Ice-covered Lakes Research Articles

Climate Change-Driven Regime Shifts in a Planktonic Food Web.

AbstractPredicting how food webs will respond to global environmental change is difficult because of the complex interplay between the abiotic forcing and biotic interactions. Mechanistic models of species interactions in seasonal environments can help understand the effects of global change in different ecosystems. Seasonally ice-covered lakes are warming faster than many other ecosystems and undergoing pronounced food web changes, making the need to forecast those changes especially urgent. Using a seasonally forced food web model with a generalist zooplankton grazer and competing cold-adapted winter and warm-adapted summer phytoplankton, we show that with declining ice cover, the food web moves through different dynamic regimes, from annual to biennial cycles, with decreasing and then disappearing winter phytoplankton blooms and a shift of maximum biomass to summer season. Interestingly, when predator-prey interactions were not included, a declining ice cover did not cause regime shifts, suggesting that both are needed for regime transitions. A cluster analysis of long-term data from Lake Baikal, Siberia, supports the model results, revealing a change from regularly occurring winter blooms of endemic diatoms to less frequent winter bloom years with decreasing ice cover. Together, the results show that even gradual environmental change, such as declining ice cover duration, may cause discontinuous or abrupt transitions between dynamic regimes in food webs.

Effects of climate change and industrialization on Lake Bolshoe Toko, eastern Siberia

Industrialization in the Northern Hemisphere has led to warming and pollution of natural ecosystems. We used paleolimnological methods to explore whether recent climate change and/or pollution had affected a very remote lake ecosystem, i.e. one without nearby direct human influence. We compared sediment samples that date from before and after the onset of industrialization in the mid-nineteenth century, from four short cores taken at water depths between 12.1 and 68.3 m in Lake Bolshoe Toko, eastern Siberia. We analyzed diatom assemblage changes, including diversity estimates, in all four cores and geochemical changes (mercury, nitrogen, organic carbon) from one core taken at an intermediate water depth. Chronologies for two cores were established using 210Pb and 137Cs. Sedimentation rates were 0.018 and 0.033 cm year−1 at the shallow- and deep-water sites, respectively. We discovered an increase in light planktonic diatoms (Cyclotella) and a decrease in heavily silicified euplanktonic Aulacoseira through time at deep-water sites, related to more recent warmer air temperatures and shorter periods of lake-ice cover, which led to pronounced thermal stratification. Diatom beta diversity in shallow-water communities changed significantly because of the development of new habitats associated with macrophyte growth. Mercury concentrations increased by a factor of 1.6 since the mid-nineteenth century as a result of atmospheric fallout. Recent increases in the chrysophyte Mallomonas in all cores suggested an acidification trend. We conclude that even remote boreal lakes are susceptible to the effects of climate change and human-induced pollution.

The Influence of Snow and Ice Albedo towards Improved Lake Ice Simulations

Lake ice models are a vital tool for studying the response of ice-covered lakes to changing climates throughout the world. The Canadian Lake Ice Model (CLIMo) is a one-dimensional freshwater ice cover model that simulates Arctic and sub-Arctic lake ice cover well. Modelling ice cover in temperate regions has presented challenges due to the differences in ice composition between northern and temperate region lake ice. This study presents a comparison of measured and modelled ice regimes, with a focus on refining CLIMo for temperate regions. The study sites include two temperate region lakes (MacDonald Lake and Clear Lake, Central Ontario) and two High Arctic lakes (Resolute Lake and Small Lake, Nunavut) where climate and ice cover information have been recorded over three seasons. The ice cover simulations were validated with a combination of time lapse imagery, field measurements of snow depth, snow density, ice thickness and albedo data, and historical ice records from the Canadian Ice Database (for Resolute Lake). Simulations of High Arctic lake ice cover show good agreement with previous studies for ice-on and ice-off dates (MAE 6 to 8 days). Unadjusted simulations for the temperate region lakes show good ice-on timing, but an under-representation of ice thickness, and earlier complete ice-off timing (~3 to 5 weeks). Field measurements were used to adjust the albedo values used in CLIMo, which resulted in improvements to both simulated ice thickness (~3 cm MAE compared to manual measurements), and ice-off timing, within 0 to 7 days (2 days MAE) of observations. These findings suggest regionally specific measurements of albedo can improve the accuracy of lake ice simulations, which further our knowledge of the response of temperate and High Arctic lake ice regimes to climate conditions.

