen, nutrients, dissolved organic carbon (DOC), surface-active substances (SAS), reduced sulfur species (RSS), phyto- and zooplankton have been investigated in water column of the Rogoznica Lake. During the thermohaline stratification (spring and summer), the surface water is well-oxygenated while anoxia is occurring in the bottom layer. Anoxic deep water is characterized by high concentrations of RSS (up to 10−3 M, mainly in the form of sulfide); nutrients (NH4 +, up to 150 μM; PO4 3−, up to 22 μM; SiO4 4−, up to 400 μM) and DOC (up to 6 mg/l) as a result of the pronounced remineralization of allochthonous organic matter produced in the surface water. The depth position of anoxic water layer changes seasonally and is greatly influenced by rainfall, the influence of which is visible from decreased salinity in deeper layers as well. As a result of autumn/winter mixing, bottom water rich with nutrients is coming to the surface, supporting new phytoplankton and oxygen productions. Natural eutrophication of the lake is strongly influenced by nutrient recycling under anaerobic conditions. Turnover of lake water layers in September 1997 occurred so quickly that it resulted in the appearance of total anoxia and sulfide presence throughout the water column. The aerobic flora and fauna died and added to the oxygen demand and the production of nutrients. Those long-term datasets help in recognizing the general trend from unique cases, such as the anoxic event in 1997, which was related to anthropogenic eutrophication. [Copyright &y& Elsevier]

Lake Vähä-Pitkusta is a 35-m deep meromictic kettle-hole lake in SW Finland. The topmost 15cm of the sediment stratigraphy consists of black gyttja, representing the last ca. 600years. A sediment core from the deepest point was examined with multi-proxy methods to determine the origin and evolution of the basin, the onset of the meromictic conditions and the primary factors that initiated the meromictic processes. The results indicate that Lake Vähä-Pitkusta became isolated from the Baltic basin during the Yoldia Sea stage. After initial unstable sedimentary conditions the development continued steadily until ca. 2500–2300 cal.BP when there is indication of lake level rise and/or increased surface runoff, probably resulting from a cooler and moister climate. The diatom and cladoceran results indicate only a slight rise in the trophic state following the increased erosion. Pollen evidence suggests weak anthropogenic activity in the catchment during the last 1000 years, and this had only little effect on the trophic state, which remained rather low. Thus, it is most unlikely that the trophic state was the primary reason for the origin of meromixis, although it may have contributed to it. We tentatively suggest that the primary reason was the peculiar morphometric dimensions of Lake Vähä-Pitkusta (small, deep, sheltered from winds) which made it very sensitive and that even a very slight increase in soluble electrolytes may push it over the edge into meromixation. It is possible that this did not occur during the cold/moist period around ca. 2300 cal.BP but during the Medieval Climate Anomaly. Studies on sediment geochemistry are needed to verify this assumption. [ABSTRACT FROM AUTHOR]

Physical and chemical profiles of a shallow (c. 12-m-deep) subsaline (total dissolved solids 2.3–2.8 g l-1) closed-basin lake in the continental area of southwestern Greenland are described for the first time. Watercolumn data for every 5th consecutive day between April 20 and October 6, 2001, and continuous recordings of lake water level and meteorological conditions are used to infer controls on contemporary lake functioning, sediment formation and climate–lake interactions. Limnological observations demonstrate the importance of lake-ice formation and its role in haline convection and the development of meromixis. Observed lake cycling suggest that the lake at present is in a state of near-meromixis where stagnant bottom waters de-stratify through deep penetration of weak haline convective cells by the end of June. From this study, the primary reasons the shallow Greenlandic low salinity lakes develop meromixis are: (i) lack of an outflow (ii) meltwater dilution and chemical stratification of surface waters, (iii) insubstantial wind mixing, (iv) a weak winter thermohaline convective cell forced by cryoconcentration, and (v) biogeochemically enhanced solute concentrations near the sediment bed. Throughout the open water period the hydrological balance is dominated by evaporative losses. Lake surface water conductivities change from 2110 to 2890 μS cm-1 due to the combined effects of open water evaporation, meltwater dilution, diffusive exchanges over the seasonal pycnocline, and boundary mixing. Freeze-out of salts and resulting deep haline convection increase overall water column salinity during winter. Owing to deep convective mixing, plant nutrients are relatively high in the upper watercolumn with a dominant internal source of phosphorous. Extreme productivity pulses of phytoplankton are observed as soon as sub-ice radiation levels increase and directly after ice-out when sufficient wind mixing can support an intense monospecific diatom bloom of Diatoma spp. leading to the rapid depletion of dissolved silica. [ABSTRACT FROM AUTHOR]

