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ABSTRACTDepletion of deepwater dissolved oxygen (DO) in lakes has become increasingly prevalent and severe because of many external stressors, potentially threatening human-derived ecosystem services ranging from drinking water quality to fisheries. Using year-round, high-frequency DO data from 12 dimictic lakes, we compared 3 measures of deepwater DO depletion during winter and summer: DO depletion rate, DO minimum, and hypoxia duration. Hypoxia (DO < 3 mg L−1) occurred in over half of the lakes and persisted an average of 83% longer in summer than in winter. While we found no difference in DO depletion rates between winter versus summer, these rates were significantly related to lake morphology in winter but trophic state in summer. In assessing cross-seasonal linkages, we found limited evidence for significant legacy effects in deepwater DO availability. Only fall mixing efficacy significantly responded to the previous summer’s minimum DO saturation, but it always reached moderate to high DO replenishment levels (>65%) regardless of the previous summer’s DO depletion severity. This lack of ecological memory in deepwater DO depletion across seasons suggests that deepwater DO largely resets during spring and fall mixing periods in most years in these dimictic lakes. Understanding the patterns and drivers in deepwater DO depletion in both winter and summer is a key step forward for predicting future chemical and biological consequences of seasonal DO depletion and managing lake ecosystem health, as well as the effects that climate change may have on these patterns.

Lake ecosystems are jeopardized by the impacts of climate change on ice seasonality and water temperatures. Yet historical simulations have not been used to formally attribute changes in lake ice and temperature to anthropogenic drivers. In addition, future projections of these properties are limited to individual lakes or global simulations from single lake models. Here we uncover the human imprint on lakes worldwide using hindcasts and projections from five lake models. Reanalysed trends in lake temperature and ice cover in recent decades are extremely unlikely to be explained by pre-industrial climate variability alone. Ice-cover trends in reanalysis are consistent with lake model simulations under historical conditions, providing attribution of lake changes to anthropogenic climate change. Moreover, lake temperature, ice thickness and duration scale robustly with global mean air temperature across future climate scenarios (+0.9 °C °Cair–1, –0.033 m °Cair–1and –9.7 d °Cair–1, respectively). These impacts would profoundly alter the functioning of lake ecosystems and the services they provide.

The intensity and frequency of storms are projected to increase in many regions of the world because of climate change. Storms can alter environmental conditions in many ecosystems. In lakes and reservoirs, storms can reduce epilimnetic temperatures from wind‐induced mixing with colder hypolimnetic waters, direct precipitation to the lake's surface, and watershed runoff. We analyzed 18 long‐term and high‐frequency lake datasets from 11 countries to assess the magnitude of wind‐ vs. rainstorm‐induced changes in epilimnetic temperature. We found small day‐to‐day epilimnetic temperature decreases in response to strong wind and heavy rain during stratified conditions. Day‐to‐day epilimnetic temperature decreased, on average, by 0.28°C during the strongest windstorms (storm mean daily wind speed among lakes: 6.7 ± 2.7 m s−1, 1 SD) and by 0.15°C after the heaviest rainstorms (storm mean daily rainfall: 21.3 ± 9.0 mm). The largest decreases in epilimnetic temperature were observed ≥2 d after sustained strong wind or heavy rain (top 5thpercentile of wind and rain events for each lake) in shallow and medium‐depth lakes. The smallest decreases occurred in deep lakes. Epilimnetic temperature change from windstorms, but not rainstorms, was negatively correlated with maximum lake depth. However, even the largest storm‐induced mean epilimnetic temperature decreases were typically <2°C. Day‐to‐day temperature change, in the absence of storms, often exceeded storm‐induced temperature changes. Because storm‐induced temperature changes to lake surface waters were minimal, changes in other limnological variables (e.g., nutrient concentrations or light) from storms may have larger impacts on biological communities than temperature changes.