The magnitude and climate sensitivity of isotopic fractionation from ablation of Antarctic Dry Valley lakes

ABSTRACT There has been extensive research on the effects of evaporation on the isotopic ratio of lacustrine and marine water bodies; however, there are limited data on how ablation or sublimation from lake or sea ice influences the isotopic ratio of the residual water body. This is a challenging problem because there remains uncertainty on the magnitude of fractionation during sublimation and because ablation can involve mixed-phase processes associated with simultaneous sublimation, melting, evaporation, and refreezing. This uncertainty limits the ability to draw quantitative inferences on changing hydrological budgets from stable isotope records in arctic, Antarctic, and alpine lakes. Here, we use in situ measurements of the isotopic ratio of water vapor along with the gradient diffusion method to constrain the isotopic ratio of the ablating ice from two lakes in the McMurdo Dry Valleys, Antarctica. We find that during austral summer, the isotopic fractionation of ablation was insignificant during periods of boundary layer instability that are typical during midday when latent heat is highest. This implies that the loss of mass during these periods did not yield any isotopic enrichment to the residual lake mass. However, fractionation increased after midday when the boundary layer stabilized and the latent heat flux was small. This diurnal pattern was mirrored on synoptic timescales, when following warm and stable conditions latent heat flux was low and dominated by higher fractionation for a few days. We hypothesize that the shifting from negligible to large isotopic fractionation reflects the development and subsequent exhaustion of liquid water on the surface. The results illustrate the complex and nonlinear controls on isotopic fractionation from icy lakes, which implies that the isotopic enrichment from ablation could vary significantly over timescales relevant for changing lake volumes. Future work using water isotope fluxes for longer periods of time and over additional perennial and seasonal ice-covered lake systems is critical for developing models of the isotopic mass balance of arctic and Antarctic lake systems.

ОПЫТ ПРИМЕНЕНИЯ ВЫСОКОЧАСТОТНОГО ГЕОРАДАРА ДЛЯ ЛАНДШАФТНОЙ СНЕГОМЕРНОЙ СЪЁМКИ В ОКРЕСТНОСТЯХ ГОРОДОВ КИРОВСК (ХИБИНЫ) И АПАТИТЫ

The results of processing of a profile snow-measuring survey of snow cover in the Khibiny Mountains are presented. The survey was performed during the period of maximum snow accumulation (March of 2020) on the main elements of the landscape: mixed forest on the plain, open woodlands at the bottom of valleys, plateaus, wooded slopes, and upper slopes without woody vegetation. The averaged values of snow storage for different types of the landscapes were obtained for the period of the maximum snow accumulation in the snowy winter of 2019/20. The maximum snow storage (> 700 mm w.e.) was determined for areas on the high plateaus and open woodlands at the bottom of valleys. Minimum snow storage (> 400 mm w.e.) was recorded in areas of mixed forest on the plain and on an ice cover of lakes. Measurements of snow depth were carried out by the standard method (a handspike) and the ground-based radio-echo sounding using georadar with the frequency of 1600 MHz. The accuracy of this method allows measuring of the snow depth with accuracy of 1 cm for a dense snow and 2 cm for a loose one. Thus, the accuracy of measuring the snow depth with the radar is comparable to the accuracy of a handspike. A large number of radar measurements of snow depth on the profiles makes possible to determine the spatial variability of this value and its statistical characteristics. As a result, a vertical gradient of snow accumulation was defined as 25 mm w.e. per 100 m. The smallest spatial variability of snow depth was observed on profiles in the forests on the plain, in woodlands, and on the upper slopes. On profiles with complex relief (plateau, lower slopes), the spatial variability of snow depth is significant – the standard deviation was within limits of 30%. Based on the results of processing the field data, a map of snow storage over the studying area during the period of maximum snow accumulation was constructed. When constructing the map, we took into account the averaged data of the measurements for each type of landscape, the boundaries of woody vegetation, the height, steepness of slopes, and the high-altitude gradient of snow accumulation. It was found that features of the spatial distribution of snow cover were primarily due to the location of natural landscape complexes. The role of changes in snow storages with altitude was found to be insignificant.

