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Adaptive efforts to achieve water quality objectives by modifying nutrient loading can have attendant impacts on fish habitats and fisheries. Thus, coordinating fishery and water quality management depends on knowledge of fish behavioral responses to habitat change. This study combined acoustic telemetry of fish with water quality modeling to understand how water quality management might impact fishery management. We examined habitat use of a native demersal fish, lake whitefish Coregonus clupeaformis , in Lake Erie. We focused on the summer stratified period when habitat was expected to be most limiting and used a forecast model to predict temperature and oxygen in the hypolimnion when fish were detected. As hypothesized, lake whitefish occupied a subset of available conditions with occupied habitats characterized by a cool, normoxic, hypolimnion. On some occasions fish were detected when the hypolimnion was predicted to be hypoxic, suggesting that fish were either displaced vertically or horizontally into marginal habitats or uncertainty in model predictions was high. Still, when hypolimnetic conditions were hypoxic, fish tended to move toward normoxia as expected, but when initial conditions were cold with high dissolved oxygen, fish movements were toward lower oxygen (but still normoxic) conditions. We also observed a high affinity for fish to remain near the southern shore in eastern Ohio, Pennsylvania, and New York. If current nutrient reduction objectives are achieved and the extent and severity of hypoxia is reduced, an expansion of lake whitefish habitat and distribution may have significance to the spatial regulation of fishing effort in Lake Erie. [ABSTRACT FROM AUTHOR]

The sinkholes along the Dead Sea (DS) shores form by dissolution of an 11–10 kyr old subsurface salt layer (hereafter named the 'Sinkholes Salt') that precipitated on the lake's floor during periods of negative water balance, water level decline and salinity increase. We analyze the variations in absolute elevation and thickness of this layer in 40 boreholes along the western shores of the DS, reconstruct water-body stratification, past lake levels, and paleo-bathymetry during salt deposition, and comment on the role of the salt-layer elevation in future sinkhole formation. In the northern basin of the DS, maximum thickness of salt (~ 23 m) is found where salt top and bottom elevations are below ~ 440 meters below sea level (mbsl) and ~ 465 mbsl, respectively. Above these elevations the salt layer gradually thins out until 416 mbsl, above which it is no longer found. These relationships suggest that thermohaline stratification, with a thermocline at 25–30 m depth, similar to the present day dynamics of the DS, developed annually during the salt-precipitation period, giving rise to uniform salt accumulation below the thermocline and partial to full dissolution above it. Salt accumulation was controlled by the bathymetry of the lake and its configuration relative to the thermocline, and locally hampered by discharge of subaqueous under-saturated groundwater. The truncation of the salt layer at elevation of 416 mbsl is attributed to salt dissolution down to this elevation by a relatively diluted upper water layer that developed following inflow of fresh surface water at the end of the salt period. This event also marks the change to a positive water balance and lake level rise from its lowest stand of ~ 405 mbsl, as determined from limnological considerations. [ABSTRACT FROM AUTHOR]

Lake stratification and mixing processes can influence gas and energy transport in the water column and water–atmosphere interactions, thus impacting limnology and local climate. Featuring the largest high-elevation inland lake zone in the world, comprehensive and comparative studies on the evolution of lake stratification and mixing and their driving forces are still quite limited. Here, using valuable temperature chain measurements in four large lakes (Nam Co, Dagze Co, Bangong Co, and Paiku Co) and a "small lake" adjacent to Nam Co, our objectives are to investigate the seasonal and diurnal variations of epilimnion depth (Ep, the most important layer in stratification and mixing process) and to analyze the driving force differences between "small lake" and Nam Co. Results indicate that Ep estimated by the methods of the absolute density difference (<0.1 kg m−3) from the surface and the Lake-Analyzer were quite similar, with the former being more reliable and widely applicable. The stratification and mixing in the four large lakes showed a dimictic pattern, with obvious spring and autumn turnovers. Additionally, the stratification form during heat storage periods, with Ep quickly locating at depths of approximately 10–15 m, and, after that, increasing gradually to the lake bottom. Additionally, the diurnal variation in Ep can be evidenced both in the large and small lakes when temperature measurements above 3 m depth are included. For Nam Co, the dominant influencing factors for the seasonal variation of Ep were the heat budget components (turbulent heat fluxes and radiation components), while wind speed only had a relatively weak positive correlation (r = 0.23). In the "small lake", radiation components and wind speed show high negative (r = −0.43 to −0.59) and positive (r = 0.46) correlation, with rare correlations for turbulent heat flux. These reported characteristics have significance for lake process modeling and evaluation in these high-elevation lakes. [ABSTRACT FROM AUTHOR]

Shallow lakes and ponds undergo frequent temporary thermal stratification. How this affects greenhouse gas (GHG) emissions is moot, with both increased and reduced GHG emissions hypothesised. Here, weekly estimation of GHG emissions were combined with high-resolution temperature and oxygen profiles of an 11 hectare shallow lake to investigate how thermal stratification shapes GHG emissions. There were three main stratification periods with profound anoxia in the bottom waters occurring quickly upon isolation from the atmosphere. Average diffusive emission of methane (CH4) and nitrous oxide (N2O) were larger and more variable in stratified phase, whereas carbon dioxide (CO2) was on average lower. CH4 ebullition was an order of magnitude greater in the stratified phase. In addition, there was a large efflux of CH4 and CO2 when the lake mixed after periods of extended (circa 14 days) thermal stratification. These two turnover events were estimated to have released the majority of the CH4 emitted between May and September. These results highlight the role of turnover emissions resulting from temporary thermal stratification and also the need high frequency measurements of GHG emission in order to accurately characterise emissions from these temporarily stratifying lakes. [ABSTRACT FROM AUTHOR]

A comparative analysis of climatic conditions is carried out according to the data of the Solovki and Kargopol weather stations in order to identify their role in the formation of the hydrochemical and hydrobiological regime of the Bolshoi Solovetskii Island lakes and its likely trends in a changing climate. According to the data of hydrological studies of 15 lakes in July–August 2021, the features of the vertical distribution of temperature, electrical conductivity, and water turbidity in them are analyzed. [ABSTRACT FROM AUTHOR]

