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Extreme weather events can pervasively influence ecosystems. Observations in lakes indicate that severe storms in particular can have pronounced ecosystem-scale consequences, but the underlying mechanisms have not been rigorously assessed in experiments. One major effect of storms on lakes is the redistribution of mineral resources and plankton communities as a result of abrupt thermocline deepening. We aimed at elucidating the importance of this effect by mimicking in replicated large enclosures (each 9 m in diameter, ca. 20 m deep, ca. 1300 m3 in volume) a mixing event caused by a severe natural storm that was previously observed in a deep clear-water lake. Metabolic rates were derived from diel changes in vertical profiles of dissolved oxygen concentrations using a Bayesian modelling approach, based on high-frequency measurements. Experimental thermocline deepening stimulated daily gross primary production ( GPP) in surface waters by an average of 63% for >4 weeks even though thermal stratification re-established within 5 days. Ecosystem respiration ( ER) was tightly coupled to GPP, exceeding that in control enclosures by 53% over the same period. As GPP responded more strongly than ER, net ecosystem productivity ( NEP) of the entire water column was also increased. These protracted increases in ecosystem metabolism and autotrophy were driven by a proliferation of inedible filamentous cyanobacteria released from light and nutrient limitation after they were entrained from below the thermocline into the surface water. Thus, thermocline deepening by a single severe storm can induce prolonged responses of lake ecosystem metabolism independent of other storm-induced effects, such as inputs of terrestrial materials by increased catchment run-off. This highlights that future shifts in frequency, severity or timing of storms are an important component of climate change, whose impacts on lake thermal structure will superimpose upon climate trends to influence algal dynamics and organic matter cycling in clear-water lakes. [ABSTRACT FROM AUTHOR]

The hydrology of the lake-rich Tibetan Plateau is important for the global climate yet little is known about the thermal regime of Tibetan lakes due to scant data. We (i) investigated the characteristic seasonal temperature patterns and recent trends in thermal and stratification regime of lakes on the Tibetan Plateau and (ii) tested theperformance of the one-dimensional lake parameterization scheme FLake for the Tibetan lake system. For this purpose we combined three years of in situ lake temperature measurements, several decadesof satellite observations and the global reanalysis data. We chose the two largest freshwater Tibetan lakes - Ngoring and Gyaring - as study sites. The lake model FLake faithfully reproduced the specific features of the high-altitude lakes and was subsequently applied to reconstruct the vertically resolved heat transport in both lakes during the last four decades. The modelsuggested Ngoring and Gyaring were ice-covered forabout 6 months and stratified in summer for about 4 months per year with a short spring overturn and longer autumn overturn. In summer the surface mixed boundary layer extended to 6-8 m depth and was about 20% shallower in the more turbid Gyaring. The thermal regime of transparent Ngoring responded more strongly to atmospheric forcing than Gyaring, where the higher turbidity dampened the response. According to reanalysis data, air temperatures and humidity increased, whereas solar radiation decreased since the 1970s. Surprisingly, mean lake temperatures did not change, nor did the phenology of ice cover or stratification. Lake surface temperatures in summerincreased only marginally. The reason is that the increase in air temperature was offset by the decrease in radiation, probably due to increasing humidity. This study demonstrates that air temperature trends are not directly coupled to lake temperatures and underscores the importance of short-wave radiation for the thermal regime of high-altitude lakes. [ABSTRACT FROM AUTHOR]

Hutchinson and Löffler's (1956) classification of lakes based on the seasonal thermal mixing regime has become a cornerstone of any analysis of lakes as elements of the earth surface. Until now however the lake classification has lacked a physically sound quantitative criterion distinguishing between two fundamental lake types: thermally stratified during a large portion of the year (mono- and dimictic) and predominantly mixed to the bottom (polymictic). Using the mechanistic balance between potential and kinetic energy we derived a generalized scaling for seasonal stratification in a closed lake basin. The scaling parameter is the critical mean basin depth, Hcrit, that delineates lakes that mix regularly from those that stratify seasonally based on lake water transparency, lake length, and an annual mean estimate for the Monin-Obukhov length. The scaling criterion consistently describes the mixing regime significantly better than either the conventional unbounded basin scaling or a simple depth threshold. Thus, the generalized scaling is universal for freshwater lakes and allows the seasonal mixing regime to be estimated without numerically solving the heat transport equations. [ABSTRACT FROM AUTHOR]