CIRCADIAN VARIABILITY OF DISSOLVED OXYGEN CONTENT AS AN INDICATOR OF THE METABOLISM OF AN ICE COVERED LAKE ECOSYSTEM

Circadian variability of dissolved oxygen content is used as an indicator of the ecosystem metabolism in the small ice-covered Lake Vendyurskoe (southern Karelia). In the endof the ice season, before the onset of the spring under-ice convection, the vertical oxygen profile reflected its consumption due to organic matter destruction, which was the most intensive near the bottom. The anoxic zone was 1.5 m thick in the central deep-water part of the lake, and the dissolved oxygen concentration was 9–10.5 mg/L in the surface layer. As under-ice convection in March and April 2020 developed against the background of an increase in under-ice irradiation (up to 100 W/m2 in the daytime), dissolved oxygen oscillations with a circadian period and an amplitude of up to 0.5–0.9 mg/ L (with a minimum at night and a maximum during the daytime) were found within the convective layer. According to the working hypothesis, the decrease in dissolved oxygen concentration was due to respiration of aquatic organisms and destruction of organic matter, while the increase was due to intensification of photosynthesis of algae during the daytime. The concentration of chlorophyll “a” was the highest (up to 2.5 μg/L) in the 0.5 m under-ice water layer, reaching 1 μg/L in the convective layer, and declining to 0.1–0.3 μg/L below this layer. In the time interval from the 20s of March to the end of April, the thickness of the convective layer increased to 7.5 m, and oxygen concentration decreased by ~0.5 mg/L due to the entrainment of oxygen-depleted underlying waters. During the same period, the increase in the oxygen content of the whole water column was about 10 %, presumably due to photosynthesis of algae.

Fasting or feeding: A planktonic food web under lake ice

Abstract Zooplankton can spend winter actively under the ice cover of lakes. However, dietary resources under lake ice are both quantitatively and qualitatively limited, and feeding might not be energetically rewarding for most zooplankton species. Many zooplankters are expected to fast throughout the winter, exhausting their previously accumulated fat storage. We hypothesised that only a fraction of the actively overwintering zooplankton contributes to an active food web under lake ice, leading to few trophic linkages within the planktonic community. Zooplankton habitats and feeding were investigated under the ice of Lake Onego. Zooplankton habitats and migrations were studied by coupling zooplankton sampling around the clock to measurements of particle movement using an acoustic Doppler current profiler. Secondly, fatty acid‐specific stable isotope compositions were used to determine whether and which zooplankton fatty acids ultimately came from the assimilation of under‐ice seston. The algal biomass was low under ice and mostly dominated by large diatoms. Copepods dominated the zooplankton community. Species present as late copepodite and adult instars were confined to the deeper layers, while nauplii occupied the surface layer. Diel vertical migration by Cyclops was the most tangible observation of persistent feeding under the ice. Previously accumulated fat storage represented most of zooplankton fatty acids, with few, yet detectable, fatty acids recently acquired by feeding under the ice. Although some zooplankton taxa maintained feeding activity under the ice of Lake Onego, the food source available beneath the ice was not sufficiently rewarding to leave an isotopic imprint upon the dominant fatty acids of bulk zooplankton. The seston fatty acids that were passed on to zooplankton from feeding under ice were not provided by diatoms, although they made up most of the phytoplankton biovolume. Instead, the zooplankton food web was supported by mixotrophic phytoplankton (i.e. cryptophytes and chrysophytes) that represented <5% of the under‐ice biovolume. Consequently, the planktonic food web under the ice of Lake Onego had few trophic linkages, and thereby low connectance. Environmental conditions under the ice of Lake Onego do not depart significantly from those observed in lakes of similar latitudes. Therefore, low connectance food webs could be relatively common under ice for lakes above 60° latitude. Our methodological approach is applicable to other lakes and could thus disclose the variability of under‐ice food webs. This would provide a much more complete picture of annual dynamics of food webs in lakes that ice over.

