HS10.9
Lakes and Inland Seas in a Changing Environment

HS10.9

Lakes and Inland Seas in a Changing Environment
Convener: Georgiy Kirillin | Co-conveners: Giulia Valerio, Tom Shatwell, Peter Zavialov, Damien Bouffard
Presentations
| Fri, 27 May, 13:20–16:40 (CEST)
 
Room 2.44

Presentations: Fri, 27 May | Room 2.44

Chairpersons: Giulia Valerio, Tom Shatwell, Peter Zavialov
13:20–13:23
13:23–13:29
|
EGU22-272
|
ECS
|
Highlight
|
On-site presentation
Mahdi Akbari, Björn Klöve, Omolbanin Faramrazzadeh, and Ali Torabi Haghighi

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.

13:29–13:35
|
EGU22-492
|
ECS
|
On-site presentation
|
Iva Dominović, Marija Marguš, Sunčana Geček, Tatjana Bakran-Petricioli, Donat Petricioli, Mathieu Dutour Sikirić, and Irena Ciglenečki

Lake Rogoznica (also known as "Dragon's Eye") is a karstic, marine lake on the Gradina peninsula located at the Adriatic coast at 43° 32' N and 15° 58' E. Most of the time the lake is stratified, with an upper oxic layer, an anoxic bottom layer, and a chemocline in between. Every few years the stratification suddenly breaks down and the entire water column becomes mixed, anoxic, and euxinic, with HS- presence throughout the water column. This leads to mass mortality of aerobic populations in the lake, which require long periods of time without mixing to recover. Rogoznica residents confirmed that the sudden overturning of layers had been occurring even before continuous research began in 1992, but also that it used to happen less frequently. In the last 30 years, five such events of complete anoxia have been recorded: in September 1997, October 2011, October 2016, October 2020, and October 2021. As the sudden mixing now occurs year after year, the lake's ecosystem does not have nearly enough time to recover. Previous work has indicated that the main trigger for the abrupt mixing is a sudden drop in surface temperature caused by an overpassing low-pressure system. Nevertheless, the process of overturning and sudden release of bottom-layer sulfides is a very delicate one, and determining other biological, physical, and chemical triggers is an important question that remains to be answered. Another key question is whether the increase in the overturn frequency is solely a part of the natural life cycle of the lake, a result of the changing climate with more extreme weather events, or a more direct consequence of human activities in the area.

Comparison of the most recent water level measurements from June 2021 with those from 2013 indicate that the tidal signal in the lake requires a somewhat different analytical approach than the standard ocean tidal analysis procedure. Moreover, measurements at the boundaries of the lake show that the water entering the lake from the karst at high tide is not only colder but also has a lower salinity. Additionally, in this work we present new insights into the physicochemical properties of the lake's water column (σT-stratification, dissolved oxygen concentration) and the direct influence of atmospheric wet deposition on the lake's surface layer.

How to cite: Dominović, I., Marguš, M., Geček, S., Bakran-Petricioli, T., Petricioli, D., Dutour Sikirić, M., and Ciglenečki, I.: Physicochemical properties of a marine lake in the central Adriatic (Lake Rogoznica): interaction with the atmosphere, the sea and the surrounding karst, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-492, https://doi.org/10.5194/egusphere-egu22-492, 2022.

13:35–13:41
|
EGU22-1165
|
ECS
|
Highlight
|
On-site presentation
David Rudberg, Jonathan Schenk, Gustav Pajala, Henrique Sawakuchi, Anna Sieczko, Jan Karlsson, Sally MacIntyre, John Melack, Ingrid Sundgren, and David Bastviken

Lakes emit CO2 to the atmosphere at magnitudes significant for the global carbon cycle, in the form of diffusive CO2 flux (FCO2). As direct FCO2 measurements are time-consuming, FCO2 is often estimated from the air-water CO2 concentration gradient (ΔpCO2) and the gas transfer velocity (k), representing the two components considered to regulate FCO2. 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 ΔpCO2 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 FCO2 and ΔpCO2, 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 ΔpCO2 and k to FCO2 variability over time; and ii) if differences in the contributions of ΔpCO2 and k to FCO2 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.

13:41–13:47
|
EGU22-1550
|
Highlight
|
On-site presentation
Pavel Kishcha, Yury Lechinsky, Boris Starobinets, and Pinhas Alpert

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.

13:47–13:53
|
EGU22-2191
|
ECS
|
Virtual presentation
Niki Simonović, Marija Marguš, and Irena Ciglenečki

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.

13:53–13:59
|
EGU22-2787
|
ECS
|
Virtual presentation
|
Margarita Choulga, Souhail Boussetta, Gianpaolo Balsamo, and Joe McNorton

In 2015 lake parametrization was introduced at European Centre for Medium-Range Weather Forecasts (ECMWF) to take into account the impact of lake on the boundary layer and near surface atmosphere. All inland water bodies (lakes, reservoirs, rivers, coastal waters) have constant size distribution and are simulated by the Fresh-water Lake model FLake.

To introduce water seasonality and reduce errors caused by the constant water distribution, time-varying water maps based on Global Surface Water Explorer (GSWE) data are being introduced in the ECMWF ECLand system. Techniques used to adapt GSWE to create monthly water fraction maps for the use of global NWP modelling and first evaluation results will be presented.

How to cite: Choulga, M., Boussetta, S., Balsamo, G., and McNorton, J.: Representing seasonal water in ECMWF ECLand system, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2787, https://doi.org/10.5194/egusphere-egu22-2787, 2022.