Understanding the fate of organic carbon (OC) inputs in aquatic ecosystems is of primaryimportance for comprehending their functioning, in particular in relation to their carboncycle. A large part of terrigenous OC, coming from the watershed, enters aquatic ecosystemsin the form of refractory organic matter (ROM), which are difficult to oxidise. Recent studiesin the literature have suggested, however, that ROM mineralisation rates may be enhanced inpresence of labile organic matter (LOM), making ROM compounds easier to degrade, with aconsequent alteration of the carbon budget in those systems. This process refers to as primingeffect. In lakes, zooplankton carcasses represent an abundant and high quality LOM substrate formicroorganisms that has the potential of priming ROM mineralisation in water column andcreating new and overlooked pathways in the lake carbon cycle. To investigate this, it iscrucial to measure the in-situ sedimentation rate and residence time of dead zooplankton, as afunction of hydrodynamic conditions, microbial decomposition and organisms’characteristics (such as size and density). Current velocimetry acoustic methods cannot measure, however, velocities of singlemillimetre-sized carcasses because particles are too small and measurements are not accuratedue to the low correlation of the acoustic pings. To overcome this, we propose an inexpensiveparticle tracking velocimetry (PTV) system to estimate the instantaneous 3D velocity ofparticles in the field. From the data it is possible to measure sinking rates of deadzooplankton and potentially infer carbon fluxes in lakes in relation to the primingeffect. The PTV rig consists of two GoPro cameras arranged in a frontal parallel arrangementand installed in a waterproof case. The case is mounted on a stainless-steel frame with alaminate blank screen placed in front of the cameras to restrict the system sampling volume.A torch was also attached on the frame to enhance lighting underwater conditions. Thecameras are set up to acquire stereoscopic films of sinking particles at 48 frames per secondand calibrated in a calibration tank to estimate their distortion and rectification parameters. Asoftware package was also developed with new stereo algorithms to sync the videos,detect and track carcasses, as they move in the water, and measure their velocity andlength. In this study, we also present early results of sedimentation rates from the PTV system,estimated from 2 deployments in summer 2018 in the mesocosm of Lake Stechlin (Germany)at 7m. Finally, the settling rates are compared with (1) those inferred from the Stokes’ lawand (2) the daily settling rates estimated from sedimentation traps installed in the mesocosmat the same depth. [ABSTRACT FROM AUTHOR]

Climate forecasts project a global increase in extreme weather events, but information on the consequences for ecosystems is scarce. Of particular significance for lakes are severe storms that can influence biogeochemical processes and biological communities by disrupting the vertical thermal structure during periods of stratification. An exceptional storm passing over northern Germany in July 2011 provided an opportunity to assess the consequences and underlying mechanisms of such extreme events on the interplay between the physics and ecological characteristics of a deep, nutrient-poor lake. Wind speeds were among the most extreme on record. A suite of variables measured throughout the event consistently indicates that a cascade of processes pushed the clear-water lake into an exceptionally turbid state. Specifically, thermocline deepening by the storm-entrained cyanobacteria of a deep chlorophyll maximum located at about 8 m depth into the surface mixed layer. Released from light limitation, intense photosynthesis of the cyanobacteria boosted primary production, increased algal biomass, raised the pH and thus induced massive calcite precipitation to a level never observed within three decades of lake monitoring. As a consequence, water transparency dropped from 6.5 to 2.1 m, the minimum on record for 40 years, and the euphotic zone shrank by about 8 m for several weeks. These results show that cyanobacterial blooms not only are promoted by climate warming, but can also be triggered by extreme storms. Clear-water lakes developing a deep chlorophyll maximum appear to be particularly at risk in the future, if such events become more intense or frequent. [ABSTRACT FROM AUTHOR]

