Cyprus lows are the main reason for precipitation over Lake Kinneret during the rainy season (December–May): these lows are centered over the Mediterranean Island of Cyprus. Cyprus lows are responsible for cold weather conditions when westerly winds transport cold moist air from the eastern Mediterranean into north Israel (including Lake Kinneret). Such cold weather conditions, accompanied by rainfall and a decrease in solar radiation (due to an increase in cloudiness over the lake), could cause cooling of Kinneret and eastern Mediterranean water temperature (WT) in rainy seasons.

However, in the Eastern Mediterranean and in Lake Kinneret, surface water temperature is increasing by ~1.5 ^oC over the last 40 years. Moreover, climate model predictions showed a reduction by one third in the appearance of Cyprus lows by the end of the 21st century. This suggests a reduction in cooling by Cyprus lows of water in the Eastern Mediterranean and in Lake Kinneret. Therefore, a comprehensive investigation of the influence of Cyprus lows on water temperature in subtropical Lake Kinneret and the Eastern Mediterranean is environmentally important.

Comparisons, conducted on a monthly basis, between high-precipitation (HP) years and low-precipitation (LP) years led to our main findings, which are as follows: Cyprus lows are instrumental in the cooling of surface and epilimnion water in subtropical Lake Kinneret and in the cooling of eastern Mediterranean surface water (Kishcha et al., 2022). In particular, comparison between HP and LP years of Kinneret surface water temperature (SWT) and epilimnion water temperature (WT) have shown water cooling of up to 2 °C in HP years, in the daytime. This study was carried out using the 21-year period of satellite and in-situ data: (1) MODIS 1 km × 1 km resolution records of SWT, and (2) shipboard measurements of WT vertical profiles down to a depth of 40 m (2000–2020). We found that the decrease in solar radiation (caused by Cyprus lows due to an increase in cloudiness) was the main factor contributing to Kinneret water cooling. In spring (March–April), SWT and epilimnion WT, averaged over the HP years, was lower by ~2 °C and ~1.4 °C, respectively, than SWT and epilimnion WT averaged over the LP years. This was when SR increased and became the main factor contributing to water heating. In situ shipboard measurements of WT at a depth of 1 m and 5 m, at five monitoring sites within Lake Kinneret, showed similar patterns of the WT difference between HP and LP years. This is evidence that cooling by Cyprus lows of Kinneret water was evenly distributed within the lake. Water cooling by Cyprus lows was also observed in eastern Mediterranean surface water. In particular, in the spring months (March–April), Mediterranean SST averaged over the same HP years was lower by ~1.2 oC than that averaged over LP years. This is evidence of the regional character of the water-cooling phenomenon caused by Cyprus lows.

Reference:

Kishcha et al. (2022).  Remote Sensing 2022, 14, 4709. https://doi.org/10.3390/rs14194709

How to cite: Kishcha, P., Lechinsky, Y., and Starobinets, B.: Cooling by Cyprus Lows of Surface and Epilimnion Water in Subtropical Lake Kinneret in Rainy Seasons, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1577, https://doi.org/10.5194/egusphere-egu23-1577, 2023.

On May 31, 2012, thousands of dead fish were found along the north-western shore of the Sea of Galilee. Analysis of fish gill tissue revealed no evidence of poisoning, and the fish looked healthy. This event adds to reports of similar fish kills at the same location, from the early 1990s, from May 2007, and a subsequent event on June 27, 2012, a month after the May 31 event. The common hypothesis for the massive kill suggests that a seiche induced by strong winds caused the upwelling of colder and anoxic hypolimnetic water along the western shores of the lake. Still, this hypothesis has not yet been tested.

