HS10.9

During the past three decades, seawater level (SWL) in the Caspian Sea has declined by about 2 m, and sea area has decreased by about 15 000 km². This has affected coastal communities, the environment and economically important sea gulfs (e.g., Dead Kultuk). We simulated SWL using total inflow from feeder rivers and precipitation and evaporation over the sea to assess the effects of coastline change and evaluate zones vulnerable to desiccation. We determined the potential of coastal vulnerability over the past 80 years by comparing the minimum and maximum annual water body maps (for 1977 and 1995). We then determined the linear regression between SWL rise and covered potential vulnerable area (CVA), using annual Normalised Difference Water Index (NDWI) maps and SWL data from 1977 to 2018. Combining SWL-CVA regression and SWL simulation model enabled us to determine desiccated areas in different regions of the Caspian Sea due to changes in precipitation, evaporation and total inflow. The results showed that 25 000 km² of the sea is potentially vulnerable to SWL fluctuations to be desiccated. Also, we found 70% of this vulnerable area is in Kazakhstan. Potential vulnerable area per kilometer coastline was found to be 6 km² in Kazakhstan, 4 km² in Russia and the whole of the Caspian Sea, 1.5 km² in Iran, 1 km² in Azerbaijan, and 0.5 km² in Turkmenistan. The results also indicated that SWL in the Caspian Sea is sensitive to evaporation and that, e.g., a 37.5 mm decrease in mean annual net precipitation would lead to an 1875 km² decrease in the sea area, while a 1 km³ decrease in mean annual inflow would lead to a 1400 km² decrease in the sea area. Thus the developed framework enabled the spatial consequences of changes in water balance parameters on sea area to be quantified. It can assess future changes in SWL and sea area due to anthropogenic activities and climate change.

How to cite: Akbari, M., Klöve, B., Faramrazzadeh, O., and Torabi Haghighi, A.: Response of the Caspian Sea Shoreline to hydro-climatic Drivers Variation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-272, https://doi.org/10.5194/egusphere-egu22-272, 2022.

Lakes emit CO₂ to the atmosphere at magnitudes significant for the global carbon cycle, in the form of diffusive CO₂ flux (F_CO2). As direct F_CO2 measurements are time-consuming, F_CO2 is often estimated from the air-water CO₂ concentration gradient (Δp_CO2) and the gas transfer velocity (k), representing the two components considered to regulate F_CO2. However, extrapolating measurements of ΔpCO₂ and k to whole-year estimates require understanding of their variability in time and across different types of lakes, which is often insufficient. As a result, simple linear interpolations are typically used in extrapolations which risk producing bias as spatiotemporal variability is not included. Further insight to the variability of Δp_CO2 and k may contribute to more representative extrapolations and provide guidance for focusing sampling campaigns on capturing times of high variability. We used floating flux chambers and surface water samples to measure F_CO2 and Δp_CO2, respectively, both within-weeks and over seasons during the open water period at 12 locations in each of 15 boreal lakes across a latitudinal gradient in Sweden. We combined these measurements to derive spatially resolved values of k in order to identify: i) the contributions of Δp_CO2 and k to F_CO2 variability over time; and ii) if differences in the contributions of Δp_CO2 and k to F_CO2 variability can be related to lake characteristics. The results presented are relevant for improved modelling of lake CO₂ emissions.

How to cite: Rudberg, D., Schenk, J., Pajala, G., Sawakuchi, H., Sieczko, A., Karlsson, J., MacIntyre, S., Melack, J., Sundgren, I., and Bastviken, D.: Relative contribution of surface water concentrations (pCO2aq) and gas transfer velocity (k) to CO2 flux variability in boreal lakes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1165, https://doi.org/10.5194/egusphere-egu22-1165, 2022.

In the summer months, characterized by the absence of precipitation and by limited cloud cover, subtropical lakes are particularly sensitive to atmospheric warming which causes increasing heating of surface water. Therefore, these lakes are best suited to the investigation of this phenomenon.