• Lake evaporation projections using a multi-model approach. • The model ensemble generally captured intra-annual variability and seasonality. • Projected increase in evaporation and decrease in precipitation this century. Lake evaporation plays an important role in the water budget of lakes. Predicting lake evaporation responses to climate change is thus of paramount importance for the planning of mitigation and adaption strategies. However, most studies that have simulated climate change impacts on lake evaporation have typically utilised a single mechanistic model. Whilst such studies have merit, projected changes in lake evaporation from any single lake model can be considered uncertain. To better understand evaporation responses to climate change, a multi-model approach (i.e., where a range of projections are considered), is desirable. In this study, we present such multi-model analysis, where five lake models forced by four different climate model projections are used to simulate historic and future change (1901–2099) in lake evaporation. Our investigation, which focuses on sub-tropical Lake Kinneret (Israel), suggested considerable differences in simulated evaporation rates among the models, with the annual average evaporation rates varying between 1232 mm year−1 and 2608 mm year−1 during the historic period (1901–2005). We explored these differences by comparing the models with reference evaporation rates estimated using in-situ data (2000–2005) and a bulk aerodynamic algorithm. We found that the model ensemble generally captured the intra-annual variability in reference evaporation rates, and compared well at seasonal timescales (RMSEc = 0.19, R = 0.92). Using the model ensemble, we then projected future change in evaporation rates under three different Representative Concentration Pathway (RCP) scenarios: RCP 2.6, 6.0 and 8.5. Our projections indicated that, by the end of the 21st century (2070–2099), annual average evaporation rates would increase in Lake Kinneret by 9–22 % under RCPs 2.6–8.5. When compared with projected regional declines in precipitation, our projections suggested that the water balance of Lake Kinneret could experience a deficit of 14–40 % this century. We anticipate this substantial projected deficit combined with a considerable growth in population expected for this region could have considerable negative impacts on water availability and would consequently increase regional water stress. [ABSTRACT FROM AUTHOR]

Known as “Asian Water Tower”, the Tibetan Plateau (TP) contains the largest glacierized area outside the polar regions and contains more than half of China’s lakes. However, these glaciers are retreating rapidly under the influence of global warming. Many studies have attempted to explain the spatial heterogeneity of glacier retreat in the TP and its impacts on local lakes, but few studies have focused on the feedback of lakes on glaciers, especially in summer with intense lake effects and glacier ablation. Using an air-lake coupled model, the potential summer climatic impacts of the lake clusters on glacier behavior over TP are investigated based on two experiments with and without the lakes. Away from the lake-rich area of Inner TP, glaciers along the Himalayas are retreating rapidly with climate warming. The most pronounced glacier ablation occurs in southeastern TP, where TP lakes reduce snowfall. However, in the Inner TP, the influence of climate warming on glaciers is partially offset by the lakes through different lake-related mechanisms. The glaciers on the Western Nyainqentanglha Range are preserved mainly by the local cooling and snowfall-increase caused by nearby Nam Co. In turn, the numerous small lakes in the Eastern Inner TP exert a cumulative effect on preserving the glaciers through cooling and moistening the atmospheric boundary layer and thus increasing snowfall. The glaciers in the western Kunlun Mountains benefit from the large-scale impacts of the TP lakes, which intensified westerlies and lead to regional temperature decrease and snowfall increase. [ABSTRACT FROM AUTHOR]

Citizen science projects have a long history in ecological studies. The research usefulness of such projects is dependent on applying simple and standardized methods. Here, we conducted a citizen science project that involved more than 3500 Swedish high school students to examine the temperature difference between surface water and the overlying air (Tw-Ta) as a proxy for sensible heat flux (QH). If QH is directed upward, corresponding to positive Tw-Ta, it can enhance CO2 and CH4 emissions from inland waters, thereby contributing to increased greenhouse gas concentrations in the atmosphere. The students found mostly negative Tw-Ta across small ponds, lakes, streams/rivers and the sea shore (i.e. downward QH), with Tw-Ta becoming increasingly negative with increasing Ta. Further examination of Tw-Ta using high-frequency temperature data from inland waters across the globe confirmed that Tw-Ta is linearly related to Ta. Using the longest available high-frequency temperature time series from Lake Erken, Sweden, we found a rapid increase in the occasions of negative Tw-Ta with increasing annual mean Ta since 1989. From these results, we can expect that ongoing and projected global warming will result in increasingly negative Tw-Ta, thereby reducing CO2 and CH4 transfer velocities from inland waters into the atmosphere. [ABSTRACT FROM AUTHOR]

Recent discovery of oxic methane production in sea and lake waters, as well as wetlands, demands re-thinking of the global methane cycle and re-assessment of the contribution of oxic waters to atmospheric methane emission. Here we analysed system-wide sources and sinks of surface-water methane in a temperate lake. Using a mass balance analysis, we show that internal methane production in well-oxygenated surface water is an important source for surface-water methane during the stratified period. Combining our results and literature reports, oxic methane contribution to emission follows a predictive function of littoral sediment area and surface mixed layer volume. The contribution of oxic methane source(s) is predicted to increase with lake size, accounting for the majority (>50%) of surface methane emission for lakes with surface areas >1 km 2 .