McGuire HE, Dunnington DW, Loder AL, Spooner IS, Mallory ML, McLellan NR, Su, C-C. 2021. Evaluating the multidecadal response of historic seawater incursion events and salinity-induced meromixis at Laytons Lake, Nova Scotia, Canada. Lake Reserv Manage. 37:378–390. Long-term monitoring data from freshwater lakes can be challenging to retrieve yet are valuable for assessing lakes at risk of saltwater intrusion. We demonstrate the use of an applied paleolimnological method to identify the response of a freshwater lake to seawater incursion and salinity-induced meromixis by examining the geochemistry of lake sediments deposited over the past century. Laytons Lake is located 1.5 km from the Bay of Fundy coastline, and experienced documented seawater incursion events in 1944 and 1949 leading to a period of extended meromixis (>40 yr). Our limnological measurements indicate that meromixis no longer exists at Laytons Lake, and hypolimnetic phosphorus (P) concentrations and conductivity values have declined since 1982. Meromictic conditions, the establishment of the chemocline, and high sedimentation rates collectively preserve a geochemical record of seawater incursion, which was identified as a short-duration, positive chloride (Cl-) excursion followed by the precipitation of iron sulfide (FeS2). A decline in Cl- concentration after the incursion events in the paleorecord may represent the deterioration of the chemocline, and the reestablishment of thermal stratification and mixing. We suspect that seawater incursions can have a multidecadal impact on aquatic systems, and that bulk geochemical analysis of paleolimnological records is a useful, low-cost tool that can provide a long-term perspective on the limnological consequences of such events. These long-term data can inform the assessment of aquatic systems at risk of saltwater incursion due to sea-level rise. [ABSTRACT FROM AUTHOR]

Cyanobacteria are important primary producers in temperate freshwater ecosystems. However, studies on the seasonal and spatial distribution of cyanobacteria in deep lakes based on high-throughput DNA sequencing are still rare. In this study, we combined monthly water sampling and monitoring in 2019, amplicon sequence variants analysis (ASVs; a proxy for different species) and quantitative PCR targeting overall cyanobacteria abundance to describe the seasonal and spatial dynamics of cyanobacteria in the deep hard-water oligo-mesotrophic Lake Tiefer See, NE Germany. We observed significant seasonal variation in the cyanobacterial community composition (p < 0.05) in the epi- and metalimnion layers, but not in the hypolimnion. In winter—when the water column is mixed—picocyanobacteria (Synechococcus and Cyanobium) were dominant. With the onset of stratification in late spring, we observed potential niche specialization and coexistence among the cyanobacteria taxa driven mainly by light and nutrient dynamics. Specifically, ASVs assigned to picocyanobacteria and the genus Planktothrix were the main contributors to the formation of deep chlorophyll maxima along a light gradient. While Synechococcus and different Cyanobium ASVs were abundant in the epilimnion up to the base of the euphotic zone from spring to fall, Planktothrix mainly occurred in the metalimnetic layer below the euphotic zone where also overall cyanobacteria abundance was highest in summer. Our data revealed two potentially psychrotolerant (cold-adapted) Cyanobium species that appear to cope well under conditions of lower hypolimnetic water temperature and light as well as increasing sediment-released phosphate in the deeper waters in summer. The potential cold-adapted Cyanobium species were also dominant throughout the water column in fall and winter. Furthermore, Snowella and Microcystis -related ASVs were abundant in the water column during the onset of fall turnover. Altogether, these findings suggest previously unascertained and considerable spatiotemporal changes in the community of cyanobacteria on the species level especially within the genus Cyanobium in deep hard-water temperate lakes. [ABSTRACT FROM AUTHOR]

Our study explores the variability of chlorophyll a and phycocyanin as well as the taxonomic and functional structure of phytoplankton in epilimnetic and metalimnetic strata regarding the main environmental features of the water column. The maximum phytoplankton pigment concentrations are located differently depending on the type of thermal stratification and light availability. In general, there were three situations observed during the summer stratification: the euphotic zone did not extend to the thermocline and the highest chlorophyll concentration developed above the thermocline level, at the bottom of the euphotic zone (type I); the euphotic zone reached beyond the thermocline, and the maximum chlorophyll concentration occurred above the thermocline (type II); or the euphotic zone was much greater than the thermocline depth and the peak phytoplankton density was located below the thermocline (type III). In metalimnetic samples, the most dominant cyanobacteria belonged to the Reynolds Functional Groups (RFG) H1, S1 , and Lo ; their occurrence differed depending on the types of light and thermal regimes. It was assumed that vertical heterogeneity affects the ecological status assessment based on phytoplankton. Calculating the two applied indices (TI index and Q index) based on metalimnetic and epilimnetic data (as opposed to only epilimnetic data) yielded worse ecological status in > 70% of cases. [ABSTRACT FROM AUTHOR]