Within the Jordan Rift valley there are two lakes: the fresh-water Lake Kinneret (Sea of Galilee) (a surface area of 106 km² and a maximal depth of 40 m) and the hypersaline Dead Sea (a surface area of 605 km² and a maximal depth of 300 m). We investigated water surface temperature (WST) and its trends in the two lakes. This was carried out using MODIS 1 km x 1 km resolution records on board Terra and Aqua satellites together with in-situ measurements, during the period (2003 – 2019). In fresh-water Lake Kinneret, we found that, in summer when evaporation is maximal, despite the presence of increasing atmospheric warming, satellite data revealed the absence of WST trends (Kishcha et al., 2021). The absence of WST trends in the presence of increasing atmospheric warming is an indication of the influence of steadily increasing evaporation on WST. Increasing water cooling, due to steadily increasing evaporation, compensated for increasing heating of surface water by regional atmospheric warming. This resulted in the obtained statistically-insignificant WST trends. During the study period (2003 – 2019), in summer, in contrast to satellite data, in-situ measurements of near-surface water temperature (at a depth of 0.1 m) in Lake Kinneret showed an increasing trend of 0.7 ^oC  decade^-1. This trend in near-surface water temperature reflected the presence of increasing atmospheric warming in the absence of evaporation.

In contrast to fresh-water Lake Kinneret, in the hypersaline Dead Sea (located only 100 km apart), MODIS showed an increasing statistically-significant trend of 0.8 ^oC decade^-1 in summer WST. This fact was obtained during the same study period (Kishcha et al., 2021). The increasing WST trend, in the presence of atmospheric warming, is evidence of the absence of increasing evaporation in the Dead Sea. This fact is supported by a constant rate of ~1 m/year of Dead Sea water level drop during the last 25-year period (1995 – 2020). The absence of increasing evaporation could be explained by surface water salinity in the Dead Sea skin layer. Increasing surface water salinity suppresses further increases in evaporation. As a result, there was no acceleration in Dead Sea water level drop in the presence of an increasing SWT trend of 0.8 ^oC decade^-1. We consider that this is a characteristic feature of the hypersaline Dead Sea, which is not present in the fresh-water Lake Kinneret.

Reference:

Kishcha et al. (2021). Absence of surface water temperature trends in Lake Kinneret despite present atmospheric warming: Comparisons with Dead Sea trends. Remote Sensing, 13, 3461. https://doi.org/10.3390/rs13173461

How to cite: Kishcha, P., Lechinsky, Y., Starobinets, B., and Alpert, P.: Absence of surface water temperature trends in the presence of atmospheric warming as evidence of increasing evaporation in fresh-water Lake Kinneret, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1550, https://doi.org/10.5194/egusphere-egu22-1550, 2022.

The karstic, stratified marine lake (Lake Rogoznica, RL) on the eastern Adriatic coast (43°32'N, 15°58'E) is a unique environment. It oscillates between a stratified water column with euxinic conditions below the chemocline and a holomictic euxinic water column under certain physicochemical conditions (1). Given the specific physicochemical, microbiological, and biochemical properties of the water column, the lake proves to be an ideal test site to track environmental changes indicative of climate change. Climate change will further increase water column temperature and enhance deoxygenation in the epilimnion while promoting the accumulation of toxic sulphide, ammonium in the hypolimnion, and organic matter (OM) throughout the water column (2). Since the early 1990s, when exploration of the lake began, the volume of the anoxic water has increased several times. The stronger stratification has led to an enrichment of dissolved organic matter (DOC) in the euxinic hypolimnion due to the anoxic conditions, while the concentration of DOC in the oxic epilimnion (0-8 m depth) decreases. At the same time, the concentration of the most reactive DOC fraction (surface active substances- SAS) (3) increases in the upper layer, while a decreasing trend in SAS is observed below 8 m depth. In addition, there is evidence of accumulation of particulate organic matter (POC) in the water column and an increase in the fraction of POC in total organic carbon (TOC).

In RL, vertical mixing events occur in early fall that can end with holomictic conditions that affect lake biogeochemistry (4), including organic matter properties and dynamics. Over the past 30 years, these events are becoming more frequent and intense. Each holomictic event is associated with a subsequent high production of POC and a change in composition DOC. On a long-term scale (1992-2021), this study presents a unique time series of organic matter content (DOC, POC, SAS) showing a noticeable change in its quantity and quality within the RL water column as an indication of the pronounced eutrophication escalated by global change.

This work was result of research activities within the MARRES project, IP-2018-01-1717.

[1] I. Ciglenečki, Z. Ljubešić, I. Janeković, M. Batistić, in R.D. Gulati, E.S. Zadereev, A.G. Degermendzhi (eds) “Ecology of meromictic lakes”. Springer 2017, Cham, p 125−154.