The WRF (The Weather Research and Forecasting) atmospheric model was recently coupled with the ocean model MITgcm (MIT general circulation model). The coupled model was named SKRIPS (Scripps–KAUST Regional Integrated Prediction System). The two SKRIPS model components (WRF and MITgcm) are well-tested at high resolution, allowing us to investigate the physical mechanism of the fish-kill event in an interactive system. To test the hypolimnetic water upwelling hypothesis for the massive fish-kill, we have set up and integrated the SKRIPS model for the May 31, 2012, event at a horizontal grid resolution of 400 m², both for the atmospheric and lake component of the model.

In this talk, I will present results from a high-resolution coupled atmosphere-lake regional simulation indicating an upwelling of cold anoxic hypolimnetic water into the surface during the event. The upwelling of cold water is increased close to the shore. The discussion will be supplemented by field data of temperature and oxygen concentrations, collected before, during, and after the fish-kill event. Our simulation results agree with the field observations, adding confidence to the anoxic hypolimnetic water upwelling hypothesis. Such fish-kill events may explain the biblical ‘miraculous catch of fish’ and the ‘miracle of the loaves and fish’. Also, it may provide a possible seasonal time frame (spring) for their occurrence in the past.

How to cite: Strobach, E., Amitai, Y., Assouline, S., Hamilton, D., Nishri, A., and Zohary, T.: Seiche-induced fish-kills in the Sea of Galilee may explain the biblical miraculous catch of fish, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5873, https://doi.org/10.5194/egusphere-egu23-5873, 2023.

The flow regime of the largest lake in Southeast Asia, Tonle Sap Lake, is driven by the reverse flow phenomenon caused by its link with the Mekong River. This reverse flow makes the lake one of the most productive aquatic ecosystems globally and thus provides important economic opportunities for local communities. The recent human activities in the upstream, as well as climate variations, have resulted in unforeseen alterations in the flow regime of the Mekong River and Tonle Sap Lake. However, little is known about the explicit attribution of different parts of the upstream basin to these variations, which would be essential for transboundary water management. To unveil these attributions, we developed a novel modeling setup consisting of hydrodynamic, hydrological, and machine learning models. This modeling setup allowed us to separate the impacts of a) climate variation, b) human activities in the Chinese part of the basin, and b) the lower part of the basin (i.e., Laos, Thailand, and Vietnam). During the 2001–2009 baseline, when human modifications to the flow were still minimal, we found that Tonle Sap Lake received, on average, 42.4 km³/yr water from the Mekong, 48.2 % of the total inflow to the lake. During the period of increased human activities, 2010–2020, this decreased due to climate variation to 40.1 km³/yr (a 5.7 % drop), which was further exacerbated by the increased human activities in the upstream parts of the basin (China ~ 7.3%, Laos, Thailand, and Vietnam ~ 9 %). Additionally, during the flow period when water flows from the lake towards the Mekong, on average, 31 % of the total inflow into the Mekong Delta originated from the lake during the baseline period. Climate variation decreased this by 4 percentage points (pp), i.e., to 27 %, while the human activities in China and lower parts of the basin decreased this by 1.6 pp (25.4 %) and 1.9 pp (25.1 %), respectively. Our findings unveiled the attributions of different drivers on Tonle Sap Lake’s hydrology and will facilitate transboundary water management in the basin. The impacts of future plans on different parts of the basin should be carefully evaluated together with existing anthropogenic impacts, as well as climate change, to minimize the further impacts on the lake and Mekong River.

How to cite: Morovati, K., Shi, L., Zhang, K., Tian, F., Tudaji, M., and Nakhaei, P.: Contributions from climate variation and human activities to flow regime change of Tonle Sap Lake and Mekong River from 2001 to 2020, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10658, https://doi.org/10.5194/egusphere-egu23-10658, 2023.