The discovery of methane (CH4) accumulation in oxic marine and limnic waters has redefined the role of aquatic environments in the regional CH4cycle. Although CH4accumulation in oxic surface waters became apparent in recent years, the sources are still subject to controversial discussions. We present high-resolution in situ measurements of CH4concentration and its stable isotope composition in a stratified mesotrophic lake. We show that CH4accumulation in surface waters originates from a highly dynamic interplay between (oxic) CH4production and emission to the atmosphere. Laboratory incubations of different phytoplankton types and application of stable isotope techniques provide a first unambiguous evidence that major phytoplankton classes in Lake Stechlin per se produce CH4under oxic conditions. Combined field and lab results show that the photoautotroph community is an important driver for CH4production and its highly dynamic accumulation in oxic surface waters.

The increase in strength and duration of summer stratification in temperate lakes is a well-described sentinel of climatic change reported in numerous lake studies. The definitions of the onset of thermal stratification in early spring and its breakup during the autumn overturn are usually binary thresholds chosen on a subjective basis. However, the transition phases between mixing and stagnation (start of summer stratification) and between stagnation and mixing (end of summer stratification) are continuous processes. Therefore, the time limits of the stratified period are definition-dependent, which complicates comparison of results from different studies. An experimental determination of the duration of summer stratification needs unified criteria to decide when stratification is established and when it terminates. We demonstrate that the termination of the stratified period (start of autumn mixing) can be detected with sufficient accuracy from near bottom temperature records at the deepest point of the lake. Nevertheless, it takes days after this moment until the whole water column is mixed completely and a stability threshold is required for the end of the stratified period. To define the beginning of the stratified period, thresholds for different stability indices were suggested previously. Although the threshold choice influences the estimated duration of stratification each year, it appears not to significantly affect the long-term trends of stratification. More important than the selection of stability thresholds is therefore the quality of observational data used for analysis. Using measured data from two dimictic Lakes in Northern Germany we show that the estimation of the stratification length depends strongly on the temporal resolution of the measurements. For comparative studies which aim to verify a shift in future temperate lake stratification experimentally, we suggest (i) to use data from high-resolution water temperature probes (discarding completely data sets with weekly, biweekly or sparser sampling period), (ii) to use a consistent stability threshold (e.g. a fixed value of Schmidt stability) to derive the duration of the stratified period uniformly, and (iii) to apply standardized algorithms in large intercomparison studies to determine indices of stratification.

The Qinghai‐Tibet Plateau possesses the largest alpine lake system, which plays a crucial role in the land‐atmosphere interaction. We report first observations on the thermal and radiation regime under ice of the largest freshwater lake of the Plateau. The results reveal that freshwater lakes on the Tibetan Plateau fully mix under ice. Due to strong solar heating, water temperatures increase above the maximum density value 1–2 months before the ice break, forming stable thermal stratification with subsurface temperatures >6°C. The resulting heat flow from water to ice makes a crucial contribution to ice cover melt. After the ice breakup, the accumulated heat is released into the atmosphere during 1–2 days, increasing lake‐atmosphere heat fluxes up to 500 W m−2. The direct biogeochemical consequences of the deep convective mixing are aeration of the deep lake waters and upward supply of nutrients to the upper photic layer. The Qinghai‐Tibet Plateau possesses the largest alpine lake system, which plays a crucial role in the land‐atmosphere interaction. Data on thermal properties of Tibetan lakes during the ice‐covered season are extremely scarce. The first observations on the thermal and radiation regime under ice of the largest freshwater lake of the Plateau reveal that freshwater (and apparently the majority of brackish) lakes gain an extremely large amount of solar radiation penetrating the highly transparent ice cover. As a result, lakes fully mix under ice and get heated up to >6°C. The accumulated heat makes a crucial contribution to ice cover melt. The quick release of the heat to the atmosphere during 1–2 days after the ice breakup strongly increases lake‐atmosphere heat fluxes, potentially affecting regional weather conditions. Warm and well‐mixed conditions are favorable for aquatic life in the extreme environment of Tibet. An abnormal thermal regime under the ice cover of Tibetan lakes is revealedThe lakes get heated above their maximum density temperature by extremely high level of solar radiation penetrating the ice coverThe stored heat shortens the ice‐covered period and is quickly released into the atmosphere after ice‐off, affecting local climate An abnormal thermal regime under the ice cover of Tibetan lakes is revealed The lakes get heated above their maximum density temperature by extremely high level of solar radiation penetrating the ice cover The stored heat shortens the ice‐covered period and is quickly released into the atmosphere after ice‐off, affecting local climate