[2] M. Čanković, J. Žučko, I. Dupčić Radić, I. Janeković, I. Petrić, I. Ciglenečki, G. Collins,  Syst. Appl. Microbiol. 42 (2019) 126016.

[3] I. Ciglenečki, I. Vilibić, J. Dautović, V. Vojvodić, B. Ćosović, P. Zemunik, N. Dunić, H. Mihajlović, Sci. Total Environ. 730 (2020) 139104.

[4] M. Čanković, J. Žučko, I. Petrić, M. Marguš, I. Ciglenečki, Aquat. Microb. Ecol. 84 (2020) 141-154.

How to cite: Simonović, N., Marguš, M., and Ciglenečki, I.: Long-term monitoring of organic matter in an eutrophic marine lake that fluctuates between stratified and holomictic euxinic conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2191, https://doi.org/10.5194/egusphere-egu22-2191, 2022.

Even low productive, high-altitude lakes experience deep water hypoxia under ice-cover. While the changing ice phenology is expected to ripple on the magnitude of under-ice hypoxia, the lack of a mechanistic framework linking the physical impact of ice loss to biogeochemical properties has led to seemingly contradictory conclusions.  

Biogeochemical and physical processes constrain the Dissolved Oxygen (DO) dynamic at the sediment-water interface under lake ice. On the one hand, the biogeochemical hypothesis envisions a primary control of DO decay under the ice by sediment oxygen uptake, which arises from benthic microbial respiration and the release of reduced compounds. On the other hand, the physical hypothesis assumes a greater DO decay when sediment heat release reinforces the inverse stratification; the stronger is the sediment heat release, the more the bottom layer, from which oxygen is consumed, gets isolated from potential diffusive resupply from the upper layers. The outcome of a shorter ice-cover on the under-ice DO dynamics depends on the dominance of either biogeochemical or physical processes.

Based on in-situ observations of DO and temperature, we assessed the relative share of biogeochemical and physical processes on decay under the ice of 14 high-altitude lakes in the French Alps. We found highly variable DO decay rates across the different lakes and years, with exponential coefficients ranging from 1.10^-3 to 6.10^-2 d^-1.  The under-ice DO decay rates increased, within years and lakes, with sediment heat release, while biogeochemical factors played only a marginal role. We tested through a reaction-diffusion model on an archetypal, testbed lake the individual effects of biogeochemical versus physical processes on DO decay. We confirmed that the sediment heat flux at ice-on is a major driver of DO decay under the ice, explaining one mechanism by which shallower or more transparent lakes experience greater DO decay under the ice^.

How to cite: Perga, M.-E., Minaudo, C., Ulloa, H., Doda, T., Perolo, P., Escoffier, N., Arthaud, F., Obrador, B., and Bouffard, D.: Controls on oxygen depletion under lake ice, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4917, https://doi.org/10.5194/egusphere-egu22-4917, 2022.

Global surface water bodies suffer multitude of human induced pressures that lead to the deterioration of water status through conditions of anoxia, eutrophication, pollution, depletion, among others, which compound water scarcity. Climatic and socio-economic changes will most likely exacerbate the water crises. However, effective water governance strategies can be deployed to mitigate water crisis and guarantee sustainable water use and ecosystem health.  Such strategies can be developed and implemented using a holistic modelling approach.  In this study, we aim at developing – through re-analysis and modelling – an adaptive water governance framework that can be utilized to ensure sustainable water-use and ecosystem health. The study will be piloted at the Möhne reservoir in the North Rhine-Westphalia state, Germany – representing a multi-decadal time machine for hydroclimatic changes, socioeconomic dynamics and water governance by the Ruhrverband. The study will address five key questions; i) How did the drivers and pressures of the reservoir water status change over time? ii) How did the water governance structures change/respond over time? iii) How did the water quality and quantity change over time? iv) What are the likely future drivers, pressures and governance structures/responses? v) Are there strategies to compare with or transfer to/from other systems?

In order to reconstruct the drivers, pressures, status and impacts of the Möhne reservoir waters, with the respective management and policy responses over time, we start by re-analyzing the multi-decadal trajectory of a few representative variables. In this poster we highlight this using air temperature and precipitation, reservoir water level and population changes in the catchment.