Lake Level Change From Satellite Altimetry Over Seasonally Ice-Covered Lakes in the Mackenzie River Basin

Variations in water levels of seasonally ice-covered subarctic lakes are indicators of environmental and climatic change. Satellite altimetry enables remote sensing of these lakes, but the lake phenology is problematic as radar reflection surfaces include water, snow, and ice. Reflection from multiple surfaces gives rise to two-peak waveforms across ice-covered lakes. Misinterpretation of the altimetric height has caused extracted water levels to be low compared with gauge data. In this study, a modified retracker is used to determine heights from the first altimetric subwaveform. Using <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> snow depth and ice thickness, the first reflection surface is shown to correspond closely to the snow/ice interface when the lake is frozen. The modified retracker is applied to the Great Bear Lake (GBL), Great Slave Lake (GSL), and Lake Athabasca (ATL) of the Mackenzie River Basin for the period 1992–2020. Standard deviations (Std) of differences between lake levels from Jason-2 waveforms and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> data across GBL and GSL are 0.06 m with the new methodology compared with 0.11 and 0.08 m, respectively, using the standard Ice retracker. With an Std of 0.11 m between altimetric and gauge lake levels, TOPEX/Poseidon is less accurate than the combined Jason missions (Std: 0.07 m).

Microbial Diversity of Pinnacle and Conical Microbial Mats in the Perennially Ice-Covered Lake Untersee, East Antarctica.

Antarctic perennially ice-covered lakes provide a stable low-disturbance environment where complex microbially mediated structures can grow. Lake Untersee, an ultra-oligotrophic lake in East Antarctica, has the lake floor covered in benthic microbial mat communities, where laminated organo-sedimentary structures form with three distinct, sympatric morphologies: small, elongated cuspate pinnacles, large complex cones and flat mats. We examined the diversity of prokaryotes and eukaryotes in pinnacles, cones and flat microbial mats using high-throughput sequencing of 16S and 18S rRNA genes and assessed how microbial composition may underpin the formation of these distinct macroscopic mat morphologies under the same environmental conditions. Our analysis identified distinct clustering of microbial communities according to mat morphology. The prokaryotic communities were dominated by Cyanobacteria, Proteobacteria, Verrucomicrobia, Planctomycetes, and Actinobacteria. While filamentous Tychonema cyanobacteria were common in all mat types, Leptolyngbya showed an increased relative abundance in the pinnacle structures only. Our study provides the first report of the eukaryotic community structure of Lake Untersee benthic mats, which was dominated by Ciliophora, Chlorophyta, Fungi, Cercozoa, and Discicristata. The eukaryote richness was lower than for prokaryote assemblages and no distinct clustering was observed between mat morphologies. These findings suggest that cyanobacterial assemblages and potentially other bacteria and eukaryotes may influence structure morphogenesis, allowing distinct structures to form across a small spatial scale.

Assessment of machine learning classifiers for global lake ice cover mapping from MODIS TOA reflectance data

The topic of satellite remote sensing of lake ice has gained considerable attention in recent years. Optical satellite data from the Moderate Resolution Imaging Spectroradiometer (MODIS) allow for the monitoring of lake ice cover (an Essential Climate Variable or ECV), and dates associated with ice phenology (freeze-up, break-up, and ice cover duration) over large areas in an era where ground-based observational networks have nearly vanished in many northern countries. Ice phenology dates as well as dates of maximum and minimum ice cover extent (for lakes that do not form a complete ice cover in winter or do not totally lose their ice cover in summer) are useful for assessing long-term trends and variability in climate, particularly due to their sensitivity to changes in near-surface air temperature. Existing knowledge-driven (threshold-based) retrieval algorithms for lake ice cover mapping that use top-of-atmosphere (TOA) reflectance products do not perform well under lower solar illumination conditions (i.e. large solar zenith angles), resulting in low TOA reflectance. This research assessed the capability of four machine learning classifiers (i.e. multinomial logistic regression, MLR; support vector machine, SVM; random forest, RF; gradient boosting trees, GBT) for mapping lake ice cover, water and cloud cover during both break-up and freeze-up periods using the MODIS/Terra L1B TOA (MOD02) product. The classifiers were trained and validated using samples collected from 17 large lakes across the Northern Hemisphere (Europe and North America); lakes that represent different characteristics with regards to area, latitude, freezing frequency, and ice duration. Following an accuracy assessment using random k-fold cross-validation (k = 100), all machine learning classifiers using a 7-band combination (visible, near-infrared and shortwave-infrared) were found to be able to produce overall classification accuracies above 94%. Both RF and GBT provided overall and class-specific accuracies above 98% and a more visually accurate depiction of lake ice, water and cloud cover. The two tree-based classifiers offered the most robust spatial transferability over the 17 lakes and performed consistently well across ice seasons. However, only RF was relatively insensitive to the choice of the hyperparameters compared to the other three classifiers. The results demonstrate the potential of RF for mapping lake ice cover globally from MODIS TOA reflectance data.