Cyanobacteria are important primary producers in temperate freshwater ecosystems. However, studies on the seasonal and spatial distribution of cyanobacteria in deep lakes based on high-throughput DNA sequencing are still rare. In this study, we combined monthly water sampling and monitoring in 2019, amplicon sequence variants analysis (ASVs; a proxy for different species) and quantitative PCR targeting overall cyanobacteria abundance to describe the seasonal and spatial dynamics of cyanobacteria in the deep hard-water oligo-mesotrophic Lake Tiefer See, NE Germany. We observed significant seasonal variation in the cyanobacterial community composition (p < 0.05) in the epi- and metalimnion layers, but not in the hypolimnion. In winter—when the water column is mixed—picocyanobacteria (Synechococcus and Cyanobium) were dominant. With the onset of stratification in late spring, we observed potential niche specialization and coexistence among the cyanobacteria taxa driven mainly by light and nutrient dynamics. Specifically, ASVs assigned to picocyanobacteria and the genus Planktothrix were the main contributors to the formation of deep chlorophyll maxima along a light gradient. While Synechococcus and different Cyanobium ASVs were abundant in the epilimnion up to the base of the euphotic zone from spring to fall, Planktothrix mainly occurred in the metalimnetic layer below the euphotic zone where also overall cyanobacteria abundance was highest in summer. Our data revealed two potentially psychrotolerant (cold-adapted) Cyanobium species that appear to cope well under conditions of lower hypolimnetic water temperature and light as well as increasing sediment-released phosphate in the deeper waters in summer. The potential cold-adapted Cyanobium species were also dominant throughout the water column in fall and winter. Furthermore, Snowella and Microcystis-related ASVs were abundant in the water column during the onset of fall turnover. Altogether, these findings suggest previously unascertained and considerable spatiotemporal changes in the community of cyanobacteria on the species level especially within the genus Cyanobium in deep hard-water temperate lakes.

Oxygen depletion in lake bottom waters has adverse impacts on ecosystem health including decreased water quality from release of nutrients and reduced substances from sediments, and the reduction of fish growth and reproduction. Depletion occurs when oxygen is consumed during decomposition of organic matter, and oxygen replenishment is limited by water column stratification. Arctic–alpine lakes are often well mixed and oxygenated, but rapid climate change in these regions is an important driver of shifts in catchment vegetation that could affect the mixing and oxygen dynamics of lakes. Here, we analyze high‐resolution time series of dissolved oxygen concentration and temperature profiles in 40 Swedish arctic–alpine lakes across the tree line ecotone. The lakes stratified for 1−125 days, and during stratification, near‐bottom dissolved oxygen concentrations changed by −0.20 to +0.15 mg L−1 day−1, resulting in final concentrations of 1.1−15.5 mg L−1 at the end of the longest stratification period. Structural equation modeling revealed that lakes with taller shoreline vegetation relative to lake area had higher dissolved organic carbon concentrations and oxygen consumption rates, but also lower wind speeds and longer stratification periods, and ultimately, lower near‐bottom dissolved oxygen concentrations. We use an index of shoreline canopy height and lake area to predict variations among our study lakes in near‐bottom dissolved oxygen concentrations at the end of the longest stratification period (R2 = 0.41). Upscaling this relationship to 8392 Swedish arctic–alpine lakes revealed that near‐bottom dissolved oxygen concentrations drop below 3, 5, and 7 mg L−1 in 15%, 32%, and 53% of the lakes and that this proportion is sensitive (5%−22%, 13%−45%, and 29%−69%) to hypothetical tree line shifts observed in the past century or reconstructed for the Holocene (±200 m elevation; ±0.5° latitude). Assuming space‐for‐time substitution, we predict that tree line advance will decrease near‐bottom dissolved oxygen concentrations in many arctic–alpine lakes. [ABSTRACT FROM AUTHOR]

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

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Abstract Dissolved oxygen is a key player in water quality. Stratified water bodies show distinct vertical patterns of oxygen concentration, which can originate from physical, chemical or biological processes. We observed a pronounced metalimnetic oxygen minimum in the low-nutrient Rappbode Reservoir, Germany. Contrary to the situation in the hypolimnion, measurements of lateral gradients excluded the sediment contact zone from the major sources of oxygen depletion for the metalimnetic oxygen minimum. Instead, the minimum was the result of locally enhanced oxygen consumption in the open water body. A follow-up monitoring included multiple chlorophyll a fluorescence sensors with high temporal and vertical resolution to detect and document the evolution of phytoplankton. While chlorophyll fluorescence sensors with multiple channels detected a mass development of the phycoerythrin-rich cyanobacterium Planktothrix rubescens in the metalimnion, this species was overlooked by the commonly used single-channel chlorophyll sensor. The survey indicated that the waning P. rubescens fluorescence was responsible for the oxygen minimum in the metalimnion. We hypothesize that pelagic processes, i.e., either oxygen use through decomposition of dead organic material originating from P. rubescens or P. rubescens extending its respiration beyond its photosynthetic activity, induced the metalimnetic oxygen minimum. The deeper understanding of the oxygen dynamics is mandatory for optimizing reservoir management. Graphical abstract Image 1 Highlights • A metalimnetic oxygen minimum (MOM) down to 40% saturation was observed in summer. • The MOM was not imported from the sediment at the side walls. • The MOM was a consequence of pelagic processes, such as respiration in the metalimnion. • We hypothesize that Planktothrix rubescens in the metalimnion caused the MOM. [ABSTRACT FROM AUTHOR]

Human activities simultaneously alter nutrient levels, habitat structure, and levels of parasitism. These activities likely have individual and joint impacts on food webs. Furthermore, there is particular concern that nutrient additions and changes to habitat structure might exacerbate the size of epidemics and impacts on host density. We used a well-studied zooplankton–fungus host–parasite system and experimental whole water column enclosures to factorially manipulate nutrient levels, habitat structure (specifically: mixing), and presence of parasites. Nutrient addition increased infection prevalence, density of infected hosts, and total host density. We hypothesized that nutrients, mixing, and parasitism were linked in multiple ways, including via their combined effects on phytoplankton (resource) abundance, and we used structural equation modeling to disentangle these pathways. In the absence of the parasite, both nutrients and mixing increased abundance of phytoplankton, whereas host density was negatively related to phytoplankton abundance, suggesting a mixture of bottom-up and top-down control of phytoplankton. In the presence of the parasite, nutrients still increased phytoplankton abundance but mixing no longer did, and there was no longer a significant relationship between host density and phytoplankton. This decoupling of host–resource dynamics may have resulted from reduced grazing due to illness-mediated changes in feeding behavior. Overall, our results show that the impact of one human activity (e.g., altered habitat structure) might depend on other human impacts (e.g., parasite introduction). Fortunately, carefully designed experiments and analyses can help tease apart these multifaceted relationships, allowing us to understand how human activities alter food webs, including interactions between hosts and their parasites and resources. [ABSTRACT FROM AUTHOR]