How to cite: Nakulopa, F., Borchardt, D., Marcé, R., Rinke, K., and Bärlund, I.: Solutions for existing and future challenges in water governance, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5429, https://doi.org/10.5194/egusphere-egu22-5429, 2022.

Convective sea-effect snowfall (snow band) can develop in the Baltic Sea when cold air masses are advected from the mainland over a relatively warm open sea. Snow bands may last for several days over the Baltic Sea and, depending on the wind direction, move towards the Finnish coast. To investigate the spatial and temporal characteristics of snow bands in Finland and statistics of conditions favoring their formation, we used a set of detection criteria together with ERA5 reanalysis at a spatial grid spacing of 0.25° (~31 km) for a 48-year time period (1973–2020). Daily changes in snow depth over land areas were studied from FMIClimGrid gridded observational data. Only snow band cases when snow fell over the Finnish mainland was considered. Based on the ERA5 and FMIClimGrid data, we found on average 16 snow band days (SBD) per year. On average, the accumulated snow depths during SBD were moderate, daily mean varied between 2 cm/day to 5 cm/day in the studied regions along the coast of Finland. The largest daily mean snow accumulation (3.5–5 cm) during SBD was observed over the southern coast, but the largest daily snow depth increase (67 cm in January 2016) in the gridded data set was detected in the western coast of Finland. Neither the annual number of snow band days nor the daily snow accumulation revealed statistically significant changes due to large variations between years. The months of November and December showed the highest frequency of SBD. However, the seasonal cycle of SBD seemed to be shifting one month forward as the decrease in the number of SBD during December as well as the increase during January and February were statistically significant in Finland. The long-term changes in sea surface temperature (SST) and air temperature at atmospheric level of 850 hPa (T850) were in line with the changes in occurrence of SBDs. SST increased in all months during 1973–2020 in northern Baltic Sea. In December, when the decrease in snow band days was largest, also the T850 increased indicating less cold air masses occurring in Finland. So, even with increased SST the temperature difference favoring snow band formation might not reach the minimum threshold (13 °C) to produce snow bands due to too warm air temperatures. On the contrary, during January and February the increased SST together with no changes in T850 could favor the formation of snow bands.

How to cite: Olsson, T., Luomaranta, A., Jylhä, K., and Nyman, H.: Statistics of sea-effect snowfall in Finland based on ERA5 and FMIClimGrid, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8356, https://doi.org/10.5194/egusphere-egu22-8356, 2022.

Within the framework of this study, a three-dimensional numerical model of biochemical processes was developed, which complements the model of thermohydrodynamics of the lake. The proposed model includes equations to describe the transport, diffusion, and reactions of dissolved gases. To compare the calculations with the measurement data, the bottom topography and atmospheric forcing were taken into account. For the flux of short-wave radiation, the parameterization of the extinction coefficient was implemented. To assess the contribution of density stratification and velocity shear to the processes of small-scale turbulence in lakes, a modification of the standard two-parameter k-epsilon closure was proposed. The basis of the parameterization is the model that reproduces mutual transformation of the kinetic and potential energy of turbulent pulsations.

The work was supported by the RFBR (20-05-00776), by Moscow Center of Fundamental and Applied Mathematics (agreement with the Ministry of Science and Higher Education 075-15-2019-1621), and by grant of the RF President’s Grant for Young Scientists (MD-1850.2020.5)

How to cite: Gladskikh, D., Mortikov, E., and Stepanenko, V.: Three-dimensional simulation of biochemical processes in inland waters, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8439, https://doi.org/10.5194/egusphere-egu22-8439, 2022.

The features of turbulent heat and mass transfer in a stratified fluid exposed by periodical inhomogeneous volumetric heating are of great practical and fundamental interest. Such phenomena take place in geophysical flows, for example, in ice-covered boreal lakes in spring, where the mechanisms and efficiency of mixing of water masses has a great effect on chemical and biological processes in lakes [1, 2]. Detailed numerical modeling of such flows coupled with experimental observations makes it possible to reveal some important aspects of the structure and parameters of under-ice turbulence, the nature and properties of its anisotropy, difference in the spectra of vertical and horizontal pulsations, and features of energy transfer. This work presents preliminary results of both experimental and numerical investigations of the radiatively-driven free turbulent under-ice convection. The aim of this work is to study the initial stages of the formation and development of a convective mixed layer as well as comparison with obtained experimental data. Numerical simulation is based on the results presented in [3], where the LES study of the development of the convective mixed layer under constant radiation heating was considered. In this study, the radiation heat flux at the ice-water interface is a periodic function evaluated by approximation of the experimental data presented at [4]. These data were obtained during investigations of the under-ice convection in the lake Vendyurskoe at springtime of 2020. The computations were carried out using the in-house finite-volume «unstructured» code SINF/Flag-S developed at Peter the Great St. Petersburg Polytechnic University. We show that the results on the rates of temperature increase and deepening of the convective mixed layer are in good agreement with our experimental data.