This study investigated concentration changes of dissolved and particulate organic carbon in a meromictic lake to determine the lake's productivity. It was revealed that the dominant form of organic carbon was the dissolved form (DOC). Maximum production activity was observed in summer (with the peak in June). At the same time, the factors restricting that process were determined, such as: strong pressure of the bacteria competing with the phytoplankton for mineral nutrients, and simultaneous precipitation of phosphates on HS-Fe complexes. The generated organic matter was settling intensively, and one of the transport methods of the dissolved organic matter was adsorption on calcium carbonate and bacterial transformation of the part of DOC into POC. [ABSTRACT FROM AUTHOR]

The lake Hunnebotn (A=1 km2, zmax=11 m) has become separated from the sea by isostasis, but a 1.5-m-deep channel is held open by periodic dredging. Water exchange is minor due to the small tidal range (21 cm) and the length of the channel (1.8 km), but there may be an inflow of seawater at spring tides. The terrestrial watershed runoff may be fertilized by sewage and agricultural runoff despite some measures taken to prevent this. Historical and new records of biological communities indicate variations in sensitivity to different phases of the isolation and eutrophication process. The native oyster was most sensitive and disappeared first. Later, the eelgrass disappeared possibly due to overgrowth by epiphytic algae, and finally there was a massive littoral–sublittoral invasion of green algae. The lake should not be left in this condition, for aesthetic reasons and because marine inlets and eutrophic brackish water may serve as refugia for spore populations of toxic microalgae. Improving surface water quality will require better control of anthropogenic sources, but bursts and leaks of nutrients from the anoxic monimolimnion cannot be controlled. [Copyright &y& Elsevier]

Abstract Lake Lugano is located at the border between Italy and Switzerland and is divided into three basins by two narrowings. The geomorphologic characteristics of these basins are very different. The catchment area is characterized by calcareous rock, gneiss and porphyry; the population amounts to approximately 290 000 equivalent inhabitants. The external nutrient load derives from anthropogenic (85%), industrial (10%) and agricultural (5%) sources. The limnological studies carried out by Baldi et al. (1949) and EAWAG (1964) revealed early signs of eutrophication, with a phosphorous concentration of about 30–40 mg m–3 and an oxygen concentration of less than 4 g m–3 in the deepest hypolimnion. Subsequently Vollenweider et al. (1964) confirmed these data and was the first to point out the presence of a meromictic layer in the hypolimnion of the northern basin. From the 1960s, as a result of an increase in the population and internal migration, the lake became strongly eutrophic with the P concentration reaching 140 mg m–3 and the oxygen in the hypolimnion reduced to zero. Fifty-five per cent of the P was from metabolic sources and 45% from detergents and cleaning products. In 1976, a partial diversion of waste water from the northern to the southern basin was begun, and gradually eight waste water treatment plants came into operation using mechanical, chemical and biological treatments. In 1986, Italy and Switzerland began to eliminate the P in detergents and cleaning products. Since 1995, the main sewage treatment plants have improved their efficiency by introducing P post-precipitation, denitrification and filtration treatments. The recovery of the lake is due to be completed by the year 2005. Altogether, during the last 20 years recovery measures have reduced the external P load from about 250 to 70–80 tonnes year–1; the goal to be reached is 40 tonnes year–1. In-lake phosphorous concentrations have decreased from 140 to 50–60 mg m–3, with the target at 30 mg m–3. Dissolved oxygen concentration is satisfactory only between the depths of 0 and 50 m, falling rapidly to zero in the deepest layers. Below a depth of 90 m, high CH4, HS–, NH4+, Fe2+ and Mn2+ concentrations exist. Primary production has decreased from 420 to 310 g Cass m–2 year–1, notwithstanding an increase in the thickness of the trophogenic layer. Structure and dynamic biomass show marked changes: phytoplankton dry weight has decreased from 16 to 7 g m–2, while zooplankton dry weight has increased from 3 to 4.5 g m–2. Chlorophyll concentration has fallen from 14 to 9 mg m–3 and Secchi disk transparency has increased from 3.5 to 5.5 m. The current sources of the external load are uncollected small urban conglomerations, storm-water overflows from outfall sewers, and the residual load from sewage treatment plants, particularly those without P post-precipitation. [ABSTRACT FROM AUTHOR]