Hydrological processes are known as major driving forces in structuring wetland plant communities, but the specific relationships are not always well understood. The recent dry conditions of Poyang Lake (i.e., the largest freshwater lake in China) are having a profound impact on its wetland vegetation, leading to the degradation of the entire wetland ecosystem. We developed an integrated framework to quantitatively investigate the relationship between the spatial distribution of major wetland herbaceous communities and the hydrological regimes of Poyang Lake. First, the wetland herbaceous community classification was built using a support-vector machine and simultaneous parameter optimization, achieving an overall accuracy of over 98%. Secondly, based on the inundation conditions since 2000, four hydrological drivers of the spatial distribution of these communities were evaluated by canonical correspondence analysis. Finally, the hydrological niches of the communities were quantified by Gaussian regression and quantile methods. The results show that there were significant interspecific differences in terms of the hydrological niche. For example, Carex cinerascens Ass was the most adaptable to inundation, while Triarrhena lutarioriparia + Phragmites australis Ass was the least. Our integrated analytical framework can contribute to hydrological management to better maintain the wetland plant community structure in the Poyang Lake area.

How to cite: Huang, W., Hu, T., Mao, J., Montzka, C., Bol, R., Wan, S., Li, J., Yue, J., and Dai, H.: Hydrological Drivers for the Spatial Distribution of Wetland Herbaceous Communities in Poyang Lake, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2406, https://doi.org/10.5194/egusphere-egu23-2406, 2023.

Urban lakes provide essential ecosystem services (hotspot of biodiversity, stormwater management, reduction of pollutant loadings…).
During late summer and early autumn 2021, two whole-lake anoxia events occurred in Lake Champs-sur-Marne (Great Paris, France).  This sandpit lake, principally fed by groundwater from the nearby Marne River, is continuously monitored by an autonomous station equipped with underwater sensors (water temperature, oxygen, Chlorophyll-a, CDOM, Nitrate). At the same point, a meteorological station is installed on a buoy, above the lake surface.
During the two anoxia events (end of August and mid-September), oxygen concentration dropped from supersaturation level corresponding to a high peak of phytoplankton biomass, to 0% within a few days. During this summer period, successive heavy rainfall events occurred, causing a flood of the Marne River and rising the watertable level to unusual values in Summer. This resulted in high water and nutrient fluxes from the river towards the lake. 
The observed whole-lake anoxia can be explained according to the following assumptions: (1) the groundwater nutrient loading, favored by the high level of the Marne River, caused a huge phytoplankton production; (2) then, the phytoplankton decline was associated to an intense mineralization of the biomass organic carbon; (3) the lake oxygen was completely exhausted, leading to a massive fish kill. 
These results highlight the severe impact of a non-extreme but high and long hydro-meteorological event on a lake ecosystem. In Lake Champs-sur-Marne, the nutrient limitation of phytoplankton production generally occurs during late summer. In 2021, the limitation was removed by the Summer exceptional nutrient loading. In temperate regions, summer algal blooms are not limited by water temperature but by nutrient availability. Climate change is expected to increase the frequency of extreme hydro-meteorological events.  Higher frequency of summer heavy rainfall may trigger repeated phytoplankton blooms, deteriorating the ecological status of lake ecosystems.

How to cite: Vinçon-Leite, B., Calabro-Souza, G., Breton, F., Casenave, C., Saad, M., Dubois, P., Lemaire, B. J., and Piccioni, F.: Impact of extreme hydro-meteorological events on the anoxia dynamics in a small urban lake, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6490, https://doi.org/10.5194/egusphere-egu23-6490, 2023.