The study is supported by the Russian Science Foundation under grants no. 21-17-00262 “Mixing in boreal lakes: mechanisms and its efficiency”.

REFERENCES

1. Bouffard, D., Wüest, A., 2019. Convection in Lakes. Ann. Rev. of Fluid Mechanics 51: 189-215.

2. Bouffard, D., Zdorovennova, G., Bogdanov, S. et al, 2019. Under-ice convection dynamics in a boreal lake. Inland Waters 9: 142-161.

3. Mironov, D., Terzhevik, A., Kirillin, G., et al, 2002. Radiatively driven convection in ice-covered lakes: Observations, scaling, and a mixed layer model. J. Geophys. Res. 107: 1-16.

4. Bogdanov, S.R., Zdorovennov, R.E., Palshin N.I. et al, 2021. Deriving of turbulent stresses in a convectively mixed layer in a shallow lake under ice by coupling two ADCPS. Fundamental and Applied Hydrophysics 14: 17-28.

How to cite: Smirnov, S., Smirnovsky, A., Bogdanov, S., Zdorovennov, R., Palshin, N., Efremova, T., Terzhevik, A., and Zdorovennova, G.: The radiatively-driven turbulent convection in ice-covered lake: numerical and observational study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9182, https://doi.org/10.5194/egusphere-egu22-9182, 2022.

Coastal regions accumulate particulate matter, including pollutants, that are brought into the lake from surrounding watersheds. In order to assess dispersion of these substances into the lake interior, it is important to understand the near-shore hydrodynamics and resuspension processes. In winter, cascading of near-shore cold water, caused by differential cooling, induces relatively intense density currents down the sloping bottom lake boundary. Previous field measurements in Lake Geneva have shown signs of resuspension in this cascading flow, which transports near-shore water towards the deep lake interior. However, the importance of sediment resuspension and transport could not be determined because hydrodynamic studies of those cross-shore flows were lacking the necessary resolution. With the recent advances in instrument capabilities, we were able to collect detailed field data in the near-shore bottom boundary layer in Lake Geneva. Using a unique high spatial and temporal resolution dataset, we present results on the hydrodynamics of winter cascading and their implications for sediment resuspension and transport.

Acoustic Doppler Current Profilers (ADCPs) and vertical thermistor lines were deployed during winter on the northern shore of Lake Geneva on the shallow shelf and along the sloping lakebed. In addition, CTD (Conductivity-Temperature-Depth) profiles were taken during periods of strong cooling in order to obtain a broader view of the temperature field along a cross-shore transect. After a cold, calm night, strong differential cooling develops between the shallow shelf and the open lake, initiating the flow of cold dense water from the shelf as pulses down the sloping lakebed. Analysis shows that the maximum in the velocity profile is relatively close to the bed, as expected for a density current. During large and intense pulses, the flow is thick enough to reveal details of the velocity profiles close to the boundary. As expected for a boundary flow, the measured profiles are logarithmic. From the profile shape, the bottom shear stress can be estimated by applying the law of the wall, thus assessing the potential of sediment resuspension with the classic incipient motion approach from Shields.

We find that within the cascading flow, favorable conditions for resuspension are intermittent which is supported by ADCP backscattering. At 30-m depth, sediment transport towards the lake interior is more likely to occur during cascading flow events than under other flow conditions during winter, including those linked to strong wind events. Indeed, during the measurement period (mid-December to March), highest near-bottom cross-shore currents with bottom shear stresses exceeding the threshold of motion were recorded during cascading flows. They are also more frequent than cross-shore currents induced by wind-driven events. Results of this study suggest that winter cascading is very efficient in renewing near-shore waters and that during weakly-stratified periods, near-shore sediment could be transported into the deep interior by strong cascading events.