Qinghai Lake is the largest lake in China. However, its influence on the local climate remains poorly understood. By using an atmosphere‐lake coupled model, we investigated the impact of the lake on the local climate. After the adjustment of four key parameters, the model reasonably reproduced the lake‐air interaction. Superimposed by the orographic effects on lake‐land breeze circulation, the presence of the lake enhanced precipitation over the southern part of the lake and its adjacent land, while slightly reduced precipitation along the northern shore of the lake. The lake effect on local precipitation revealed a distinct seasonal and diurnal variability, reducing precipitation in May (−6.6%) and June (−4.5%) and increasing it from July (5.7%) to November (125.6%). During the open water season, the lake's daytime cooling effect weakened and the nighttime warming effect strengthened, affecting spatial distribution and intensity of lake‐induced precipitation. In early summer, precipitation slightly decreased over the north part of the lake due to the lake's daytime cooling. In turn, lake‐induced nighttime warming increased precipitation over the southern section of the lake and its adjacent land. With the start of the autumn cooling in September, heat and moisture fluxes from the lake resulted in precipitation increase in both daytime and nighttime over the entire lake. In October, the background atmospheric circulation coupled with the strong lake effects lead to a small amount but high proportion of lake‐induced precipitation spreading evenly over the lake. The coupled lake‐atmosphere model after adjusting four key parameters performs well in quantifying the lake's regional climate effectsQinghai Lake decreased the local precipitation during April–June while increased that during July–NovemberThroughout the simulation period, precipitation declined slightly in the north and rose greatly in the south due to the lake's presence

Ice cover plays a critical role in physical, biogeochemical, and ecological processes in lakes. Despite its importance, winter limnology remains relatively understudied. Here, we provide a primer on the predominant drivers of freshwater lake ice cover and the current methodologies used to study lake ice, including in situ and remote sensing observations, physical based models, and experiments. We highlight opportunities for future research by integrating these four disciplines to address key knowledge gaps in our understanding of lake ice dynamics in changing winters. Advances in technology, data integration, and interdisciplinary collaboration will allow the field to move toward developing global forecasts of lake ice cover for small to large lakes across broad spatial and temporal scales, quantifying ice quality and ice thickness, moving from binary to continuous ice records, and determining how winter ice conditions and quality impact ecosystem processes in lakes over winter. Ultimately, integrating disciplines will improve our ability to understand the impacts of changing winters on lake ice. Lakes are experiencing accelerated rates of warming, including shorter seasonal duration of ice cover, later ice‐on, earlier ice‐off, and in some years no ice cover at all. Lake ice has been historically studied independently by four subdisciplines: observations by in situ and remote sensing scientists, controlled mesocosm experiments by limnologists, and process‐based models by physical modelers. Here, we highlight opportunities for collaboration between disciplines and provide guidelines to successfully integrate the disciplines to tackle the most urgent questions surrounding lake ice loss in warming climates. Ripe opportunity to merge disciplines and harmonize data streams to further understand lake ice dynamics in changing wintersDevelop integrated plans for future data collection by moving from ice cover and phenology to multifaceted descriptions of ice conditionsOperationalize development of skill sets to work in multidisciplinary teams by constructing a workforce and developing a common language