Chemical and isotopic vertical profiles from the volcanic lakes of Sete Cidades, Santiago, Fogo, Congro and Furnas (Island of São Miguel, Azores Archipelago, Portugal) were studied to investigate the biogeochemical processes acting at different depths, with a focus on the CO2 and CH4 dynamics. These lakes are fed by meteoric water affected by seawater spray and interacting with volcanic rocks at a relatively low extent. In addition to volcanogenic gas inputs, the biogeochemical processes are influenced by microbial activities since the lakes offer specialized ecological niches for oxic and anoxic metabolism. The lakes were sampled in two extreme conditions of (partial) mixing (winter) and stratification (summer), respectively. The seasonal thermal stratification favored the development of anaerobic hypolimnia, showing relatively high concentrations of NH4+, NO3-, P and other minor species (Fe, Mn, Zn, As) controlled by microbial activity and minerogenetic processes occurring within the lake sediments. The strongly negative d13C-TDIC values measured in almost all the studied lakes suggest dominant contribution of organic carbon. Dissolved gases were mostly consisting of atmospheric compounds with significant concentrations of CO2 and CH4. The d13C-CO2 values were intermediate between those measured in the hydrothermal fluids and those typical of biogenic CO2. Dissolved CH4, which was the most abundant extra-atmospheric gas in the anoxic waters, was measured at significant concentrations even in the aerobic layers, especially in the winter season. This unexpected feature may tentatively be explained by admitting i) convective mixing of shallow and deep waters, and/or ii) aerobic CH4 production. Further investigations, focusing on the recognition of microbial populations able to produce CH4 at different redox conditions, may be useful to corroborate these intriguing hypotheses.

Chemical and isotopic vertical profiles from the volcanic lakes of Sete Cidades, Santiago, Fogo, Congro and Furnas (Island of São Miguel, Azores Archipelago, Portugal) were studied to investigate the biogeochemical processes acting at different depths, with a focus on the CO2 and CH4 dynamics. These lakes are fed by meteoric water affected by seawater spray and interacting with volcanic rocks at a relatively low extent. In addition to volcanogenic gas inputs, the biogeochemical processes are influenced by microbial activities since the lakes offer specialized ecological niches for oxic and anoxic metabolism. The lakes were sampled in two extreme conditions of (partial) mixing (winter) and stratification (summer), respectively. The seasonal thermal stratification favored the development of anaerobic hypolimnia, showing relatively high concentrations of NH4+, NO3-, P and other minor species (Fe, Mn, Zn, As) controlled by microbial activity and minerogenetic processes occurring within the lake sediments. The strongly negative d13C-TDIC values measured in almost all the studied lakes suggest dominant contribution of organic carbon. Dissolved gases were mostly consisting of atmospheric compounds with significant concentrations of CO2 and CH4. The d13C-CO2 values were intermediate between those measured in the hydrothermal fluids and those typical of biogenic CO2. Dissolved CH4, which was the most abundant extra-atmospheric gas in the anoxic waters, was measured at significant concentrations even in the aerobic layers, especially in the winter season. This unexpected feature may tentatively be explained by admitting i) convective mixing of shallow and deep waters, and/or ii) aerobic CH4 production. Further investigations, focusing on the recognition of microbial populations able to produce CH4 at different redox conditions, may be useful to corroborate these intriguing hypotheses. [ABSTRACT FROM AUTHOR]

From the 1970s to 1990s, more stringent air quality regulations were implemented across North America and Europe to reduce chemical emissions that contribute to acid rain. Surface water pH slowly increased during the following decades, but biological recovery lagged behind chemical recovery. Fortunately, this situation is changing. In the past few years, northeastern US fish populations have begun to recover in lakes that were historically incapable of sustaining wild fish due to acidic conditions. As lake ecosystems across the eastern United States recover from acid deposition, the stress to the most susceptible populations of native coldwater fish appears to be shifting from acidification effects to thermal impacts associated with changing climate. Extreme summer temperature events - which are expected to occur with increasing frequency in the coming century - can stress and ultimately kill native coldwater fish in lakes where thermal stratification is absent or highly limited. Based on data from northeastern North America, we argue that recovery from acid deposition has the potential to improve the resilience of coldwater fish populations in some lakes to impacts of climate change. This will occur as the amount of dissolved organic carbon ( DOC) in the water increases with increasing lake pH. Increased DOC will reduce water clarity and lead to shallower and more persistent lake thermoclines that can provide larger areas of coldwater thermal refuge habitat. Recovery from acidification will not eliminate the threat of climate change to coldwater fish, but secondary effects of acid recovery may improve the resistance of coldwater fish populations in lakes to the effects of elevated summer temperatures in historically acidified ecosystems. This analysis highlights the importance of considering the legacy of past ecosystem impacts and how recovery or persistence of those effects may interact with climate change impacts on biota in the coming decades. [ABSTRACT FROM AUTHOR]

Volcanic lakes provide windows into the interior of volcanoes as they integrate the heat flux discharged by a magma body and condense volcanic gases. Volcanic lake temperatures and geochemical compositions therefore typically serve as warnings for resumed unrest or prior to eruptions. If acidic and hot, these lakes are usually considered to be too convective to allow any stratification within their waters. Kawah Ijen volcano, featuring the largest hyperacidic lake on Earth (volume of 27 million m 3 ), is less homogeneous than previously thought. Hourly temperature measurements reveal the development of a stagnant layer of cold waters (<30 °C), overlying warmer and denser water (generally above 30 °C and density ∼1.083 kg/m 3 ). Examination of 20 yrs of historical records and temporary measurements show a systematic thermal stratification during rainy seasons. The yearly rupture of stratification at the end of the rainy season causes a sudden release of dissolved gases below the cold water layer which appears to generate a lake overturn, i.e. limnic eruption, and a resonance of the lake, i.e. a seiche, highlighting a new hazard for these extreme reservoirs. A minor non-volcanic event, such as a heavy rainfall or an earthquake, may act as a trigger. The density driven overturn requires specific salinity-temperature conditions for the colder and less saline top water layer to sink into the hot saline water. Spectacular degassing occurs when the dissolved gases, progressively stored during the rainy season due to a weakened diffusion of carbon dioxide in the top layer, are suddenly released. These findings challenge the homogenization assumption at acidic lakes and stress the need to develop appropriate monitoring setups. [ABSTRACT FROM AUTHOR]