How to cite: Mettra, F., Reiss, R. S., Lemmin, U., and Barry, D. A.: Sediment resuspension and transport from near-shore zones towards the deep interior of Lake Geneva caused by winter cascading, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10100, https://doi.org/10.5194/egusphere-egu22-10100, 2022.

Wetlands represent 15% of global organic carbon storage and act as natural “blue carbon”, playing a significant role in the global carbon sink. However, due to climate change and anthropogenic activities (such as desiccation), they can become an important atmospheric CO₂ source.  In many arid areas, lagoons may constitute the greatest part of the natural waters in temperate latitudes. They are usually very shallow or even temporary since evaporation exceeds precipitation. The most common lagoons are saline lakes in endorheic basins, which are strongly dependent on the hydrological budget. “Fuente de Piedra” lake (hereafter FdP) is a shallow and saline endorheic lagoon located in the province of Málaga, in the south of Spain. It is an important nature reserve because of its population of nesting flamingos in the Western Mediterranean, and is also the largest lagoon in Andalusia and is part of Ramsar since 1999. Based on previous studies, we hypothesize that FdP will be a net sink of CO₂ but probably a yearly source of CH₄ and N₂O. However, its magnitude is still to be determined. Regarding the effect of drought, due to the contradictory results found in the literature, it is difficult to predict how GHG balances will behave during periods of drought and flooding. Therefore, the main objective of this study is to quantify CO₂, CH₄ and N₂O fluxes in FdP and their seasonal variability. In this regard, the Picarro G2508 spectrometer, is being used every 15 days in four locations over a transect (dry sediments, wet sediments, shore and lagoon) to measure CO₂/CH₄/N₂O fluxes since March 2021. At the same time, the eddy covariance technique is being used since August 2021 to quantify CO₂, CH₄ and H₂O exchanges at the ecosystem level. Positive values of fluxes denote a net release to the atmosphere, while negative values indicate a net uptake. Preliminary results of Picarro measurements show that, during the drought period there is a significant effect of salinity for CO₂ emissions with maximum value 0.3 µmol m^-2 s^-1 when sediments are covered by salt and 2.8 µmol m^-2 s^-1 when salt was removed. Regarding measurements of the transect, during the flooding period CO₂ and CH₄ fluxes ranged respectively from -3 µmol (CO₂) m^-2 s^-1 and 0.008 µmol (CH₄) m^-2 s^-1 in the lake to 1.7 µmol (CO₂) m^-2 s^-1 and zero (CH₄) µmol m^-2 s^-1 in the dry sediments. On the other hand, no N₂O emissions where detected. Regarding the eddy covariance measurements at the ecosystem level, CO₂ and CH₄ flux values ranged from 10 µmol (CO₂) m^-2 s^-1 and 0.01µmol (CH₄) m^-2 s^-1 to zero µmol (CO₂) m^-2 s^-1 to -0.05 (µmol CH₄ m^-2 s^-1) during drought period (no measurements for the flooding period were taken yet). As a preliminary conclusion FdP seems to act as a source of CO₂ during the drought period, while for CH₄, FdP seems to act as a slight sink. However, more measurements are needed in order to provide stronger conclusions about the drought and the flooding period.

How to cite: Alfadhel, I., Reche, I., Sánchez-Cañete, E. P., Peralta, I., David Aguirre-García, S., Abril-Gago, J., Kowalski, A. S., Domingo, F., and Serrano-Ortiz, P.: The influence of drought and salinity on Greenhouse Gas emissions in “Fuente de Piedra” endorheic lagoon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10108, https://doi.org/10.5194/egusphere-egu22-10108, 2022.

Topic:

Wetlands provide important ecosystem services,  e.g. for biodiversity and water resources. The salt pans in the Neusiedler See – Seewinkel National Park are a unique ecosystem for animal and plant species especially adapted to the extreme conditions prevailing in and around the salt pans. The conservation of the salt pans is largely based on the interplay with the water balance of the area and the influence of physical as well as anthropogenic factors. The large number of lakes, which are often only filled with water for a short time due to their shallow depth, makes monitoring by means of installed gauges more difficult. Remote sensing, on the other hand,  is an important source of consistent  information in space and time. In the FEMOWinkel project, which is funded within the framework of the Austrian StartClim 2021 program, long time series of multispectral satellite data are exploited for monitoring and data-driven modeling of water extent in the salt pans.

