Temperate aquatic ecosystems with seasonal ice-cover are increasingly subjected to stressors through global warming and reduced ice duration. Chlorophyll a (Chl-a), as a proxy of photosynthesis and ecosystem function, is heavily impacted by these stressors. However, studies on dynamics of Chl-a under ice was lack coverage of the ice-melting period due to sampling safety consideration, despite the melting period being a crucial link between the winter and spring. We addressed this gap using an autonomous floating platform equipped with sensors to record high-frequency data. The dataset (comprising 9188 automatically monitored samples and 310 manual samples) captures the temporal variations of Chl-a and its associated factors in a temperate coastal brackish lake, an area that remains understudied. Causality analysis of time series was used to determine the key drivers governing Chl-a dynamics. The results revealed high Chl-a concentration, with an average of (14.12 +/- 5.30) mu g/L in Hanzhang Lake, China. Chl-a experienced a rapid increase during the ice-melting period. Light was identified as the primary driver influencing other factors, ultimately affecting Chl-a. Path analysis revealed that light availability directly influenced the dynamics of Chl-a, and indirectly influenced it by warming the water and altering nutrient supply under ice. These observations enhance our understanding of the interaction between winter and spring ecological processes. Under global warming, shorter ice duration increases irradiance and elevates spring water temperatures, which may impact the ecosystems in temperate coastal areas.

Abstract Aquatic systems exchange large amounts of carbon dioxide (CO2) and methane (CH4) with the atmosphere. The microbubble hypothesis suggests that microbubbles preferentially enhance air−water transfer of sparingly soluble gases like CH4 over CO2, explaining elevated normalized gas transfer velocity ratios (k600,CH4:k600,CO2) observed in many inland waters. This hypothesis remains to be empirically tested. We tested this hypothesis in meso-oligotrophic Lake Stechlin (NE Germany) using acoustic bubble spectroscopy to quantify microbubble concentrations, floating chambers to measure CH4 and CO2 fluxes, and dissolved gas concentrations to estimate k600 for CH4 and CO2. Contrary to our hypothesis, k600,CH4:k600,CO2 averaged near unity and was independent of microbubble concentration. Theoretical microbubble-mediated CH4 fluxes contributed ∼0.05% to total CH4 fluxes. Thus, microbubbles affected gas transfer velocities weakly under the relatively low-wind, posteutrophication conditions of our sampling campaign. Instead, k600,CH4:k600,CO2 was more likely explained by gas-specific near-surface concentration gradients, surfactant-induced surface immobilization, and chemical enhancement of CO2 exchange. Microbubble concentrations were mainly linked to wind-induced turbulence and were elevated in littoral areas. Our results limit the applicability of the microbubble hypothesis and suggest that larger microbubble gas fluxes likely require stronger wind-induced forcing than observed here, or site-specific sources, such as littoral vegetation.

Lake ice phenology in Northeast China mediates regional climate interactions and sustains winter tourism and fisheries. However, sparse long-term observations limit our understanding of ice cover trends under climate warming. Here, we combine a one-dimensional thermodynamic lake model with machine-learning residual correction and train and validate the model with satellite-derived ice phenology data from 32 lakes during 2001–2014 to simulate lake ice phenology from 1901–2100 through historical hindcasts and CMIP6 projections (SSP126, SSP370, and SSP585). Results show a marked shift toward later freeze-up, earlier break-up, and shorter ice duration beginning in the 1970s. Across the three scenarios, regional air temperatures are projected to increase by 1.2–8.1 °C century−1, whereas the mean ice duration decreases by 13.6–49.8 d century−1. Under SSP585, historically extreme conditions are projected to become the new phenological state, implying increasing pressure on ecosystem management and seasonal planning for winter economic activities.