As a result of reductions in sulfate deposition and changing climate, dissolved organic carbon (DOC) concentrations have increased in many lakes situated in forests of northeastern North America and northern Europe since the 1990s. Although this increase is well documented, the associated ecological implications remain unclear. In particular, DOC strongly influences the vertical temperature structure of lakes, with increasing DOC often leading to a shallower epilimnion. We investigated the effect of increased DOC concentrations on lake thermal structure using fossil diatom records from six remote Maine lakes. Sedimentary diatom profiles from three pairs of small (<0.5 km) lakes were compared, with each pair containing one lake with a documented significant increase in DOC and the other experiencing no change in DOC since the early 1990s. Lake thermal structure was inferred from changes in the relative abundance of Discostella stelligera and Aulacoseira species, two diatom taxa that are associated with changes in thermal stratification. The three lakes without increasing DOC showed no change in diatom-inferred mixing depth over the past few decades. Of the lakes with documented increases in DOC, two showed the highest turnover in diatom community structure over time. Profiles from both of these lakes also indicated directional change in diatom-inferred mixing depth over the past 20 years, but the direction of change differed. This study demonstrates that recent increases in DOC have the potential to alter the physical and biological structure of lakes, but that these responses may differ across lakes. [ABSTRACT FROM AUTHOR]

Using airborne thermal infrared imaging and horizontally resolved in situ temperature monitoring at the lake surface, we estimated strength and duration of regular wind-driven upwelling of dense deep water to the lake surface in two small (in terms of Rossby radius) temperate lakes during the initial phase of summer thermal stratification. The onset and duration of the upwelling events correlated well with the balance between stratification (in terms of Schmidt stability) and wind forcing, as expressed by Lake and Wedderburn numbers. The period of regular upwelling appearances lasted 7-15 days, identified by Schmidt stabilities around 30 J m−2 and Lake numbers between 0 and 1, and resulted in persistent temperature gradients of up to 2°C across the lake surface. Our results suggest that spring upwelling should inevitably take place in all freshwater temperate lakes with mean temperatures crossing the maximum density value of freshwater on annual cycle, whereas duration and intensity of the upwelling would vary depending on lake morphometry and weather conditions. Our results suggest major contribution of upwelling in nutrient supply to the upper waters, oxygenation of the deep water column, and air-lake gas exchange, in particular, the release of the sediment-produced methane into the atmosphere. [ABSTRACT FROM AUTHOR]

Recent climate change represents one of the most serious anthropogenic threats to lake ecosystems in Canada. As meteorological and hydrological conditions are altered by climate change, so too are physical, chemical and biological properties of lakes. The ability to quantify the impact of climate change on the physical properties of lakes represents an integral step in estimating future chemical and biological change. To that end, we have used the dynamic reservoir simulation model, a one-dimensional vertical heat transfer and mixing model, to hindcast and compare lake temperature-depth profiles against 30 years of long-term monitoring data in Harp Lake, Ontario. These temperature profiles were used to calculate annual (June-September) thermal stability values from 1979 to 2009. Comparisons between measured and modelled lake water temperature and thermal stability over three decades showed strong correlation ( r2 > 0.9). However, despite significant increases in modelled thermal stability over the 30 year record, we found no significant change in the timing of the onset, breakdown or the duration of thermal stratification. Our data suggest that increased air temperature and decreased wind are the primary drivers of enhanced stability in Harp Lake since 1979. The high-predictive ability of the Harp Lake dynamic reservoir simulation model suggests that its use as a tool in future lake management projects is appropriate. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Biogeochemical dynamics under seasonal ice cover were investigated in the shallow (<10 m) water column of highly productive Georgetown Lake, western Montana, USA. This high altitude (1,800 m) reservoir is well-mixed in summer, but becomes strongly stratified under ice cover (mid-November-mid-May). A rapid drop in dissolved oxygen (DO) concentration and rise in dissolved inorganic carbon (DIC) concentration was observed after the onset of ice, with a corresponding increase in δO-DO and decrease in δC-DIC, likely caused by respiration (R) of organic carbon. Photosynthesis/respiration ratios (P/R) estimated from simultaneous measurement of DO and δO-DO were near unity prior to ice formation but then systematically decreased with time and depth in the lake under ice cover. P/R in the water column was higher at a shallower monitoring site compared to a deeper site near the dam outlet, which may have been important for over-winter survival of salmonids. By March, the bottom 3 m of the water column at both sites was anoxic, with the bottom 1 m being euxinic. Elevated concentrations of dissolved sulfide, ammonium, phosphate, Fe, and Mn in deep water suggest coupling of organic carbon degradation with reduction of a number of electron acceptors (e.g., Fe, $${\text{NO}}_{3}^{ - } ,\;{\text{SO}}_{4}^{2 - }$$ ). The concentrations and δS values of HS in the deep water and $${\text{SO}}_{4}^{2 - }$$ in the shallow water were similar, indicating near-complete reduction of sulfate in the euxinic zone. Late in the winter, an influx of isotopically heavy DIC was noted in the deep water coincident with a buildup of dissolved CH to concentrations >1 mM. These trends are attributed to acetoclastic methanogenesis in the benthic sediments. This pool of dissolved CH was likely released from the lake to the atmosphere during spring ice-off and lake turnover. [ABSTRACT FROM AUTHOR]