The water bodies are delineated based on image time series of the Landsat satellites, which have been providing data almost continuously since the 1980s. Cloud gaps will partly be filled with radar-based information provided by European Copernicus – Programme. The derived time series of water body extent and number are validated by comparison with aerial photographs and – where available – water levels. The salt pans are characterized with respect to their seasonal variability and their reaction to longer-term changes in water availability by comparison with ancillary data, e.g., surface and groundwater levels, climate data and other remote sensing products, such as soil moisture and vegetation indices. In the third step, a data-driven modeling of the lake extent is carried out using machine learning methods. We will also address the question of whether it is possible to predict the effects of dry or wet winters and springs on water body extent in the following summer using these methods.

Preliminary results of the time series analysis show a pronounced dynamic in the extent of the water bodies over the course of the study time. Periods, e.g., from 1990 to 1993 and 2001 to 2007, in which some of the lakes fell dry, alternated with wetter periods, e.g., from 1994 to 1999, in which the salt pans remained at least partially filled even in summer. These differences correlate with drought indicators such as the Standardised Precipitation-Evapotranspiration Index (SPEI). The remote sensing-based approach will make it possible to transfer the applied methods to other similar ecosystems located in steppe regions.

How to cite: Schauer, H., Schlaffer, S., Büechi, E., and Dorigo, W.: Remote sensing-based monitoring of the water surfaces in the Neusiedler See – Seewinkel National Park, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11157, https://doi.org/10.5194/egusphere-egu22-11157, 2022.

The Haringvliet is a former estuary in the Rhine-Meuse Delta. Since 1970, seaward outflow is regulated with floodgates, while seawater is kept out. To improve fish migration and the ecological quality of the Rhine-Meuse system, limited seawater inflow during flood has been reintroduced again in 2018. The incoming salt water progresses through the former tidal channels, arrives in deep pits and is partially flushed out again by the outflow, especially during high river discharge. However, the remaining salt water can gradually spread through the system due to wind-induced mixing and circulations, especially when the gates are closed also for outflow during low river discharge. As this can threaten fresh water intakes, inflow, flushing and dispersion of salt need to be well understood and carefully managed.

In this study we analyse velocity measurements from ADCPs at two former tidal channels in the Haringvliet, together with salinity time series and profiles at multiple locations. The salinity profiles show that the system tends to be strongly stratified. Using the ADCP backscatter, we estimated the time development of the interface level to relate this to the local velocity, floodgate discharge and wind. For peak discharges and low wind speed, the velocities show a clear relation with the discharge and the interface can lower abruptly. However, for lower discharges and higher wind speed, the relations are less clear, and the profiles are highly affected by the wind. In case of wind but closed sluices, flow against the wind was found for wind in the systems longitudinal direction. We explain this from the large area of (former) shoals, leading to flow with the wind in shallow parts and against the wind in deep parts due to a local imbalance between stress divergence and pressure gradient. This turns out to be an important driver of landward salt transport, as increased salt concentrations were found at landward locations for seaward wind. Next to that, indications were found of exchange between former tidal channels and transport over sills due to wind driven tilting of the salinity interface.

Enhanced understanding of the salt transport dynamics in this former estuary after reintroduction of limited seawater inflow is an essential element to manage this system, protect the fresh water availability and keep the confidence of critical stakeholders, which is essential for the success of this ecosystem improvement program.

How to cite: Kranenburg, W., Tiessen, M., Blaas, M., and van Veen, N.: Circulation, stratification and salt dispersion in a former estuary after reintroduction of seawater inflow to improve fish migration, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11727, https://doi.org/10.5194/egusphere-egu22-11727, 2022.

Randomly distributed patches of smooth or rough/rippled surfaces are readily observed on most water bodies. Smooth surface patches are called natural slicks and typically form under low wind conditions (< 6 m s^-1) when biogenic surfactants in the surface microlayer accumulate above a certain threshold. They may have spatial scales from tens of meters to kilometers. Slicks suppress the formation of wind-induced Gravity-Capillary Waves (GCW), leading to altered surface reflectance of light and microwaves and can also affect near-surface turbulent motions. Therefore, it is of interest to determine the effect slicks can have on the air-water exchange of momentum, heat, and gas, which can influence the biogeochemical dynamics in the near-surface layer of lakes and oceans.

