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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]

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]

The seasonal dynamics of the vertical structure of small saline Lake Uchum, located in the steppe arid zone of the south of Siberia (Krasnoyarsk krai), has been studied in detail for the first time. This lake is a meromictic water body. We have revealed a heterogeneous vertical distribution of plankton organisms and a dense population of purple sulfuric bacteria in the redox zone. The taxonomic composition and seasonal dynamics of phyto- and zooplankton are described. Presumably, the meromixis of Lake Uchum is due to the inflow of fresh water to the surface of the saline water body during the rise of its level in the early 20th century, similarly to lakes Shira and Shunet located nearby. The processes of salt displacement into the solution during the formation of ice, as well as the precipitation of salts in the winter, also contribute to the maintenance of permanent stratification. The information on the current state of the lake can be useful for reconstructing the climate by bottom sediments, as well as for creating a model of water quality and investigating the therapeutic properties of lake mud. [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]

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]

There are numerous anthropogenic water reservoirs in the vicinity of the Žulovská pahorkatina (northern Czech Republic). The vast majority of them have developed as a result of the flooding of the abandoned quarries of crystalline rocks (granites or granodiorites). The surface area of these water bodies is small and is <6000 m. These are, however, very deep reservoirs of up to 25 m. Permanent water stratification has been recorded in some of them; these are meromictic reservoirs. The development of the phenomenon of meromixis in the reservoirs that were studied was conditioned by the supply of organic matter from the catchment. The biochemical process of organic matter decomposition led to the release of ions, mainly NH. The accumulation of dissolved substances in the benthic zone initiated the meromixis phenomenon. Therefore, the water bodies that were tested can be classified as reservoirs of the biogenic type of meromixis. This article presents the stages of the evolution of a holomictic reservoir into a meromictic one. Three water bodies in which meromixis was recorded were selected for the study. The control object was an abandoned quarry reservoir with no meromixis. [ABSTRACT FROM AUTHOR]

The data on changes in the vertical distribution of free-living ciliates from a freshwater meromictic pond (city of Samara) within a year are presented. The specific character of the waterbody is that the taxonomic diversity of ciliates of pelagic plankton is mainly formed not by usual euplanktonic species, but by ciliates of highly specialized ecological groups such as mixotrophs, 'benthic migrants,' and sapropelic species. The ciliate fauna is similar to the fauna of thermally stratified lakes with anaerobic hypolimnion. The maximal abundance and biomass of ciliates during the year and their high absolute values are determined by the existence of stable gradients of abiotic factors and, in the first turn, concentrations of oxygen, hydrogen sulfide, and oxidation-reduction potential. [ABSTRACT FROM AUTHOR]

Following the technical closure of the brown lignite Meirama mine (NW Spain) in April 2008, the reclamation of the mined area is being accomplished with the controlled flooding of its large pit. During the first 7 months of flooding, the sequential arrest of the ground water dewatering system led to the growth of an acidic water body of about 2 hm. Since October 2008, the surface waters from some local streams have been diverted towards the pit so that these have become the major water input in the flooding process. Surface water has promoted a major change in the chemical composition of the lake water so that, at present, its surface has a circum neutral pH, net alkalinity, and low conductivity. At present, the lake has slightly more than one half of its final volume, and it is expected the overflow in 3 to 3.5 years. The lake is meromictic, with a sharp chemocline separating the acidic monimolimnion (pH ≈ 3.2, acidity ≈ 150 mg CaCO/L, κ ≈ 2.4 mS/cm) from the main water body (pH ≈ 6.5, alkalinity ≈ 15 mg CaCO/L, κ ≈ 0.3 mS/cm). Oxygen is being depleted at the bottom of the lake so that the monimolimnion became anoxic in January 2011. Above the chemocline, the composition of the lake is similar, but not identical, to that of the flooding stream waters. Close to the surface, some constituents (pH, metals) show strong seasonal variations in coincidence with the phytoplankton growing periods. Those parameters whose limits are legally prescribed comply with the corresponding water quality standards, and they are also consistent with the forecasting results obtained in early modeling. At present, a project considering the construction of an uptake tunnel to exploit the lake is being developed for the emergency water supply of the metropolitan area of A Coruña. [ABSTRACT FROM AUTHOR]