Climate change increases the magnitude and frequency of extreme weather events. This includes severe summer storms altering lake physical structure, biodiversity and ecosystem processes. However, insights into lake responses to extreme storms and the underlying mechanisms primarily rest on unreplicated and observational case studies, without separating effects of physical forcing from secondary drivers such as external nutrient and dissolved organic matter inputs. In a large-scale replicated experiment conducted in a unique enclosure facility mimicking realistic environmental conditions, we tested how storm-induced mixing entails changes in lake ecosystems. Consequences include altered phytoplankton composition, nutrient, oxygen and carbon dynamics, with potential negative feedbacks on climate through organic matter sequestration. These experimental results are reflected in a minimal dynamical model and are also supported by observations made during a natural severe storm. An important practical implication is that efforts to abate lake eutrophication needs to accommodate the projected increases in extreme weather situations.

Lakes store large quantities of carbon in their sediments, contributing to climate regulation. Yet the fate of this carbon after lake desiccation remains unclear. Using a space-for-time substitution approach, combining sediment cores, carbon dioxide flux measurements, and remote sensing, we quantified organic carbon losses from the world's largest desiccated lake, the Aral Sea. Since 1960, exposed lake bed sediments have released 204 ± 53 teragrams of carbon (Tg C), with vegetation growth offsetting less than 1%. Incorporating these emissions alters the regional carbon budget, transforming the Aral Sea basin from a presumed land-use-change carbon sink into a net source. Reflooding the sea could prevent an additional 165 ± 13 Tg C release, reframing restoration not only as an ecological and humanitarian imperative but also as a climate mitigation opportunity.

Density stratification is a distinctive feature of lakes characterized by a less dense layer (epilimnion) on top of a denser water (hypolimnion) separated by a strong density jump (pycnocline) between them. While the main driver of thermal lake stratification is temperature, this phenomenon changes the vertical particles distribution, which in turn may affect lake stratification, when suspended particles (including both non-organic and organic ones) cause an overall increase in water column density. Sinking of particles to denser layers changes the sinking rates and may produce particle accumulation at the density interface (pycnocline) having important consequences for organic matter turnover. To investigate the interaction of the physical water properties and distribution of particles as a consequence of the stratification, we used in this study the particle tracking system UVP 6 (Under Vision Profiler) for particle characterization in a stratified Lake Stechlin, Berlin, Germany. The preliminary results, as expected, show that the particle abundance changes in concordance with temperature, which proves the dependency of particle characteristics (size and concentration) on the vertical temperature distribution.

The effects of freshwater river runoff on dynamics of ice-covered brackish lakes have not been adequately studied to date. Compared to freshwater lakes, the circulation patterns in brackish lakes are complicated by non-linear effects of temperature and salinity on density stratification and mixing, and as a result on the ice melt. Quantifying these effects is essential for understanding circulation of large endorheic lakes in cold regions and their ecological and physical characteristics. We present modeling results on circulation caused by river runoff in a typical ice-covered brackish lake obtained with the Regional Ocean Modeling System (ROMS). The lake water salinity was set to 14 practical salinity units (PSU). In the initial state, the water temperature increased linearly from the freezing point at the surface to the temperature of maximum density, at the bottom, both accounting for the water salinity. Mixing of cold freshwater river inflow with the warmer saline waters produces negative buoyancy and downslope flow of dense currents near the river inlets with a secondary geostrophically-balanced circulation throughout the lake. We use the modeling results to quantify the contribution of this circulation mechanism on deep lake circulation and ventilation of the near-bottom waters.