Lake Neusiedl and adjacent lakes at the Austrian/Hungarian boundary represent a unique wetland, situated at the geodynamical and geomorphological boundary between the Alps, Carpathians and the Pannonian Basin, and therefore represents an important transition zone concerning terrain, climate, vegetation and fauna. The more than 700 km² large area is one of the flattest regions of Austria with less than 17 m relief variation. Today, Lake Neusiedl measures some 320 km² with less than 1.8 m depth and a tectonic origin is widely accepted. East of the lake, about 30 shallow lakes still exist, of which the largest measures less than 2 km length and less than 1 m depth. At present, the water level of Lake Neusiedl shows the lowest values since 1965 and all the shallow lakes were dry during the summer 2022. We use high-resolution topographical, sedimentological, geomorphological as well as historical maps and historical charters to investigate the formation end evolution of Lake Neusiedl and adjacent lakes. Our data show that the present-day conditions and processes of Lake Neusiedl strongly diverge from conditions in the past. The earliest preserved record of modification of the lake´s hydrological conditions is from 1568, followed by increasing drainage efforts and the building of a dam road in the southeast, finished in 1780, which subsequently cut off the lake from its most important tributaries from the south. This palaeohydrological reconstruction of Lake Neusiedl and Hanság also implies an episodic lower salt content of the water compared to modern values, especially during flood periods. Therefore, it is not useful to compare the hydrological situation of Lake Neusiedl before 1780 (or even 1568) and after. The documented variation of the water level of Lake Neusiedl between desiccation and highest flood level is around 4.2 m. These variations affected enormous areas in this low-relief region, with a huge impact on the landscape, fauna, vegetation, human settlement patterns, land use, communication routes and even possible occurrence of malaria – which should be considered in regional archaeological, historical and biological interpretations. The numerous shallow lakes and presently dry basins, detected in the high-resolution airborne laser scanning data in the Seewinkel originally formed as thermokarst lakes during permafrost degradation after the end of the Last Glacial Maximum. In total, more than 370 enclosed depressions are visible. The depressions of eastern Austria represent one of the first Latest Pleistocene thermokarst lakes documented in central Europe. Although man-made climate change has a clear impact on the water balance of the lakes, they reinforce past hydrological modifications.

How to cite: Draganits, E., Weißl, M., Zámolyi, A., and Doneus, M.: Lake Neusiedl wetland (Austria/Hungary) in a changing environment: anthropogenic versus anthropogenic effects, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16150, https://doi.org/10.5194/egusphere-egu23-16150, 2023.

Lake Surface Temperature (LST) is a key characteristic that reflects meteorological and climatological influences on lakes. In general, there is limited LST data from high elevation lakes available as these areas are remote and not part of regular monitoring programs. Nonetheless, for the development of effective management strategies for high-altitude lakes, it is important to understand their response to climate warming. This study aims at both the reconstruction of LST back to 1998 and the projection of LSTs for 21 alpine lakes (1500-2300 m a.s.l.) in the Niedere Tauern region in Austria until the year 2100. For the determination of the relationship between atmospheric variables (temperature and precipitation), near-lake snow depth and observed LST, general additive models were trained with a daily temporal resolution for the years 1998-2003, and 2019-2020. We subsequently employed the model with the highest fit to reconstruct LSTs for the whole period 1998 to 2003. Furthermore, we estimate LST until 2100 using an ensemble of regional climate projections for the RCP2.6 (in-line with the COP 21 Paris Agreement), RCP4.5 and RCP8.5 (“worst-case”) scenario. Under the RCP8.5 scenario, the average rise for August lake surface temperatures in the far future (2071-2100) is predicted to increase by 2.3 °C compared to temperatures in the near future (2020-2049).  Consequently, the ice-free period is expected to rise on average 1-1.2—fold in the near future (2031-2060) and 1-1.5-fold in the distant future. These alterations in the lakes’ temperature regime probably affect multiple limnological parameters related to ecological quality such as primary productivity and trophic state.

How to cite: Enigl, K., Pritsch, H., and Kurmayer, R.: Development of surface temperatures of alpine lakes in Austria under climate change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7630, https://doi.org/10.5194/egusphere-egu23-7630, 2023.

Lake Neusiedl, a shallow brackish lake of Austria and Hungary, is the westernmost steppe lake of Europe with an area of ca. 320 km² and the second largest coherent reed population in Europe. Half of Lake Neusiedl consists of a wetland ecosystem dominated by Phragmites australis, which forms a seasonally varying mosaic of water, reed and sediment patches, is highly sensitive to climate variations and which we investigated here.