Recruitment from dormant stages in the benthos can provide a critically important inoculum for surface populations of phytoplankton, including bloom-forming cyanobacteria. For example, water-column populations of the large (1-3-mm diameter) colonial cyanobacterium Gloeotrichia echinulata (Smith) P. Richter can be strongly subsidized by benthic recruitment. Therefore, understanding controls on recruitment is essential to an investigation of the factors controlling Gloeotrichia blooms, which are increasing in low-nutrient lakes across northeastern North America. We quantified surface abundances and recruitment from littoral sediments at multiple near-shore sampling sites in oligotrophic Lake Sunapee, New Hampshire, USA, during the summers of 2005-2012 and used this data set-the longest known record of cyanobacterial recruitment-to investigate potential drivers of interannual differences in Gloeotrichia recruitment. We found extensive spatiotemporal variability in recruitment. Recruitment was higher at some sites than others, and within seasons, recruitment into replicate traps at the same site was generally more similar than recruitment at different sites. These data suggest that local factors, such as substrate quality or the size of the seed bank, may be important controls on recruitment. Benthic recruitment probably accounted for <4%, but possibly up to 8%, of pelagic populations, within the range observed in previous studies. Across years, higher seasonal recruitment rates were associated with greater lake mixing during August, including deeper thermoclines, lower Schmidt stability, lower minimum air temperatures, and greater variability in water temperature. Taken together, our data suggest that interannual variation in Gloeotrichia recruitment may be related to regional climatic variability. [ABSTRACT FROM AUTHOR]

We report on the calibration of the one-dimensional hydrodynamic lake model Dynamic Reservoir Simulation Model to simulate the water temperature conditions of the pre-alpine Lake Ammersee (southeast Germany) that is a representative of deep and large lakes in this region. Special focus is given to the calibration in order to reproduce the correct thermal distribution and stratification including the time of onset and duration of summer stratification. To ensure the application of the model to investigate the impact of climate change on lakes, an analysis of the model sensitivity under stepwise modification of meteorological input parameters (air temperature, wind speed, precipitation, global radiation, cloud cover, vapour pressure and tributary water temperature) was conducted. The total mean error of the calibration results is −0.23 °C, the root mean square error amounts to 1.012 °C. All characteristics of the annual stratification cycle were reproduced accurately by the model. Additionally, the simulated deviations for all applied modifications of the input parameters for the sensitivity analysis can be differentiated in the high temporal resolution of monthly values for each specific depth. The smallest applied alteration to each modified input parameter caused a maximum deviation in the simulation results of at least 0.26 °C. The most sensitive reactions of the model can be observed through modifications of the input parameters air temperature and wind speed. Hence, the results show that further investigations at Lake Ammersee, such as coupling the hydrodynamic model with chemo-dynamic models to assess the impact of changing climate on biochemical conditions within lakes, can be carried out using Dynamic Reservoir Simulation Model. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Highlights: [•] We studied effects of nuclear plant thermal pollution and climate warming on a lake. [•] Temperature model FLake and 50-years data from Lake Stechlin, Germany were used. [•] Thermal pollution counteracts the stability increase due to climate warming. [•] Thermal pollution in winter increases the summer hypolimnion temperatures. [•] Both thermal pollution and climate warming prolong the summer stagnation period. [ABSTRACT FROM AUTHOR]

This paper focuses on the chemical and isotopic features of dissolved gases (CH and CO) from four meromictic lakes hosted in volcanic systems of Central-Southern Italy: Lake Albano (Alban Hills), Lake Averno (Phlegrean Fields), and Monticchio Grande and Piccolo lakes (Mt. Vulture). Deep waters in these lakes are characterized by the presence of a significant reservoir of extra-atmospheric dissolved gases mainly consisting of CH and CO. The δC-CH and δD-CH values of dissolved gas samples from the maximum depths of the investigated lakes (from −66.8 to −55.6 ‰ V-PDB and from −279 to −195 ‰ V-SMOW, respectively) suggest that CH is mainly produced by microbial activity. The δC-CO values of Lake Grande, Lake Piccolo, and Lake Albano (ranging from −5.8 to −0.4 ‰ V-PDB) indicate a significant CO contribution from sublacustrine vents originating from (1) mantle degassing and (2) thermometamorphic reactions involving limestone, i.e., the same CO source feeding the regional thermal and cold CO-rich fluid emissions. In contrast, the relatively low δC-CO values (from −13.4 to −8.2 ‰ V-PDB) of Lake Averno indicate a prevalent organic CO. Chemical and isotopic compositions of dissolved CO and CH at different depths are mainly depending on (1) CO inputs from external sources (hydrothermal and/or anthropogenic); (2) CO-CH isotopic exchange; and (3) methanogenic and methanotrophic activity. In the epilimnion, vertical water mixing, free oxygen availability, and photosynthesis cause the dramatic decrease of both CO and CH concentrations. In the hypolimnion, where the δC-CO values progressively increase with depth and the δC-CH values show an opposite trend, biogenic CO production from CH using different electron donor species, such as sulfate, tend to counteract the methanogenesis process whose efficiency achieves its climax at the water-bottom sediment interface. Theoretical values, calculated on the basis of δC-CO values, and measured δC values are not consistent, indicating that CO and the main carbon-bearing ion species (HCO) are not in isotopic equilibrium, likely due to the fast kinetics of biochemical processes involving both CO and CH. This study demonstrates that the vertical patterns of the CO/CH ratio and of δC-CO and δC-CH are to be regarded as promising tools to detect perturbations, related to different causes, such as changes in the CO input from sublacustrine springs, that may affect aerobic and anaerobic layers of meromictic volcanic lakes. [ABSTRACT FROM AUTHOR]