When a solid inclined surface is submerged in a quiescent stratified fluid, the combined effects of buoyancy forces and diffusion generate an upward gravity flow along the slope. Thermally stratified ice-covered lakes remain in a nearly quiescent state and are potentially prone to this effect. We use three-dimensional hydrodynamic modeling to investigate the diffusion-gravity flow and its impact on lake-wide circulation in idealized ice-covered lakes. The qualitative characteristics of the boundary flow were adequately simulated by the model, supported by a good agreement with theoretical predictions. In enclosed lakes, the modeled diffusion-driven boundary flow generates residual circulation, which overturns the entire lake water column within 1 to 6 months, suggesting a significant contribution of this mechanism to heat and mass transport in lakes with long ice-covered seasons. When the insulation boundary condition is lifted and additional buoyancy is produced by heat flux from lake sediment, a counterflow emerges, resulting in a circulation pattern characterized by the superposition of two opposing boundary flows. At flux magnitudes exceeding one watt per square meter, the counterflow can entirely replace the diffusion-driven circulation. Due to the small magnitudes of these flows, the Coriolis effect substantially influences circulation, partially transforming radial flow into rotational lake-wide "gyres." The number and rotational direction of these gyres depend on the relative contribution of bottom heat flux. The results provide a framework for designing field studies in real lakes and investigating circulation effects on the transport of dissolved matter, such as nutrients, oxygen and greenhouse gases in ice-covered lakes.

The rate of technological innovation within aquatic sciences outpaces the collective ability of individual scientists within the field to make appropriate use of those technologies. The process of in situ lake sampling remains the primary choice to comprehensively understand an aquatic ecosystem at local scales; however, the impact of climate change on lakes necessitates the rapid advancement of understanding and the incorporation of lakes on both landscape and global scales. Three fields driving innovation within winter limnology that we address here are autonomous real‐time in situ monitoring, remote sensing, and modeling. The recent progress in low‐power in situ sensing and data telemetry allows continuous tracing of under‐ice processes in selected lakes as well as the development of global lake observational networks. Remote sensing offers consistent monitoring of numerous systems, allowing limnologists to ask certain questions across large scales. Models are advancing and historically come in different types (process‐based or statistical data‐driven), with the recent technological advancements and integration of machine learning and hybrid process‐based/statistical models. Lake ice modeling enhances our understanding of lake dynamics and allows for projections under future climate warming scenarios. To encourage the merging of technological innovation within limnological research of the less‐studied winter period, we have accumulated both essential details on the history and uses of contemporary sampling, remote sensing, and modeling techniques. We crafted 100 questions in the field of winter limnology that aim to facilitate the cross‐pollination of intensive and extensive modes of study to broaden knowledge of the winter period.

Shallow lakes (<2 m depth) of Central Asian, receiving strong solar radiation and low precipitation, are sensitive to atmospheric forcing because of their low heat capacity, yet their under‐ice thermal conditions remain poorly investigated. We conducted the first complete ice season monitoring of Lake Ulansu (Ulansuhai, Wuliangsuhai), revealing unique thermal behavior. The lake was salinity stratified (<3‰), stabilizing the lower water layer and allowing the water temperature to reach 10°C before break‐up. The solar radiation absorbed by the water ( Q sw ) drove the water–ice heat flux, with approximately 82% of Q sw returning to the ice base, facilitating a rapid shift from convective mixing to stable stratification. These findings provide key insights into the thermal regimes of Central Asian shallow lakes, informing climate models and ecological assessments for more than 10,000 similar lakes in the region.

Arctic and boreal lakes in the Northern Hemisphere experience annual ice cover lasting 4 to 7 months. Freshwater lakes in cold regions are sensitive to subtle environmental changes and influenced by various physical and biogeochemical factors. Our study focuses on comparative analysis of under-ice metabolism shaped by the thermal and oxygen dynamics of Arctic Lake Kilpisjarvi and Boreal Lake Paajarvi during the late winter. We aim to understand the effect of different trophic levels and light regimes on lake metabolism within cold regions by using high-frequency data on temperature, dissolved oxygen, and solar radiation for Lake Kilpisjarvi in 2019 and 2020, and Lake Paajarvi in 2022. Besides the long-term data, we compared the phytoplankton biomass and chemical parameters obtained from water samples collected from different depths. We studied the changes in the vertical distribution of lake metabolism by diel cycles by considering the strength and influence of internal motions on temperature and oxygen data. Our results demonstrate that following prolonged darkness, a significant increase in dissolved oxygen occurs in the upper water column of Lake Kilpisjarvi. The depth of the mixed layer increases with depth, ranging from 1.1 m/day to 2.3 m/day for 2019 and 2020 in Kilpisjarvi. In contrast, Lake Paajarvi has a slower and steady rate of deepening at 0.55 m/day, resulting in a comparatively shallow mixed layer.