Little is known about the effects of climate change on reed dominated wetlands and the contribution of central European reed belts as a source of greenhouse gases (GHG). The current ongoing drought periods at Lake Neusiedl affect especially the water balance but also the carbon fluxes in the reed belt. Therefore, we investigated the drought influenced carbon and water fluxes and their drivers of the reed ecosystem of a brackish lake over the last three years.

We used the eddy covariance (EC) technique to continuously quantify the vertical turbulent GHG exchange between the reed belt and atmosphere. The EC observations have been conducted near Illmitz in the natural zone of National Park Lake Neusiedl from summer 2018 to now. For taking the reed development of the studied ecosystem into account, vegetation indices data were used.

The annual CO₂ emissions decreased by 95 % from 200.5 g C m^-2 in 2019 to 9.2 g C m^-2 in 2021. Gross primary production and ecosystem respiration both increased from 2019 to 2021. The annual emissions of CH₄ decreased by 59 % from 9.0±1.0 g C m^-2 in 2019 to 3.7±1.9 g C m^-2 in 2021. The reed belt tended from a strong to a low carbon source if only the vertical flows are taken into account. One explanation is the decreasing water level in the lake between 2019 to 2021, which was followed by a drying out of the reed belt (≙ no water above surface) in the late summer of 2020 and a longer period in 2021. The second explanation is the increasing reed growth (in area and biomass) inside the reed belt which increased the photosynthetic rate. The vegetation indices like NDVI, EVI and LAI from the reed belt support this by an increasing tendency from 2019 to 2021. The third explanation is that due to the low water levels less (or almost no) lateral exchange occurred via channels between the reed belt and the open water areas of the lake in 2021 compared to 2019.

The apparent disconnection between the open water area of Lake Neusiedl and the reed belt re-directs carbon cycles and ecosystem functioning.

How to cite: Baur, P. A., Maier, A., Zechmeister, T., and Glatzel, S.: Consequences of drying-out of Lake Neusiedl on the GHG budget of the reed belt, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7710, https://doi.org/10.5194/egusphere-egu23-7710, 2023.

Soil erosion on lakeshores due to the fluctuating water levels, waves, and other factors, remain a world-wide problem. Lakeshore vegetation can be helpful in preventing erosion. We investigated the effects of drawdown rate and depth on the growth and reproduction of a keystone lakeshore plant. We hypothesised that plants with access to water for longer would grow better and have higher reproductive output.

We subjected 108 Spiny Sedge (Cyperus gymnocaulos) plants to six treatments comprising three drawdown rates (static, slow, fast) and two water depths (shallow and deep). We measured plant stem heights and numbers of flowers and bulbils weekly, and the initial and final biomass of the above ground and below ground components.

Plants in treatments without access to water for long periods had the lowest growth and reproductive output. However, if the final water level was deep but drawdown was done slowly, plants were able to maintain similar growth and reproduction rates to plants with continuous access to water.

Fluctuating water levels in lakes cause lakeshore plants at lower elevations to be inundated for longer and plants at higher elevations to be deprived of water for longer. Plants located at mid-elevations will thrive if their roots have access to water and their above ground parts are not fully submerged.

Our findings are useful to water managers and ecologists concerned about preserving lakeshores from erosion by promoting vegetation. Both the rate and depth of drawdown should be considered in managing lake water levels. Where water levels fluctuate over large depths slower rates of drawdown will enable most plants to have access to water for longer, promoting their growth and reproduction, hence, reducing the lakeshore erosion.

Keywords: lakes, vegetation, drawdown, erosion, sustainability

How to cite: Dahanayake, A., Webb, A., Greet, J., and Brookes, J.: Slower drawdown rates in lakes promote the growth and reproduction of lakeshore vegetation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1430, https://doi.org/10.5194/egusphere-egu23-1430, 2023.