The concentrations of carotenoids buried in the bottom sediments of Lake Shira (Siberia, Khakassiya) have analyzed for the period of the last 2300 years. The bottom sediments were found to contain carotenoids, which are molecular markers of the corresponding groups of Phototrophic organisms. The bottom sediments of Lake Shira were shown to be a promising object for climate reconstructions of the Late Holocene in southern Siberia. [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]

During the middle and late Holocene, the Iberian Peninsula underwent large climatic and hydrologic changes, but the temporal resolution and regional distribution of available palaeoenvironmental records is still insufficient for a comprehensive assessment of the regional variability. The high sedimentation rate in karstic, meromictic Montcortès Lake (Catalan pre-Pyrenees) allows for a detailed reconstruction of the regional palaeoecology over the last 5,340 years using diatom analysis, aquatic pollen, sedimentological data, and historic documentary records. Results show marked fluctuations in diatom species assemblage composition, mainly between dominant Cyclotella taxa and small Fragilariales. We suggest that the conspicuous alternation between Cyclotella comta and C. cyclopuncta reflects changes in trophic state, while the succession of centric and pennate species most likely reflects changes in the hydrology of the lake. The diatom assemblages were used to identify six main phases: (1) high productivity and likely lower lake levels before 2350 BC, (2) lower lake levels and a strong arid phase between 2350 and 1850 BC, (3) lake level increase between 1850 and 850 BC, (4) relatively high lake level with fluctuating conditions during the Iberian and Roman Epochs (650 BC-350 AD), (5) lower lake levels, unfavourable conditions for diatom preservation, eutrophication and erosion triggered by increased human activities in the watershed during the Medieval Climate Anomaly (900-1300 AD), and (6) relatively higher lake levels during the LIA (1380-1850 AD) and afterwards. The combined study of diatoms, algae and pollen provides a detailed reconstruction of past climate, which refines understanding of regional environmental variability and interactions between climate and socio-economic conditions in the Pyrenees. [ABSTRACT FROM AUTHOR]

In a brackish, temperate, 24-m-deep Lake Shira, the profiles of salinity, temperature, oxygen and sulfide concentrations were measured on a seasonal basis from 2002 to 2009. The lake was shown to be meromictic with autumnal overturn restricted to mixolimnion. The depth of mixolimnion and position of oxic–anoxic interface varied annually. The spring mixing processes contribute to the formation of mixolimnion in autumn. The exceptionally windy spring of 2007 caused the deepening of mixolimnion in the winter of 2008. The winter position of oxic–anoxic interface was affected by the position of lower boundary of mixolimnion in all winters. The salinity in the winter mixolimnion increased compared with the autumn because of freezing out of salts from the upper water layers meters during ice formation and their dissolution in water below. The profiles of salinity and temperature were simulated by the mathematical 1-D model of temperature and salinity conditions taking into account ice formation. The simulated profiles generally coincided with the measured ones. The coincidence implies that simplified one-dimensional model can be applied to roughly describe salinity and density profiles and mixing behavior of Lake Shira. [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]

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]