Over the 20th century, surface water temperatures have increased in many lake ecosystems around the world, but long-term trends in the vertical thermal structure of lakes remain unclear, despite the strong control that thermal stratification exerts on the biological response of lakes to climate change. Here we used both neo- and paleoecological approaches to develop a fossil-based inference model for lake mixing depths and thereby refine understanding of lake thermal structure change. We focused on three common planktonic diatom taxa, the distributions of which previous research suggests might be affected by mixing depth. Comparative lake surveys and growth rate experiments revealed that these species respond to lake thermal structure when nitrogen is sufficient, with species optima ranging from shallower to deeper mixing depths. The diatom-based mixing depth model was applied to sedimentary diatom profiles extending back to 1750 AD in two lakes with moderate nitrate concentrations but differing climate settings. Thermal reconstructions were consistent with expected changes, with shallower mixing depths inferred for an alpine lake where treeline has advanced, and deeper mixing depths inferred for a boreal lake where wind strength has increased. The inference model developed here provides a new tool to expand and refine understanding of climate-induced changes in lake ecosystems. [ABSTRACT FROM AUTHOR]

This research investigates possible impacts of enlarged water body according to dam reconstruction on the hydrodynamics and water quality of the reservoir using a laterally averaged, two-dimensional hydrodynamic and transport model, CE-QUAL-W2. The lake was formed by the artificial dam in 1983 for agricultural water supply and is currently under consideration of reconstruction so as to expand the volume of reservoir for flood control as well as water supply in downstream areas. To calibrate and validate the model, field-collected data were compared with model predictions for water level fluctuations and water temperature during the years of 2001 (from January to December) and 2003 (from March to November). The model results showed a good agreement with field measurements both in calibration and verification. Utilizing the model, impacts of dam reconstruction on the thermal hydrodynamics and turbid current were predicted. From the model results, dam reconstruction limited the depth of thermal stratification below 10 meter and formed steep temperature gradient between epilimnion and hypolimnion. The restricted thermal stratification persisted up to the end of September. This result indicated that thermal stratification would become stronger during summer and stay longer after dam reconstruction. In addition, the restricted thermal stratification caused vertical circulation of water mixing lower than 10 meter and isolated the upper water layer from the lower water layer which increased the volume of hypolimnetic water with low temperature. The vertical circulation near the surface also mitigated propagation of density plume within the depth of 10 m which would remain the hypolimnetic water clean. [ABSTRACT FROM AUTHOR]

A survey is given of the vertical distribution and seasonal changes of the chemical composition of water in Lake Verevi. The majority of data consist of 166 to 1011 measurements per parameter from 1984.2001. Statistical analysis was performed with the SAS system, Release 8.1, using a 64-parameter linear model with independent factors ‘sampling year’, ‘sampling day within the year’, and ‘sampling depth’. The depth intervals where the chemical composition of water changed significantly, confidence limits, and predicted mean values were estimated using specific contrasts tailored for the General Linear Models (GLM) procedure of the SAS program package. The hydrochemical state of the lake in summer, autumn, and winter is characterized on the basis of 18 parameters. Stratification is the strongest in summer when water temperature, dissolved oxygen, oxygen saturation, and pH decrease towards the bottom. At the same time, alkalinity, the sum of Ca and Mg, conductivity, Si, CODCr, and CODMn increase in the same direction. The most significant increase towards the bottom was shown by total P, PO43-, total N, and NH4+. The concentrations of SO42-, Cl-, NO2-, and NO3- show relatively constant values from surface to bottom. In autumn, at the time of convective mixing, differences in the water composition from surface to bottom persist but become less pronounced. According to chemical composition, the water in L. Verevi is stratified all the year round. [ABSTRACT FROM AUTHOR]

Portage Lake is situated near the center of the Keweenaw Peninsula and is connected to Lake Superior via lengthy (> 7 km) navigation channels. Using moored thermistor records and meteorological data, we examine how changes in lake stratification are related to surface winds and heat flux. Frequent episodes of full water column mixing are observed throughout the summer. Convective mixing through surface cooling appears to be an important agent responsible for these events, as all occur during cold air outbreaks and when the net heat flux is directed out of the lake (negative). However, wind-induced mixing is also implicated in contributing to some vertical mixing events, as evidenced by two events initiated during a period of strong winds and declining, but not yet negative, heat fluxes. Our analysis indicates that each time the water column restratifies, it tends to become more susceptible to convectively-driven overturn during cold air outbreaks. This tendency is quantified by the estimated time over which surface cooling due to a specified set of conditions, characteristic of a cold air outbreak, would reduce the temperature contrast between the upper and lower layers by half. This time declines by more than art order of magnitude for successive restratification events observed in the summer of 1999. Our analysis also reveals successive formation and dissolution of a diurnal surface mixed layer in an otherwise homogeneous water column during a 10-day period of August 1999. This is attributed to the combination of relatively light winds and negative daily net heat fluxes. [ABSTRACT FROM AUTHOR]

The use of a one-dimensional lake temperature model was investigated using long series of meteorological input data. These data were mainly based on one station. Effects of morphology on water temperature conditions were considered over this long period. The studied lakes are from the Finnish Lake District. Concerning their dynamics, they are small with areas ranging from 257 km² to 0.14 km² with the largest depth about 80 m, but typically less than 50 m. The strength of thermal stratification varies. Verification was made using test summers: for two larger lakes longer periods could be used. Summer stratification periods could finally be resolved satisfactorily, but only after adjusting with data from each lake. For some years the vertical temperature simulations are not as successful as for others, indicating the importance of local weather differences. For lakes with a large area to depth ratio heat is absorbed freely, while when it is small heat is blocked more into the epilimnion. For very small lakes sheltering is essential. Large total volume delays cooling, when stratification prevails. Sheltering is important and also very small lakes cool slowly due to their strong stability. [ABSTRACT FROM AUTHOR]