Lake surface water temperature (LSWT) is a physical property of lakes. LSWT is a critical parameter for evaluating lakes' water quality and biodiversity. The change in LSWT can also be an indicator of climate change. Therefore, it is crucial to monitor LSWT to improve our understanding of the spatiotemporal dynamics of LSWT for many applications. Conventionally and ideally, we can install in-situ gauge stations or monitoring sites to measure surface water temperature in lakes, and these in-situ measurements are generally the most accurate. However, in-situ measurements in lakes are often sparse and limited in terms of spatial coverage and temporal length, which leaves many lakes with no measurements or lacking long-term continuous measurements. For example, Lake Vänern (surface area of about 5,655 km², the largest lake in the European Union) has only two operational stations for measuring LSWT. The existing in-situ measurements are at irregular intervals (approximately bi-weekly) and have many data gaps. Many lakes globally have the same data situation as Lake Vänern. As a result, in-situ measurements cannot sufficiently capture the spatiotemporal dynamics of LSWT in large lakes.

Satellite remote sensing has emerged as an essential method to monitor LSWT. Thermal infrared satellite data have been widely used to estimate the surface temperature at relatively high spatial resolution (higher and up to 1 km resolution). One of the most used satellite products for surface temperature is the MODIS (Moderate Resolution Imaging Spectroradiometer) global land surface temperature product, which is available from 2000 at 1 km-daily spatial-temporal resolutions. However, many studies stressed that cloud influence could significantly degrade the quality and availability of satellite-derived surface temperature for certain lakes, calling for a dedicated investigation to address this issue. Besides MODIS data, there are many other satellite-derived LSWT products at different spatial-temporal resolutions and spatial coverage, e.g., the ones available at http://www.laketemp.net. In addition, the recent ERA5-Land, a state-of-the-art reanalysis dataset, can also provide spatially complete and temporally continuous land surface variables, including the lake temperature at 0.1 degree-hourly spatial-temporal resolutions from 1950 to present. Each of the aforementioned products has its own advantages and limitations.

Our initial analysis showed a significant data gap in LSWT from MODIS product for Lake Vänern due to cloud influence, which motivates us to conduct this study. This study aims to evaluate multiple existing LSWT products and, more importantly, to combine them with the advanced data fusion and bias correction method to develop a new spatially complete and temporally continuous LSWT dataset for Lake Vänern, Sweden. New in-situ measurements of LSWT will be collected from boats and drones at many locations of Lake Vänern; such measurements, together with existing data from the two stations, will be used to evaluate multiple LSWT products, the developed method, and the merged dataset. The newly developed LSWT dataset for Lake Vänern will benefit many applications, such as lake evaporation estimation, water balance analysis, air-lake interactions, and local climate forecasting.

How to cite: Duan, Z. and Schultze, A.: Development of a new spatially complete and daily continuous lake surface water temperature dataset for Lake Vänern, Sweden, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6928, https://doi.org/10.5194/egusphere-egu23-6928, 2023.

Numerous limnological studies have been investigating lake overturning regimes in the frame of possible future climate scenarios, mainly regarding the predicted surface air temperature change. Overturning regime shifts are expected to manifest in the stratification onset and offset timing, as well as in stratification duration. In addition, the lakes' vertical water column is expected to become more stable, and thus less inclined to overturn. Since aquatic life in the lake depends thoroughly on its phenology, the predicted regime shifts are going to influence the whole lake ecosystem, along with the microclimate of its surrounding area.

An interesting example of regime shifting can be found in the central part of the Adriatic coast (43° 32' N, 15° 58' E). Lake Rogoznica (also known as "Dragon's Eye" lake) is a marine lake that is connected to the nearby Adriatic Sea through the porous karstic landscape. Although this connection is confirmed by high salinity values that reach those found in the surrounding sea (~38 PSU) and the visible tides, the newest research suggests that the complete overturning of the lake is governed by the delicate balance between the freshwater input (precipitation and runoff) and seawater input (Adriatic Sea), together with substantial surface air temperature fluctuations. Namely, Rogoznica Lake seems to exhibit complete overturns with irregular periodicity: sudden and abrupt holomixis occurred in the Autumn of 1997, 2011, 2016, 2020, and 2021. Another peculiarity of the lake is the complete anoxia with high levels of free sulfides that accompanies every such event, and the consequent mass mortality of the lake's aerobic life - the complete opposite of the oxygenation effect that seasonal overturning usually has in lakes.