A 2.5-m-long sediment core was retrieved from Lake Somaslampi, a small lake located in a kame field on the north slope of the Scandes Mountains in Finnish Lapland. Holocene environmental changes were inferred from the lithological, geochemical, pollen, diatom and Cladocera records stored in the lake sediment. The chronology was based on six radiocarbon AMS dates supported by a palynological control chronology. The sediment profile consists of a glacial sedimentary sequence truncated by a lacustrine one. A hiatus, tentatively correlated with climate cooling and advances of glaciers during the 8.2 ka yrs BP “Finse cooling Event”, occurs between these sequences. The glacial sequence was composed of fluvioglacial clastics, smoothly changing into glacio-lacustrine diatomaceous ooze deposited in a meromictic proglacial lake that covered the kame field. The meromixis was probably caused by the greater depth of the lake, the extended ice-cover, and the microbial mats covering large areas of the lake bottom. A distinct change in the biota of the glacio-lacustrine sediments indicates higher trophic conditions than during deposition of the fluvioglacial clastics. The late-Pleistocene vegetation was characterised by subarctic birch tundra vegetation (Betula–Salix–Ericaceae) with low biodiversity gradually changing to Betula–Pinus dominance in the early Holocene. The lake was deep and had a diatom inferred pH ~ 7 indicated also by the dominance of planktonic Cladocera. The base of the lacustrine sediment sequence (6,650–6,300 cal. BP) consisted of loess-rich sediment indicating an increase in eolian activity. This is also supported by the pollen record, which is dominated by more long-distant taxa such as Alnus and Pinus, and by the increased C/N ratio of the sediment. After the initial meromictic phase of the lake, an abrupt lowering of the water level occurred. Lake Somaslampi was isolated from the larger Pre-Lake Somas basin and became holomictic, shallow, much warmer and more productive, until the deterioration of climate around 3,000 yr BP and the increased input of clastics from the tundra soils. The vegetation followed the general climatic trend by gradually changing from the dominance of Betula and Pinus to the dominance of more tundra-related vegetation like Poaceae and Cyperaceae. However, the higher frequencies of planktonic Cladocera and centric diatoms in the most recent sediments indicates higher trophic conditions, increased turbulence and a prolonged ice-free period, which can possibly be linked to the recent climate warming especially in areas of higher altitude and latitude. [ABSTRACT FROM AUTHOR]

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]

Cores of recent sediments were sampled along a depth gradient in a 23 m deep kettle lake with stagnant deep waters containing exceptionally high concentrations of dissolved iron and manganese. Sediment cores were taken on two occasions, in 1978 and 1997, before and after an incidence of full circulation. The aims of this study are to see how oxic and anoxic conditions in the water column influence stratigraphy and sediment focusing and, to compare cores from 1979 and 1998 to see how measured element fluxes and external events are reflected in the chemical stratigraphy. Element analyses show characteristic stratigraphic patterns that depend on the ability to undergo redox transformations, sorptive properties and chemical equilibria in the anoxic deep waters and porewaters. In sediments from the oxic part of the lake Al, Cr, Co, Ni, Zn Cu, Cd, and Pb were well correlated. Positive correlations are seen between elements associated with primary production and sulphur. In the anoxic part of the lake most metals were positively correlated with carbonate. Phosphorus correlated positively with iron in sediments from oxic waters and negatively with manganese and iron deep-water sediments. Porewater analyses indicate that recycling from the deep-water sediments was negligible. The stratigraphy of lead agrees with the historic variation in atmospheric input and is used as a chronological marker. Assessed deposition rates agree with measurements in sediment traps. Most elements more than double their rates of deposition towards the deepest point of the lake, while sulphur, manganese and carbonate had maxima around the depth of the redoxcline in the water. Variations in the external loading and variable redox conditions in the deep waters explain variations in the chemical composition of recent sediments. [ABSTRACT FROM AUTHOR]

Morphometric, hydrological and basic physico-chemical characteristics of three deep Alpine lakes, Traunsee, Hallstättersee and Attersee as well as their long-term bahaviour are presented. The deep Alpine lakes Hallstättersee and Traunsee have been influenced by salt mining and the traditional salt industry for over 100 years. Waste products from these activities, entering the lakes, have mainly affected the chemistry of these water bodies, especially by substantially increasing the chloride concentrations up to 170 mg L-1. As a consequence of the increased density, mixing conditions of the lakes were altered. The resulting incomplete mixing led to oxygen depletion in deeper layers. In addition, increased nutrient load from the catchment rised the trophic level in the 70s and 80s of the last century in turn, affecting the oxygen content in the hypolimnion. Finally a situation developed where the risk became high for these lakes to become meromictic induced by human activity. In fact, Hallstättersee became facultatively meromictic. This process was interrupted by increased chloride input of more than 30 mg L-1 due to accidental wash out from an upstream salt mine rendering Hallstättersee homogenous in 1978 to 1980 resulting in complete over-turn. Conditions substantially improved in both lakes after mining practices were altered and restoration measures against eutrophication were initiated. Chloride and phosphorus concentrations declined, while oxygen conditions substantially improved in the following years. Conditions in Traunsee substantially improved and chloride levels near the sediment decreased to less than 140 mg L-1. The third lake considered here, Attersee, always remained in a near-natural state although some signs of increased nutrient levels became visible in the late 1970s. Chloride concentrations of around 3 mg L-1 in this lake can be considered as background levels. Attersee can now serve as a reference site for deep Alpine lakes because of its ultra-oligotrophic and pristine nature. [ABSTRACT FROM AUTHOR]