Mass and Heat Balance of a Lake Ice Cover in the Central Asian Arid Climate Zone

To improve the understanding of the seasonal evolution of the mass and heat budget of ice-covered lakes in the cold and arid climate zone, in-situ observations were collected during two winters (2016–2017 and 2017–2018) in Lake Wuliangsuhai, Inner Mongolia, China. The mean snow thickness was 5.2 and 1.6 cm in these winters, due to low winter precipitation. The mean ice thickness was 50.9 and 36.1 cm, and the ice growth rate was 3.6 and 2.1 mm day−1 at the lower boundary of ice. Analyses of mass and heat balance data from two winters revealed that the surface heat budget was governed by solar radiation and terrestrial radiation. The net heat flux loss of the ice was 9–22 W m−2, affected by the snow and ice thickness. Compared to boreal lakes, Lake Wuliangsuhai received more solar radiation and heat flux from the water. The ice temperature had a strong diurnal variation, which was produced by the diurnal cycles of solar radiation, and air and water temperatures. These results expand our knowledge of the evolution of mass and heat balance in temperate lakes of mid-latitude arid areas.

Conditions of spatiotemporal variability of the thickness of the ice cover on lakes in the Tatra Mountains

This research aimed to identify the impact of local climatic and topographic conditions on the formation and development of the ice cover in high-mountain lakes and the representativeness assessment of periodic point measurements of the ice cover thickness by taking into consideration the role of the avalanches on the icing of the lakes. Field works included measurement of the ice and snow cover thickness of seven lakes situated in the Tatra Mountains (UNESCO biosphere reserve) at the beginning and the end of the 2017/2018 winter season. In addition, morphometric, topographic and daily meteorological data of lakes from local IMGW (Polish Institute of Meteorology and Water Management) stations and satellite images were used. The obtained results enabled us to quantify the impact of the winter eolian snow accumulation on the variation in ice thickness. This variation was ranging from several centimetres up to about 2 meters and had a tendency to increase during the winter season. The thickest ice covers occurred in the most shaded places in the direct vicinity of rock walls. The obtained results confirm a dominating role of the snow cover in the variation of the ice thickness within individual lakes.

How Did the Late 1980s Climate Regime Shift Affect Temperature-Sensitive Fish Population Dynamics: Case Study of Vendace (Coregonus albula) in a Large North-Temperate Lake

The population dynamics of fish in northern lakes is strongly influenced by climatic factors. In this study, we investigated whether there is a link between the late 1980s climate regime shift in Europe and the collapse of vendace (Coregonus albula) population at the same time in Lake Peipsi. Until the end of the 1980s, vendace was very abundant in the lake, but then its catches sharply declined. This decline inspired investigations into the extreme weather events preceding the vendace collapse using data on daily water temperatures and ice phenology together with commercial fishery statistics since 1931 and test catch data since 1986. We identified using advanced statistical methods that the hot summer of 1988, which was accompanied by a severe cyanobacterial bloom and extensive fish kill, and the subsequent non-permanent ice cover and early ice-offs in 1989 and 1990 in Lake Peipsi were the main reasons for the disappearance of vendace from catches in 1991. Moreover, a negative correlation appeared between catches of the predatory pikeperch (Sander lucioperca) and vendace. Predation pressure as well as fish habitat degradation caused by lake eutrophication may contribute to the instability of the vendace population too. Our study showed that extreme weather events such as heat waves in summer and non-permanent ice-cover in winter in consecutive years may have long-lasting harmful effects on the population abundance of cool-water fish species such as vendace whose eggs usually develop under an ice cover in north-temperate lakes.

The littoral zone of polar lakes: inshore–offshore contrasts in an ice-covered High Arctic lake

In ice-covered polar lakes, a narrow ice-free moat opens up in spring or early summer, and then persists at the edge of the lake until complete ice loss or refreezing. In this study, we analyzed the horizontal gradients in Ward Hunt Lake, located in the Canadian High Arctic, and addressed the hypothesis that the transition from its nearshore open-water moat to offshore ice-covered waters is marked by discontinuous shifts in limnological properties. Consistent with this hypothesis, we observed an abrupt increase in below-ice concentrations of chlorophyll a beyond the ice margin, along with a sharp decrease in temperature and light availability and pronounced changes in benthic algal pigments and fatty acids. There were higher concentrations of rotifers and lower concentrations of viruses at the ice-free sampling sites, and contrasts in zooplankton fatty acid profiles that implied a greater importance of benthic phototrophs in their inshore diet. The observed patterns underscore the structuring role of ice cover in polar lakes. These ecosystems do not conform to the traditional definitions of littoral versus pelagic zones but instead may have distinct moat, ice-margin, and ice-covered zones. This zonation is likely to weaken with ongoing climate change.