It is still unclear whether sudden anoxic holomixis and their frequency are a consequence of local tourist activities, regional climate variability, or climate change affecting the Mediterranean, as the effects of all mentioned could have an intertwined effect on the small, complex system of Rogoznica Lake. In this work, we present the latest findings regarding the Rogoznica Lake water column stratification with a two-sided approach: a comprehensive sea-atmosphere measurements analysis and numerical modelling attempts.

How to cite: Dominović, I., Marguš, M., Dutour Sikirić, M., and Ciglenečki, I.: Regime shifts in a euxinic marine lake, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14467, https://doi.org/10.5194/egusphere-egu23-14467, 2023.

The combined effect of gravity and viscosity forces produces a flow along any inclined solid lateral wall in a density-stratified fluid. The flow becomes an important driver of circulation in ice-covered lakes isolated from wind forcing. We present numerical results from the Regional Ocean Modeling System (ROMS) investigating three-dimensional buoyancy-driven circulation in an ice-covered lake of simplified symmetric shape. The numerical results revealed vertical current velocities of 10^-6 m s^-1 and horizontal current velocities of 10^-3 m s^-1. The model simulated an upward current along sloping boundaries, with downwelling return flow throughout the bulk of the lake's water column. For the typical internal Rossby radius of 14 km, basin-scale circulation in a lake with a horizontal dimension of 45 km turns to a counterclockwise gyre in the lake half depth. We investigated the dependence of the boundary flow and the residual lake-wide circulation on the lake size, bottom slope inclination, and earth rotation. The obtained magnitudes of the boundary flow were compared against known simplified analytical solutions. The outcomes demonstrated that the ROMS model, on the basis of the Boussinesq hydrostatic equation, is able to simulate weakly energetic flows governed by viscous forces and rotation in enclosed thermally stratified ice-covered domains. The results contribute to a better understanding of the processes driving the under-ice freshwater circulation with wider applications, including dynamics of buoyancy-driven flows affected by nonlinear effects, such as freshwater density anomaly.

How to cite: Sharifi, F. S., Hinkelmann, R., Hattermann, T., and Kirillin, G.: Three-dimensional gravity flows in an idealized stratified ice-covered lake, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-68, https://doi.org/10.5194/egusphere-egu23-68, 2023.

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.

How to cite: Asirok, E., Kirillin, G., and Grossart, H.-P.: Breathing at Sleep: Respiration in Arctic Lakes During Polar Nights, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-366, https://doi.org/10.5194/egusphere-egu23-366, 2023.

Lake evaporation (E_W) is an important component of both basin evapotranspiration (ET_B) and lake water balance for lake-basins on the inner Tibetan Plateau (IB), and it greatly influences lake water storage change (ΔS_W). However, the effects of E_W on ET_B and ΔS_W at lake-basin scale have never been reported for most basins on the IB. In this study, the E_W of 117 large lakes (area > 50 km²) in 95 closed lake-basins (area > 1000 km²) were estimated, and its effects on ET_B and ΔS_W over 2001-2018 were examined using several newly derived diagnostic equations from the aspects of E_W amount, rate, trend slope and inter-annual variability. During the study period, mean annual E_W rate and total E_W amount for the lakes are 994.25 ± 20.48 mm, and 24.83 ± 0.52 km³ respectively. The significant increasing trend (0.29 ± 0.04 km³/a) in annual E_W amount is mainly caused by the increase (224.65 km²/a) in lake area (82.13%), and the increase (2.12 ± 1.28 mm/a) in E_W rate is responsible for the rest (17.87%). E_W accounts for 23.16% ± 4.94% of the ET_B (107.24 ± 21.90 km³) for the 95 basins, and its impact has increased significantly (0.20% ± 0.09%/a) over the period. The increasing trends of E_W rate and lake area ratio (0.06%, P < 0.05) contributed 14.49% and 52.69% to the increase trend in ET_B (0.85 mm/a), and their variances contributed 1.60% ~ 4.79% and 1.64% ~ 6.50% to ET_B variance (155.44 ± 107.97 mm²), respectively. The contribution of E_W, quasi lake inflow (R_L, 23.48 km³), and lake surface precipitation (P_W, 9.18 km³) to mean ΔS_W (7.82 km³) are -43.02%, 40.84% and 15.96%, respectively. And the increasing trends of the three components (E_W, R_L and P_W) account for -58.02%, 29.59% and 12.39% of the decrease trend in annual ΔS_W (-0.81 × 10 km³/a, P > 0.05), respectively. Basin R_L, derived based on lake water balance, is significantly correlated with two independent land surface net precipitation estimates (0.57 < R < 0.86), and basin lake area ratio is a good indicator of basin E_W and lake inflow in the IB.