This paper describes a tracer experiment with sulfurhexafluoride (SF6) in the monimolimnion of the meromictic mining lake Merseburg-Ost 1b. In October 1998, 1.1 mmol (160 mg) of the conservative gas SF6 was released at the site of greatest depth 3.5 m above the sediment to observe its vertical spreading. An easy-to-use system to collect ∼0.5 L water samples using evacuated flasks was developed. The headspace technique, gas chromatographic separation and ECD-detection were used to determine SF6. The main objective is the evaluation of mean vertical diffusion coefficients for SF6 and heat from periodically measured SF6 and CTD profiles. During the study period of 11/2 years, the heat transport was estimated to be molecular in the stratified portion. A thermal flux from the sediments of 0.23 W/m2 was found necessary to balance the heat. In the region of high stability ( N 2∼10-2 s-2) diffusivities for SF6 were ∼ 10-8 m2/s, whereas in the lower part of the monimolimnion both tracers resulted in K∼ 7 · 10-6 m2/s. We found K to be approximately proportional to N -2.4 ± 0.2. [ABSTRACT FROM AUTHOR]

The main limnological characteristics of the biogenic meromictic Lake La Cruz, a small karstic lake located on a dolomitic substrate, are outlined below. The lake owes its chemical stratification mainly to high concentrations of bicarbonate and calcium in the permanent monimolimnion, located below a depth of 19 m. A massive precipitation of CaCO3 crystals ("whiting"), occurring in the euphotic zone every summer, leads to further sedimentation and redissolution in deep waters with enhanced CO2 concentrations. Other ions, like magnesium, ammonium, reduced iron, silicate, etc. also contribute to the chemically-induced stability of the monimolimnion. The patterns of stratification during two consecutive thermal stagnation periods (1987-1988) were influenced by different climatological conditions. Depending of the year, the location of the pycnoclines varied; the rainier spring of 1988 favoured a much shallower location of the oxic-anoxic boundary and a much higher primary production reflected by larger oxygen peaks. This greater phytoplanktonic development in upper waters occasioned a shadowing effect on purple and green phototrophic bacterial populations growing in anoxic waters poor in sulphide. [ABSTRACT FROM AUTHOR]

The development of Soppensee (Central Switzerland, 596 m a.s.l.) has been reconstructed using algal remains such as diatoms, chlorophytes and fossil pigments, as well as the pollen and spores of macrophytes. Sediment accumulation in Soppensee began at the end of the last glacial period, approximately 15,000 yrs ago. During the Oldest Dryas biozone (> 12,700 radiocarbon yrs B.P.) the lake had low primary productivity. After reforestation with birch and later pine, around 12,700 B.P., phases of summer anoxia occurred in the lake. These anoxic conditions were most probably caused by additional carbon input from the catchment, as well as longer phases of stratification due to reduced wind exposure caused by the sheltering effect of increased tree cover. From the Younger Dryas biozone (10,800 to 10,000 radiocarbon yrs B.P.) onwards, Soppensee became meromictic for several millennia. The fossil diatom assemblages are dominated by planktonic alkaliphilous to alkalibiontic species with mainly meso- to eutrophic preferences. Diatom-inferred total phosphorus reconstructions suggest meso- to eutrophic conditions throughout the Holocene. Eutrophic conditions are also suggested by the presence of pigments of cyanobacteria, including Oscillatoria species. First human activity in the catchment is evidenced ca. 5000 radiocarbon yrs B.P. by the occurrence of cereal pollen. Diatom-inferred total phosphorus concentrations also increased slightly during the Neolithic period. According to the fossil pigment record, meromictic conditions ended during the Iron Age. Deep-water anoxia, however, persisted at least during the stratification period. During the Middle Ages massive deforestation in the catchment and around the lake changed the limnological conditions drastically. The lack of forest increased the wind fetch and, therefore, also the mixing of the lake, while soil erosion and retting of hemp supplied additional nutrients. Because of intensive agriculture in its catchment, Soppensee has become hypertrophic and diatom assemblages have consequently changed completely in the last 50 yrs. [ABSTRACT FROM AUTHOR]