We investigated the spatiotemporal variability in momentum flux caused by slicks in Lake Geneva during several field campaigns using eddy covariance instrument setups mounted on an autonomous catamaran. The measurements were combined with aerial and shore-based imagery (both RGB and thermal). In addition, surface microlayer sampling was conducted from an accompanying boat to determine whether visually-identified smooth patches were associated with higher enrichments of fluorescent dissolved organic matter, a proxy for natural surfactants. Wavelet analysis was used to explore short-time (~1 min) averaged air-water exchange variations related to the transition from smooth slicks to rough surface areas that cannot be captured by the conventional eddy covariance analysis method.

Our results suggest that under light wind conditions and in the absence of short GCW on the surface of slicks, wind stress cannot be effectively transferred to the water, leading to reduced momentum exchange inside slicks compared to the surrounding non-slick areas. This results in lateral gradients in vertical mixing that can affect air-water exchange processes and contribute to spatial variability in surface temperature and near-surface heat content.

How to cite: Foroughan, M., Lemmin, U., and Barry, D. A.: The effect of natural surfactants on air-water momentum exchange under light wind conditions in Lake Geneva, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12027, https://doi.org/10.5194/egusphere-egu22-12027, 2022.

In many deep lakes, global warming is weakening wintertime convective cooling, thus reducing the occurrence of complete vertical overturning. At the same time, our understanding of the deepwater dynamics in deep lakes remains elusive, mainly because spatiotemporally resolved in situ measurements are lacking. We address this knowledge gap by exploring the dynamics in the deep hypolimnion of Lake Geneva (max. depth 309 m) by means of extensive field observations.

Due to its great depth and the mild central European climate, Lake Geneva remains weakly stratified during most years, with complete convective overturning only occurring during severely cold winters. Recent studies show that three-dimensional (3D) transport processes, such as cold-water density currents, coastal upwelling, and wind-driven interbasin exchange significantly impact the dynamics in Lake Geneva’s deep hypolimnion, contributing to its ventilation.

From February to July 2021, five moorings equipped with Acoustic Doppler Current Profiles (ADCPs), current meters, thermistors and Dissolved Oxygen (DO) loggers were deployed at different locations and depths across the ~300-m deep central plateau (~12 km × 6 km) of Lake Geneva. One mooring remained in place until December 2021.

The nearly year-long measurements show frequent, large temperature peaks of ~0.03-0.15°C that last ~1-10 d at ~300-m depth, indicating significant isotherm tilting and downward transport of warmer waters, corresponding to vertical excursions of ~30-80 m. During those events, near-bottom DO levels temporarily increase by ~1-2 mg l^-1. Furthermore, the different mooring sites reveal large spatial heterogeneity across the 300-m deep plateau, both in the magnitude and temporal variability of the observed peaks.

From February to December 2021, a mean warming of ~0.07°C was observed at 300-m depth. Over longer periods, a “saw-tooth” pattern was previously found in deep lakes, which consists of continuous warming over several years, interrupted by sudden cooling during particularly cold winters (not the case during our campaign). In contrast, mean DO levels at 300-m depth first increase during spring, stagnate in early summer, and then gradually decrease until late fall/early winter.

Rotary spectra of the current velocities in the deepest layers show a broad peak in the clockwise-rotating component close to, but below the local inertial period (~16.5 h), in agreement with recent findings of dominant clockwise-rotating inertial currents in Lake Geneva’s deep hypolimnion. However, rotary wavelet analysis further reveals that the broad peak in the clockwise spectra is composed of several distinct frequency bands, concentrating mainly at ~16 h and ~12-14 h. The latter is close to the internal Poincaré wave period, as reported in the literature, indicating that both inertial currents and near-inertial internal waves are important.

Altogether, these preliminary results demonstrate that Lake Geneva’s deep hypolimnion is surprisingly energetic and characterized by a strong spatial heterogeneity that can only be explained by large-scale 3D flow features, challenging the classic one-dimensional concept of deepwater renewal in large, deep lakes. In the next step, a validated 3D hydrodynamic model will be used to further investigate the observed temperature/DO peaks and trends, the ever-present oscillating currents, as well as determine the origin of these processes.

How to cite: Reiss, R. S., Lemmin, U., and Barry, D. A.: Deepwater dynamics and spatial heterogeneity observed in the deep hypolimnion of a large lake (Lake Geneva), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12536, https://doi.org/10.5194/egusphere-egu22-12536, 2022.