How to cite: Wang, L., Wang, J., and Li, X.: Lake evaporation and its effects on basin evapotranspiration and lake water storage on the inner Tibetan Plateau, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6834, https://doi.org/10.5194/egusphere-egu23-6834, 2023.

Lakes are responding rapidly to climate change and one of the most tangible responses is the increase in lake surface water temperature. Such an increase can intensify thermal stratification and dampen the intensity of vertical mixing. In turn, surface warming has the potential to alter the mixing regime of lakes, potentially leading to abrupt shifts in ecosystem functioning. Reduced mixing between the surface and bottom waters can indeed decrease the upwelling of essential nutrients from deep water to the lake surface and the oxygen transport in the opposite direction. This can result in a decrease in lake productivity and can increase the risk of anoxia at depth, respectively.

Despite the important consequences of such lake mixing anomalies, we lack a systematic overview of their occurrence, mainly due to the lack of systematic data to detect and analyze them worldwide. Remotely sensed lake surface water temperature available from ESA CCI (Climate Change Initiative) and similar sources represent spatial skin temperature gradients, but they do not resolve vertical gradients. They are hence often used to prove the lakes’ long-term warming in terms of spatial average. However, the horizontal gradients of such data could help us better understand the internal processes of lakes and the identification of lake mixing anomalies. Given that seasonal overturning often occurs at different times across the lake, the spatial character of remotely sensed data can reveal important processes in freshwater systems and can help assess the long-term variability in the overturning behaviour of large lakes in the context of climate change. Within our project, we use the spatial component of satellite Earth Observation data to reveal information about lake mixing and mixing anomalies. We apply a thermal front tracking method, a technique much more exploited in oceanography than limnology, to identify mixing anomalies in dimictic lakes worldwide.

Our study suggests that the spatially distributed property of Earth Observation can be useful to spot lake mixing anomalies in dimictic lakes worldwide. Thus, we present the first global-scale assessment of lake mixing anomalies occurrence in the last 20 years. Earth Observation data can also be used to calculate how susceptible lakes are to undergo a mixing regime shift. Interestingly, we found that lakes experiencing more mixing anomalies are also those more susceptible to undergoing a mixing regime shift. Moreover, using Earth Observation, we detected mixing anomalies that have already been documented and, more interestingly, we spotted mixing anomalies occurring in unstudied remote lakes. Although further investigations would be needed to specifically assess the impact of climate change on these remote lakes, these cases highlight that remote sensing can be used as a first screening tool to spot lake mixing anomalies worldwide. Thus, Earth Observation and our methodology can be potentially used as an early warning system for lake mixing regime shifts.

How to cite: Calamita, E., Brechbühler, M., Woolway, I., Albergel, C., and Odermatt, D.: Detecting lake mixing anomalies using Earth Observation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15065, https://doi.org/10.5194/egusphere-egu23-15065, 2023.