Glaciers on the Tibetan Plateau (TP) are retreating rapidly in response to global warming. While extensive research has focused on the spatial heterogeneity of glacier retreat across the TP, only a few studies have examined the role of TP lakes in shaping these patterns. In this study, we applied the Weather Research and Forecasting (WRF) model, coupled with a one-dimensional lake mass and energy balance scheme, to examine how TP lakes affect seasonal surface air temperature, snowfall, and water vapor flux, and how these lake-induced climatic changes influence the spatial heterogeneity and seasonal variability of glacier retreat across the region. Results show that TP lakes tend to reduce 2-m air temperature (T2m) in glacierized regions in many seasons, and are associated with increased snowfall on some alpine glaciers in the Inner TP during summer and autumn, and on marginal glaciers during winter and spring. Furthermore, the seasonal climate effects of individual lakes vary regionally, and they may exert distinct influences on adjacent glaciers, particularly in autumn in our simulations. Langa Co and Mapam Yumco may contribute to reduced T2m and enhanced snowfall over the Naimona’nyi Glacier to the south, which is consistent with the relatively slower retreat observed among glaciers in the West Himalayas. Similarly, Yamzho Yumco and Puma Yumco have comparable impacts on the Qiangyong Glacier in the Central Himalayas. Additionally, Nam Co exerts contrasting influences on the western Nyainqentanglha Range, mitigating glacier retreat on the southern slopes but intensifying it on the northern slopes during summer, with the opposite pattern occurring in autumn. Simulations indicate that TP lakes can modify regional moisture pathways, contributing to enhanced northward water vapor fluxes associated with the Indian Summer Monsoon in some cases, which may influence snowfall and glacier mass balance in regions such as the West Kunlun and Tanggula Mountains.

The restoration of the North Aral Sea was an unprecedented effort to save a large water basin by construction of a dam that separates it from the rest of the desiccating Aral Sea area. As a result, the lake volume has stabilized at 27.5 km3, the area has increased from 2800 km2 (2006) to 3400 km2 (2020), and the salinity has dropped from 18 to 10 g kg−1. The consequences of this unique experiment include highly dynamic changes in the thermal conditions, seasonal stratification, ice regime, and dissolved oxygen content and remain not fully quantified to date. We analyze the current state of the North Aral Sea with regard to stabilization of its long-term dynamics. We further consider the possible future projections in view of the global change effects on the regional hydrological regime and potential water management measures. Using data from a series of expeditions to the North Aral Sea in 2016–2019 and year-long continuous monitoring of the thermal and oxygen regime by an autonomous mooring station, we present the first comprehensive analysis of the North Aral system behavior on seasonal to interannual scales after its restoration. We demonstrate that the present seasonal mixing regime is distinguished by relatively weak summer thermal stratification occupying about 7 % of the lake volume. Salinity does not contribute to the summer density stratification, but a stable salinity stratification can develop during ice melt in late winter. On the background of weak thermal stratification, highly energetic internal waves with periods of ∼4.5 d dominate the near-bottom dynamics and facilitate mixing at the lake bottom. As a result, the bulk of the water column remains well-saturated with oxygen throughout the year. However, low oxygen conditions may develop in the deepest part of the lake in midsummer. In summary, the mixing regime of the restarted lake favors vertical transport of dissolved matter and water–sediment mass exchange, ensuring oxygenation of deep waters and supply of nutrients to the upper water column. While the North Aral Sea is restored to a well-mixed state similar to that before its desiccation started, its seasonal mixing regime is currently in unstable equilibrium, wobbling between polymictic and dimictic conditions. The fragility of this seasonal pattern is demonstrated by modeling results: slight changes in the water level or transparency may turn the Aral Sea to a steadily dimictic or polymictic state.