The seasonal and spatial variations in the community structure of bacterioplankton in the meromictic alpine Lake Cadagno were examined by temporal temperature gradient gel electrophoresis (TTGE) of PCR-amplified 16S rDNA fragments. Two different amplifications were performed, one specific for the domain Bacteria (Escherichia coli positions 8–536) and another specific for the family Chromatiaceae (E. coli positions 8–1005). The latter was followed by semi-nested reamplification with the bacterial primer set, allowing comparison of the two PCR approaches by TTGE. The TTGE patterns of samples from the chemocline and the anoxic monimolimnion were essentially identical, whereas the oxic mixolimnion displayed distinctively different banding patterns. For samples from the chemocline and the monimolimnion, dominant bands in the Bacteria-specific TTGE profiles comigrated with bands obtained by the semi-nested PCR approach specific for Chromatiaceae. This observation suggested that Chromatiaceae are in high abundance in the anoxic water layer. All dominant bands were excised and sequenced. Changes in the community structure, as indicated by changes in the TTGE profiles, were observed in samples taken at different times of the year. In the chemocline, Chromatium okenii was dominant in the summer months, whereas Amoebobacter purpureus populations dominated in autumn and winter. This change was confirmed by fluorescent in situ hybridization. [ABSTRACT FROM AUTHOR]

Sediments of Lake Fidler, a meromictic lake in south-west Tasmania, contain distinctive laminae. In order to determine their composition and formation, these laminae were studied using a combination of X- ray analysis, scanning electron microscopy, X-ray diffraction and vibrational spectrometry. Results indicated that the laminae were composed of clay originating from the adjacent Gordon River estuary. The clay was also found as part of the general sediment matrix of the core. The evidence indicates that the laminae are formed during incursions of brackish water from the adjacent Gordon River estuary into Lake Fidler which cause the flocculation of clay minerals in the water column. These clay minerals then sink rapidly to the sediments to form laminae. Although the formation of these laminae pre-dates meromixis, their mode of formation corroborates the theory that meromixis is maintained ectogenically by periodic inflows of brackish water replenishing monimolimnetic salts. [ABSTRACT FROM AUTHOR]

A sediment core from a Mediterranean karstic lake was studied through its pollen, diatom, chydorid, ostracod, charcoal and authigenic mineral composition. Information about environmental history recorded in the sediment sequence since the Middle Ages is presented. The main fluctuations of water volume and trophic status of the lake occurred during periods of great change in land management and during climatic cold phases. The synergetic effect of these two factors led to a high water level phase and triggered a rising of the trophic level which produced meromixis. The onset of meromictic conditions at about 1700 AD coincides with the Maunder minimum in the Little Ice Age as well as with a period of increasing human population, woodland clearance and agricultural expansion to the detriment of the nomadic livestock breeding or transhumance ("Mesta"). [ABSTRACT FROM AUTHOR]

Multidisciplinary palaeolimnological approaches were applied to long sediment cores from Längsee, Carinthia. Climate amelioration after deglaciation allowed the expansion of shrubs and timber-line trees and the onset of plankton and probably lake summer stratification. This phase called Pre-Bölling has an upper boundary of approx. 16 kyrs BP. The change from a steppe to a tundra-like vegetation culminated during the following less temperate, humid phase, with lower in-lake productivity (Oldest Dryas, Ia; 16-13.6 kyrs BP). At about 13.6 kyrs BP when reforestation took place, summer surface water temperature increased towards recent temperatures and hypolimnetic anoxia developed. Different anoxic levels in respect to climate change underline the dynamic type of meromixis. Levels varied, being highest during the Preboreal/Boreal, and lowered during the Atlantic towards recent levels. The contrast between low trophic status and sediments enriched in organic matter characterizes "pseudeutrophic lake stratification" before the onset of human impact. Changes in the pelagic cladoceran communities during this time seem to be related to changes in predation. Aside long-term climatic change during the Atlantic, short-term fluctuations of precipitation within climate oscillations are suggested as possible causes of erosion processes, changes in lake level and water temperature. These are indicated for the Oldest and Younger Dryas, the Boreal/Atlantic and the Subboreal/Subatlantic transitions. They most likely act as the trigger mechanism for temporary mixing and changes in redox conditions. Extensive forest clearance in the catchment during Bronze-Age settlement caused a change in sediment type and temporary nutrient enrichment. The perturbations are comparable with erosion processes during the late-glacial when climate oscillations were the only driving forces. Anthropogenic influences on the lake environment vary according to land-use type. [ABSTRACT FROM AUTHOR]