Simulation and Seasonal Characteristics of the Intra-Annual Heat Exchange Process in a Shallow Ice-Covered Lake

The intra-annual heat exchange process has a considerable influence on the energy circulation, material metabolism, and ecological succession of lakes. The input and output of heat in an ice-covered lake provide the basic dynamic force driving changes in the biochemical state of the lake. Based on the heat balance between the lake surface and the atmosphere, we established a thermodynamic model for calculating the thermodynamic factors of shallow inland lakes during the ice and open seasons. The data of the Ulansuhai Lake, Inner Mongolia, from two years (2012 and 2013) are used to analyze the seasonal characteristics and associated influences of the heat budget on the ecosystem. The results indicated that the monthly mean lake temperature over the past 10 years was 1.7–2.2 °C lower than in the previous 50 years. The absorbed solar radiation reached up to 210 W/m2 in 2012 and 179 W/m2 in 2013, and there were clear differences in the heat budget between the ice-covered and open seasons. The mean net heat fluxes in the ice season were −33.8 and −38.5 W/m2 in 2012 and 2013, respectively; while in the open season water, these fluxes were 62.5 and 19.1 W/m2. In the simulations, the wind was an important factor for intensive evaporation in summer and the main driver of the ice cover formation patterns in winter, involving the transmission and diffusion of material and energy in the lake. The results provide a theoretical foundation for simulating ice cover growth and ablation processes in shallow lakes. They also present data on the ecological evolution in these lacustrine environments.

Climate Change Drives Increases in Extreme Events for Lake Ice in the Northern Hemisphere

AbstractExtreme climate events can have significant consequences on ecosystems and by extension human populations. Over 50 million of the world's lakes typically freeze each winter, and the absence of winter ice cover, in lakes where ice has historically been present, can be characterized as an extreme event. We quantified the effects of extreme climate events on lake ice cover using 78‐year ice records from 122 lakes to show that (1) extreme ice‐free years are becoming more frequent and severe, (2) winter air temperature is a significant predictor of ice cover that was driven by large‐scale climate oscillations, (3) extremes in temperature are closely related to extremes in ice cover, and (4) ice‐free years are forecasted to result in significant loss of ice‐cover in the future. Without drastic reductions in carbon emissions, we can expect the widespread loss of lake ice cover could have significant socioeconomic and biological implications.

Variability of chlorophyll and the influence factors during winter in seasonally ice-covered lakes

Lake ice is an essential and integral part of the cryosphere and freshwater systems. The formation of lake ice affects the physical, hydrological, and biological conditions of ecological systems. Global warming may contribute to even shorter periods of ice cover in the lakes of the Frigid Zone, which adversely affects the growth of phytoplankton and primary productivity. This study was conducted for the purpose of evaluating the growth of phytoplankton and factors involved, in 28 ice-covered lakes across the Songnen Plain, in the Northeast of China, to understand how they take part in the whole-ecosystem functioning. A total of 1026 water samples were collected in April, September, and January during the period 2014–2018. In the frozen period, the concentration levels of dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorus (TP) were all comparable with the spring and autumn. Despite the limited light availability and low temperature, the phytoplankton survived in sub-ice waters during winter with a low concentration of chlorophyll a (Chla). Its average concentration was positively correlated with the concentration observed in the previous autumn (rp = 0.563, p < 0.01). According to the regression tree analysis, during the winter period, Chla was mainly related to the concentration of TN in sub-ice water (TNwater) and with the difference of concentration of TP between water and ice (TPcd). Furthermore, either in ice or in sub-ice water, the concentration of Chla was also significantly affected by total suspended matter (TSM) (p < 0.05). The levels of TNwater, TPcd, and TSM could explain the 77.8% of the variance in the concentration of Chla during winter with contributions in the ranges of 25.5%–35.0%, 9.2%–11.3%, and 21.5%–34.0%, respectively (p < 0.05). This research substantially contributes to comprehending how the existing conditions under-ice affect the whole ecosystem when the ice cover is reduced lakes or rivers.