Seasonally ice-covered Arctic lakes lack solar radiation during polar nights, and snow cover can extend the dark period up to half a year of permanent darkness. We used high-resolution temperature and dissolved oxygen data from three winters in Arctic Lake Kilpisjärvi to quantify oxygen conditions. We applied vertically resolved single-column oxygen budget and lake-averaged oxygen budget approaches to estimate drivers of deep-water oxygen depletion. Deep-water hypoxia occurred in all 3 years, with timing and magnitude strongly influenced by under-ice temperature and mixing. Basin-scale internal waves with periods exceeding diurnal lengths were persistent, generating turbulence indicated by a vertical diffusion coefficient exceeding conductive values by an order of magnitude as the only large-scale motions under ice. The major physical factor on oxygen dynamics was the ice formation date. A 1–2 weeks earlier ice-on resulted in temperatures close to the maximum density, reduced vertical mixing, and intensification of bottom gravity currents. Downslope lateral transport by gravity currents produced accumulation of low oxygenated waters in deep layers, responsible for 55–85% of the near-bottom oxygen decrease, the rest contributed by local oxygen uptake by sediment. A delayed ice-on led to stronger vertical mixing and weaker oxygen depletion. Oxygen consumption was mainly controlled by bottom oxygen flux, while water column respiration contributed about 20% of total depletion. The results highlight sensitivity of under-ice oxygen dynamics to physical transport processes. The effect of ice formation date on under-ice oxygen content suggests potentially strong response of Arctic lakes to shortening of the ice cover.

Convective turbulence driven by gravitational instability is a fundamental mixing mechanism in geophysical flows, but in situ estimation of its characteristics is obscured by the background flows and the relatively slow temporal scales. We present characteristics of the full Reynolds tensor from a convective surface boundary layer of an ice-covered lake. The results were obtained by using an original method of measuring the full set of turbulent stresses by a combined use of two ADCPs. The strong horizontal shear stress was revealed as a characteristic feature of free convection differing from the "conventional" turbulent boundary flows. The ratio of normal stresses along vertical and horizontal axes remained below 1/4, demonstrating anisotropic character of turbulence asymptotically approaching the axisymmetric two-component “pancake” form. The vertical r.m.s. velocity fluctuations obeyed the buoyancy flux scaling with the coefficient of 1/3, which is at the lower boundary of previously reported values, while horizontal fluctuations followed the same scaling with a unity coefficient

The restoration of the North Aral was an unprecedented effort to save a large water basin by construction of a dam that separates it from the rest of the desiccating Aral Sea area. As a result, the lake volume has stabilized at 27.5 km3, the area has increased from 2800 km2 in 2006 to 3400 km2 in 2020, and the salinity has dropped from 18 to 10 g kg-1. The consequences of this unique experiment include highly dynamic changes of the thermal conditions, seasonal stratification, ice regime, and dissolved oxygen content and remain not fully quantified to date. We analyze the current state of the North Aral Sea with regard to stabilization of its long term dynamics, as well as consider the possible future projections in view of the global change effects on the regional hydrological regime and potential water management measures. Using data from two year-long observations, we analyze the current seasonal mixing regime and sub-seasonal oscillations due to lake-scale internal waves in the North Aral. We found that the seasonal stratification pattern is intermediate between dimictic and polymictic, with relatively weak summer thermal stratification occupying only a small deep part of the lake. Salinity does not contribute to the summer density stratification. On the background of weak thermal stratification, highly energetic internal waves with periods of 4.5 days dominate the near-bottom dynamics and facilitate mixing at the lake bottom. As a result, the bulk of the water column remains well saturated with oxygen throughout the year. However, low-oxygen conditions may develop in the deepest part of the lake in mid-summer. In summary, the mixing regime of the restarted lake favors vertical transport of dissolved matter and water-sediment mass exchange ensuring oxygenation of deep waters and supply of nutrients to the upper water column. While the North Aral Sea is restored to the well-mixed state similar to that before its desiccation started, its seasonal mixing regime is currently in unstable equilibrium, wobbling between polymictic and dimictic conditions. The fragility of this seasonal pattern is demonstrated by modeling results: slight changes of the water level or transparency may turn the Aral Sea to steadily dimictic or polymictic state.