• Microbial community in the water column showed high heterogeneity tightly coupled with hydrochemical variation. • Dominant species existed in the surface water, whereas microbial diversity and evenness increased with depth. • Environmental selection was prominent in the surface water, and random processes occurred at greater depth. • Microbial co-occurrence was more frequent and positive interactions in the upper layer. • The pH drove the microbial diversity and community assembly in the stratified AMD lake. Acid mine drainage (AMD) lakes are typical hydrologic features caused by open pit mining and represent extreme ecosystems and environmental challenges. Little is known about microbial distribution and community assembly in AMD lakes, especially in deep layers. Here, we investigated prokaryotic microbial diversity and community assembly along a depth profile in a stratified AMD lake using 16S rRNA gene sequencing combined with multivariate ecological and statistical methods. The water column in the AMD lake exhibited tight geochemical gradients, with more acidic surface water. Coupled with vertical hydrochemical variations, prokaryotic microbial community structure changed significantly, and was accompanied by increased diversity with depth. In the surface water, heterogeneous selection was the most important assembly process, whereas stochastic processes gained importance with depth. Meanwhile, microbial co-occurrences, especially positive interactions, were more frequent in the stressful surface water with reduced network modularity and keystone taxa. The pH was identified as the key driver of microbial diversity and community assembly along the vertical profile based on random forest analysis. Taken together, environmental effects dominated by acid stress drove the community assembly and species coexistence that underpinned the spatial scaling patterns of AMD microbiota in the lake. These findings demonstrate the distinct heterogeneity of local prokaryotic microbial community in AMD lake, and provide new insights into the mechanism to maintain microbial diversity in extreme acidic environments. [Display omitted] [ABSTRACT FROM AUTHOR]

Few sedimentary archives of lake meromixis are available in palaeolimnological records, because long-term observations are limited in time and indisputable sediment proxies of hypolimnetic anoxia are still scarce. Here we use visible and near infrared (VNIR), and short-wave infra-red (SWIR) hyperspectral imaging combined with geochemical analyses to reconstruct lake stratification history, redox status and mixing conditions in the water column of Lake Son Kol (Kyrgyzstan) in Central Tian Shan during the last 8500 years. In particular, the detection of Bacteriopheophytin a (Bphe a), a pigment produced by anoxygenic phototrophic bacteria at the chemocline of meromictic lakes, emphasizes episodes of multi-decadal to centennial hypolimnetic anoxia in Lake Son Kol. Phases of hypolimnetic anoxia are inferred from the deposition of dark organic sediments within a stratified lake system, which occurred during periods of increased snowmelt (and solid winter precipitation) and warmer spring/summer temperatures that promoted floods and the export of terrestrial material from the catchment. Prolonged euxinic conditions in bottom waters (involving the development of a stable chemocline) are reported around 8500, 8400, 8200–7800, 7700–7500, 7300–7000, 6500–6100, 6000–5700 and 5500–5250 cal yr BP. At ca. 5250 cal yr BP, the chemocline abruptly vanished as Lake Son Kol tipped into a regime with predominantly cooler and well-mixed conditions (predominance of oxygenic phototrophs), coeval with higher lake levels. The disappearance of hypolimnetic anoxia in Lake Son Kol coincides with strengthened wind conditions that imply enhanced lake overturning and upward mixing of nutrients in the water column. This study reveals the strong potential of hyperspectral imaging, in combination with more classical palaeolimnological approaches, to reconstruct the lake trophic and mixing history and explore the controlling mechanisms at work on decadal to centennial timescales. Our results outline how abrupt ecosystem changes may occur even in the absence of anthropogenic climate change. • Lake Son Kol is a well-documented paleoclimatic archive in Central Tian Shan. • Hyperspectral imaging reveals Bacteriopheophytin a as a proxy for hypolimnetic anoxia. • Evidence for recurrent centennial-scale periods of chemocline development. • Warmer summers, milder winters and low wind activity during 8500–5250 cal. BP. [ABSTRACT FROM AUTHOR]

Most large temperate lakes experience overturning every spring and fall as surface water moves past 4 °C, the temperature of maximum density for freshwater. These semiannual, lake-wide overturning events play an important role regulating the thermal structure, deep-water ventilation, nutrient supply, water circulation, and nearshore water quality of the lakes. The general pattern of overturning has long been known from field observations and models, but its timing, duration, detailed spatio-temporal progression and seasonal and interannual variability remain largely undocumented, particularly in the context of recent climate-driven changes in lake thermal dynamics. Here, we used a reconstructed record of daily and spatially-explicit lake surface water temperatures (LSWT) to analyze the migration of the 4 °C thermal front as it progressed from the shorelines to the deep parts of the Laurentian Great Lakes during every overturning event between June 1995 to April 2012. The analysis revealed a strong asymmetry in the timing and duration of overturning between spring and fall, and no relationship with the lake-averaged LSWT or its rate of change. Key differences in the average spatio-temporal progression of overturning were also observed between spring and fall, with the spring progression being largely driven by latitude and water depth and the fall progression being less predictable and influenced by other factors such as wind. Narrow regions of very slow overturning progression were also identified, revealing areas of the lakes where persistent 4 °C thermal bars are likely to re-occur every year. The timing and duration of these seasonal overturning events varied between years by as much as one and two months, respectively, with a direct impact on the duration of lake-wide stratification. In 2012, Lakes Michigan and Ontario experienced an incomplete fall overturning, leading only to a partial winter stratification. Lakes Michigan and Ontario were more susceptible to experience an incomplete overturning than the other Laurentian Great Lakes, seemingly due to a combination of comparatively milder winter air temperatures and lower lake dynamic ratio (steepness of bottom slope). Overall, the duration of lake-wide winter stratification was found to be strongly correlated with mean winter air temperatures, and a simple trend analysis suggested that rising temperatures could lead to more frequent incomplete fall overturnings and partial winter stratifications in Lakes Michigan and Ontario over the next few decades. This study demonstrated that remote sensing provides an unparalleled tool for assessing the long-term variability in the overturning behavior of large lakes in the context of climate change. • Remotely sensed 4 °C isotherm facilitated tracking overturning progression in lakes. • Analysis revealed average spatio-temporal progression of overturning in Great Lakes. • Identified narrow regions of slow overturning progression (recurrent thermal bars). • Revealed large seasonal and interannual variability in overturning timing/duration. • Rising air temperatures might affect overturning behavior of lakes Michigan/Ontario. [ABSTRACT FROM AUTHOR]