Vertical distribution of fossil carotenoids in a sediment core from meromictic Mahoney Lake was studied. Besides okenone and demethylated okenone, lutein and zeaxanthin and β-carotene isomers were identified. No carotenoids typical for purple nonsulfur or green sulfur bacteria were detected. The ratio of zeaxanthin to lutein (above 1:1 in all samples) indicates a dominance of cyanobacteria over green algae in the phytoplankton assemblages of the past. Okenone, which is found exclusively in Chromatiaceae, was the dominating carotenoid in all sediment zones. The oldest sediment layers containing okenone were deposited 11 000 years ago. Between 9000 and 7000 and since 3000 years b.p., Chromatiaceae reached a considerable biomass in the lake. Vertical changes in okenone concentration were not related to changes of paleotemperatures. In contrast, okenone concentrations decreased during periods of volcanic ash input. During most of the lake history, however, mean okenone concentrations were positively correlated with sedimentation rates. This indicates that vertical changes of okenone concentration in the sediment reflect past changes of purple sulfur bacterial biomass in the lake. According to these results, the past limnology of Mahoney Lake resembled that of the present with a sulfide-containing monimolimnion and a well-developed population of okenone-bearing purple sulfur bacteria. [ABSTRACT FROM AUTHOR]

During 1986 planktonic primary production and controlling factors were investigated in a small (A = 11.8 · 10 m, Z = 11.5 m) meromictic kettle lake (Mittlerer Buchensee). Annual phytoplankton productivity was estimated to ca 120 gC · m · a (1,42 tC · lake · a). The marked thermal stratification of the lake led to irregular vertical distributions of chlorophyll a concentrations (Chl a) and, to a minor extent, of photosynthesis (A). Between the depths of 0 to 6 m low Chl a concentrations (< 7 mg · m) and comparatively high background light attenuation (k = 0,525 m, 77% of total attenuation due to gelbstoff and abioseston) was found. As a consequence, light absorption by algae was low (mean value 17,4%) and self-shading was absent. Because of the small seasonal variation of Chl a concentrations, no significant correlation between Chl a and areal photosynthesis (ΣA) was observed. Only in early summer (June-July) biomass appears to influence the vertical distribution of photosynthesis on a bigger scale. Around 8 m depth, low-light adapted algae and phototrophic bacteria formed dense layers. Due to low ambient irradiances, the contribution of these organisms to total primary productivity was small. Primary production and incident irradiance were significantly correlated with each other (r = 0.68). Although the maximum assimilation number (P) showed a clear dependence upon water temperature (Q = 2.31), the latter was of minor importance to areal photosynthesis. [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]

Meromixis, stratification and internal waves were observed 4 km NW of Kigoma Port, Lake Tanganyika. Meromixis occurred during the cool, dry season, from June to August; its end was marked by upward displacement of some isotherms. From September to early November, the water temperature increased but the thermocline was still not distinct. FromNovember onwards, stratification was enhanced and the thermocline became well defined and oscillated about 40 m. From February, while still oscillating, it gradually descended to a maximum depth of 70 m, inJune. In July 1994, the cycle of meromixis started again. Oscillations of the thermocline indicated the existence of internal waves. The periodicity and duration of the oscillations implied that they were seiches similar to those already observed elsewhere on the lake, thus indicating their lake-wide existence. Other parameters, such as conductivity, pH, turbidity, ammonia and nitrate appeared to vary in response to these oscillations; thus the oscillations probably play a vital role in the biological production in Lake Tanganyika. [ABSTRACT FROM AUTHOR]