Lakes represent a vital source of freshwater, accounting for 87

Methane (CH4) accumulation in the well-oxygenated lake epilimnion enhances the diffusive atmospheric CH4 emission. Both lateral transport and in situ oxic methane production (OMP) have been suggested as potential sources. While the latter has been recently supported by increasing evidence, quantifying the exact contribution of OMP to atmospheric emissions remains challenging. Based on a large high-resolution field data set collected during 2019-2020 in the deep stratified Lake Stechlin and on three-dimensional hydrodynamic modeling, we improved existing CH4 budgets by resolving each component of the mass balance model at a seasonal scale and therefore better constrained the residual OMP. All terms in our model showed a large temporal variability at scales from intraday to seasonal, and the modeled OMP was most sensitive to the surface CH4 flux estimates. Future efforts are needed to reduce the uncertainties in estimating OMP rates using the mass balance approach by increasing the frequency of atmospheric CH4 flux measurements.

Primary production is a key factor in assessing aquatic ecosystems and the global carbon cycle. Despite the ice-cover period lasting several months in many lakes, less attention has been paid to primary production in winter under ice compared to the open water period. In particular, the relationship between light conditions under ice and associated primary production remain not fully understood. This study, conducted in Lake Hanzhang, China, during 2022–2023, investigated the impact of under-ice light on primary production to understand how it varies during the ice-cover period and how the optical properties of lake ice affect it. The ice structure and its optical properties were analyzed, and primary production was calculated using the Vertically Generalized Production Model. During ice growth, the maximum ice thickness reached 31.6 cm, and approximately 56 % of photosynthetically active irradiance was absorbed by the ice, with only around 10 % reaching the water beneath. The optical properties of the shallow lake ice were mainly related to the bubble volume within the ice, with a positive correlation between the extinction coefficient and bubble volume. Throughout the ice-cover period, the diurnal primary production in Lake Hanzhang varied substantially, with the average primary production under ice amounting to 148 mg C·m−2·d−1, indicating that photosynthesis can remain active under ice in winter. Our study revealed that the photosynthesis of phytoplankton is not restricted by ice-cover and that the growth and melting of the ice sheet are vital for primary production in the water beneath. These findings highlight the importance of studying underwater biological processes about the shortening ice period caused by global warming.

Polar nights are unique periods to understand metabolism in ice-covered lakes from physical and biological perspectives. Despite the nearly zero primary production, polar nights cannot be disregarded as periods of biological stagnation, and recent data on biological processes and biogeochemical interactions are not yet fully understood, but can provide valuable information on lake ecosystem functioning in the absence of sunlight. Seasonally ice-covered Arctic Lake Kilpisjärvi at the latitude of 69° N experiences night time conditions from December to mid-January. Here, we use the continuous high-frequency time series of high-resolution temperature and oxygen from Kilpisjärvi to understand temperature and depth-related changes in respiration levels for three years from 2019 to 2022. In parallel, we aim to derive vertical microbial community composition by analysing water samples from different depths during the ice-covered period and correlate the outputs with oxygen dynamics to understand bacterial adaptations along biogeochemical gradients in Lake Kilpisjarvi.