Global spread of anoxia in aquatic ecosystems has become a major issue that may potentially worsen due to global warming. The reconstruction of long-term hypolimnetic anoxia records can be challenging due to lack of valid and easily measurable proxies. The sedimentary Mn/Fe ratio measured by X-ray fluorescence (XRF) is often used as a proxy for past lake redox conditions. Yet the interpretation of this ratio can be problematic when Fe and Mn accumulation is not solely redox driven. We used the varved sediments of Lake Moossee (Switzerland) to examine the partitioning of Fe and Mn in seven fractions by sequential extraction under various oxygen conditions over the last 15,000 years. We combined this data with XRF scans and an independent diagnostic proxy for anoxia given by a hyperspectral imaging (HSI)-inferred record of bacteriopheophytin, to validate the use of the XRF-Mn/Fe ratio as redox proxy. In the 15,000-year long record, Fe was bound to humins and amorphous, crystalline, sulfide and residual forms. Mn was mainly present in carbonate and amorphous forms. Higher erosion, prolonged anoxia, diagenesis and humic matter input affected Fe and Mn accumulation. Under holomixis the XRF-Mn/Fe ratio successfully reflected lake redox conditions. Periods with higher detrital Fe input obscured the applicability of the ratio. During phases of permanent anoxia, intensified early diagenetic processes trapped Mn in the sediments in carbonate, crystalline oxide and humic forms. Our study shows that the single use of the XRF-Mn/Fe ratio is often not conclusive for inferring past lake redox conditions. The application of the XRF-Mn/Fe as a proxy for anoxia requires taking into account the individual lake characteristics and changes in lake environmental conditions, which affect the accumulation of Fe and Mn in the sediments. Unlabelled Image • The XRF-Mn/Fe redox proxy is evaluated with sequential Fe and Mn fractions. • Intensive diagenetic processes limit the use of the XRF-Mn/Fe proxy. • Mn can be trapped in the sediments in authigenic forms under permanent anoxia. • Catchment processes and detrital inputs highly affect Mn/Fe ratios in the sediments. [ABSTRACT FROM AUTHOR]

Saline-alkaline lakes of the East African Rift are known to have an extremely high primary production supporting a potent carbon cycle. To date, a full description of carbon pools in these lakes is still missing. More specifically, there is not detailed information on the quality of dissolved organic matter (DOM), the main carbon energy source for heterotrophs prokaryotes. We report the first exhaustive description of DOM molecular properties in the water column of a meromictic saline-alkaline lake of the East African Rift. DOM availability, fate and origin were studied either quantitatively, in terms of dissolved organic carbon (DOC) and nitrogen (DON) or qualitatively, in terms of optical properties (absorbance) and molecular characterization of solid-phase extracted DOM (SPE-DOM) through negative electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). DOM availability was high (DOC ∼ 8.1 mM in surface waters) and meromixis imprinted a severe quantitative and qualitative change on DOM pool. At the surface, DOM was rich in aliphatic and moderately in aromatic molecules and thus mirroring autochthonous microbial production together with photodegradation. At the bottom changes were extreme: DOC increased up to 5 times (up to 50 mM) and, molecular signature drifted to saturated, reduced and non-aromatic DOM suggesting intense microbial activity within organic sediments. At the chemocline, DOC was retained indicating that this interface is a highly reactive layer in terms of DOM processing. These findings underline that saline-alkaline lakes of the East African Rift are carbon processing hot spots and their investigation may broaden our understanding of carbon cycling in inland waters at large. • First detailed description of DOM in a tropical meromictic saline-alkaline lake. • High amount of autochthonous DOM was found in water column. • Solid-phase extracted DOM is microbially derived, photodegraded and aliphatic. • Changes in DOM quantity and quality were located at the chemocline. [ABSTRACT FROM AUTHOR]

Saline emissions can elevate water densities and have the potential to alter stratification and mixing dynamics in lakes. In the light of rising anthropogenic salinization of temperate freshwater lakes, it is highly relevant to understand how saline emissions, as well as the rehabilitation from salt pollution might affect lake circulation patterns. In this study, we present the impact of industrial pollution with chloride salts on the deep Alpine lake Traunsee in Austria from 1930 to 2005 and the observed limnological changes during the recovery from salinization in the following 12 years. We assembled and analysed a unique dataset of monthly sampling profiles covering chloride, conductivity, temperature and dissolved oxygen over a 87-year period. We quantified the impact of saline emissions on the lake's total chloride content, water densities, stability indices and deepwater oxygen concentrations. Time-series of water density profiles and stability indices revealed a significant impact of salinization on seasonal stratification and mixing. Higher dilution and shallower release of saline wastes and the short water retention in Traunsee helped to prevent a long-lasting density stratification. Nevertheless, two periods of salt-induced meromixis occurred in the lake. The first was caused directly by the disposal of saline wastes into the deepwater in the 1930s and 1940s, whereas the second was induced by the recovery from salinization in recent years. The naturally faster wash-out of salts from shallower water layers reinforced the density gradient and impeded vertical water circulation after salt pollution ceased. The rehabilitation from salinization had a stronger effect on stratification intensity and deepwater oxygen concentration than the continuous salt pollution over the last century. [ABSTRACT FROM AUTHOR]

The article focuses on the phylogenetic diversity of microorganisms from chemocline of the Meromictic Soda Lake Doroninskoe in Zabaikalie, Russia. Topics discussed include recycling of sulfur into a chemocline as a result of bacterial activity, study of the microbial communities through the use of the 16S rDNA amplicon pyrosequencing, and several microorganisms which dominates the lake, including cyanobacteria, proteobacteria, and bacteroidetes