Environmental and climatic factors affecting winter hypoxia in a freshwater lake: evidence for a hypoxia refuge and for re-oxygenation prior to spring ice loss

Low dissolved oxygen, or hypoxia, is a common phenomenon in ice-covered lakes in winter. We measured dissolved oxygen (DO) before, during, and after ice-over to characterize the timing, severity, and spatial variability of winter hypoxia in Upper Red Rock Lake, Montana, home to one of the last remaining lacustrine populations of endemic Montana Arctic Grayling (Thymallus arcticus). Unlike most previous investigations of winterkill-prone lakes, we observed considerable horizontal spatial variability in DO, a non-linear winter oxygen depletion rate, and lake-wide re-oxygenation 2–4 weeks prior to spring ice loss. Parts of the upper 1 m of the lake and near stream mouths remained well-oxygenated even during late winter. DO levels were strongly associated with maximum daily air temperature. Our analysis of a 28-year weather record revealed large interannual variability in risk of winter hypoxia, with a slight declining trend in winter severity (number of days with maximum air temperatures ≤ 0°C) in Upper Red Rock Lake. The approach we used in our study provides a useful framework for quantifying and mapping the seasonal dynamics of the extent and severity of winter hypoxia, and for identifying critical winter habitats.

Solar radiation transfer for an ice-covered lake in the central Asian arid climate zone

ABSTRACT Spectral albedo and light transmittance through snow, ice, and water were measured in Lake Wuliangsuhai (40°36′–41°30′N, 108°43′–108°70′E), Inner Mongolia, China, during winter 2016. Data on the weather, structure of lake ice, and geochemistry of water were also collected during the 60-day field program. The study lake is shallow (mean depth 1.0–1.5 m) with a large wetland area. Compared with polar lakes, solar elevation is higher, snow accumulation is much lower, and the ice has more sediment. The ice was all congelation ice with a mean thickness of 36.6 cm, corresponding to a mean air temperature of −9.6 °C. The mean daily broadband albedo and photosynthetically active radiation (PAR) band transmittance were 0.54 and 0.08 (bare ice), 0.74 and 0.04 (new snow), and 0.30 and 0.12 (melting period), respectively. The level of light allowed photosynthesis to occur to the bottom of the lake. The ice acted as a grey filter for the sunlight with a mean attenuation coefficient of 2.1 m−1. These results expand our knowledge of the evolution of light transfer through ice and snow cover and its role in the ecology of lakes in temperate and arid areas.

Resonance Generation of Short Internal Waves by the Barotropic Seiches in an Ice-Covered Shallow Lake

Purpose. The observation measurements testify the fact that heat and mass transfer processes in the shallow ice-covered lakes are not limited to the molecular diffusion only. In particular, the effective thermal diffusivity exceeds the molecular one by up to a few orders of magnitude. Now it is widely accepted that the transfer processes, in spite of their low intensity, are controlled by intermittent turbulence. At the same time, its nature and generation mechanism are still studied insufficiently. The paper represents one of such mechanisms associated with resonance generation of short internal waves by the barotropic seiches. Methods and Results. The temperature measurements in a shallow lake in winter were used as an experimental base. Having been analyzed, the temperature profiles’ dynamics observed during a few weeks after freezing revealed the anomalous values of thermal diffusivity. At that the temperature pulsations’ spectra clearly demonstrate the peak close to the main mode of barotropic seiches. Counter-phase oscillations at the different depths and pronounced heterogeneity of the amplitudes of temperature pulsations over depth indicate presence of internal waves. Based on these data, the mechanism of energy transfer from the barotropic seiches to the internal waves similar to the “tidal conversion” (the latter governs resonance generation of internal tides in the ocean), is proposed. The expressions for heat flux, energy dissipation rate and effective thermal diffusivity are derived. Conclusions. Internal waves can play an essential role in the processes of interior mixing and heat transfer in the ice-covered lakes. Though direct wind-induced turbulence production is inhibited, baric perturbations in the atmosphere can give rise to barotropic seiches, which play the role of an intermediate energy reservoir and can generate short resonant internal waves resulted from interaction with the undulate lake floor. The internal wave field parameters strongly depend on the barotropic seiche amplitudes, buoyancy frequency and the bottom topography features.