13:59–14:05
|
EGU22-2858
|
ECS
|
On-site presentation
|
Agnieszka Szczerba, Sergi Pla-Rabes, Maurycy Żarczyński, and Wojciech Tylmann

Chrysophyte cysts are considered as good environmental indicators because of short generation times, seasonal replacement, and their high sensitivity to changes in physicochemical conditions. In our study, we explored the relationship between chrysophyte cysts and changes in meteorological conditions in two lakes located in northern Poland (Łazduny and Rzęśniki). We compared samples collected in sediment traps, results of on-site monitoring of limnological and hydrochemical variables, and meteorological data. Multiple statistical analyses showed that meteorological conditions indirectly influence cyst seasonality, through changes in the mixing regimes that determine nutrient and light availability in lakes. Even though the taxonomic structure and interannual variability of chrysophyte cysts are dependent on multiple variables, air temperature proved to be the most important meteorological variable influencing cyst assemblages. Multi-level pattern analysis showed that specific cyst types were indicative of different periods of lake physical structure, thus suggesting the potential of chrysophyte cysts in paleoclimatic studies.

How to cite: Szczerba, A., Pla-Rabes, S., Żarczyński, M., and Tylmann, W.: Chrysophyte cysts reflect seasonal meteorological and limnological conditions: Evidence from sediment trap study in northeast Poland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2858, https://doi.org/10.5194/egusphere-egu22-2858, 2022.

14:05–14:11
|
EGU22-4209
|
Presentation form not yet defined
The roughness length for momentum, heat and moisture for small water bodies (lakes, rivers, reservoirs)
(withdrawn)
Irina Repina, Arseniy Artamonov, and Alexander Malkov
14:11–14:17
|
EGU22-4917
|
Highlight
|
On-site presentation
Marie-Elodie Perga, Camille Minaudo, Hugo Ulloa, Tomy Doda, Pascal Perolo, Nicolas Escoffier, Florent Arthaud, Biel Obrador, and Damien Bouffard

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.

14:17–14:23
|
EGU22-5429
|
ECS
|
Highlight
|
On-site presentation
Faluku Nakulopa, Dietrich Borchardt, Rafael Marcé, Karsten Rinke, and Ilona Bärlund

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.

14:23–14:29
|
EGU22-5713
|
ECS
|
Highlight
|
Presentation form not yet defined
|
Sofia La Fuente, Iestyn Woolway, and Eleanor Jennings

Global lake evaporation is a critical and continuous process that plays an important role in the earth’s water cycle. Accurate quantification of lake evaporation dynamics is crucial to understanding lake energy budgets, land-atmosphere interactions, as well as water availability. However, despite its importance, relatively few studies have investigated the impacts of climate change on global lake evaporation. In this study, we present global lake evaporation projections from 1901-2099 using an ensemble of lake-climate projections from the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP). Our results show that global lake evaporation will increase by the end of the 21st century with the largest changes occurring in tropical regions. Furthermore, our analysis suggests that lake evaporation extremes (90th percentile) are projected to occur more frequently, with greater changes detected at low latitudes. We anticipate lake evaporation increases to have severe impacts on the water budget, and therefore, on the availability of surface freshwater this century. 

How to cite: La Fuente, S., Woolway, I., and Jennings, E.: Global lake evaporation responses to climate change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5713, https://doi.org/10.5194/egusphere-egu22-5713, 2022.

14:29–14:35
|
EGU22-5844
|
ECS
|
On-site presentation
|
Stan Thorez, Koen Blanckaert, Ulrich Lemmin, and David Andrew Barry

Negatively buoyant riverine inflows plunge when entering lakes or reservoirs and form gravity-driven currents near the bed. When a high sediment load causes the density excess, such currents are called turbidity currents. They can supply momentum, heat, oxygen, sediments, nutrients and contaminants to deep lake basins and are the main cause of reservoir storage capacity loss. Even with steady inflow, turbidity currents have been observed to exhibit regularly pulsing velocity patterns, which are likely to enhance mixing between the inflowing and the ambient waters. Previously, literature linked this pulsing to several mechanisms, including interfacial waves such as Kelvin-Helmholtz instabilities and Rayleigh-Taylor instabilities related to surface lobes along the plunge line. However, to our knowledge, field measurements of the latter have not been reported.

In the present study, field measurements of the plunging inflow of the negatively buoyant Rhône River into Lake Geneva (Switzerland/France) were carried out. Vessel-mounted ADCP measurements of the longitudinal flow field of the plunging flow and the subsequent turbidity current were combined with remote time-lapse imagery capturing related surface patterns.

The ADCP measurements confirm that the inflowing river water plunges and forms a turbidity current. At the turbidity current-ambient water interface, regularly spaced “bulges” in the velocity pattern suggest pulsing. Simultaneously taken remote time-lapse images show that at the surface, the inflowing sediment-rich water forms a distinct plume with a triangular shape leading away from the river mouth in the downstream direction towards a sharp tip. At the edge and in the immediate surroundings of this plume, a variety of intermittent lobes and vortical structures whose periodicity might be related to that of the velocity pulsing in the turbidity current, is observed.

How to cite: Thorez, S., Blanckaert, K., Lemmin, U., and Barry, D. A.: Evidence and possible causes of velocity pulsing in a turbidity current in Lake Geneva, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5844, https://doi.org/10.5194/egusphere-egu22-5844, 2022.

14:35–14:41
|
EGU22-6081
|
Highlight
|
Virtual presentation
The North (Small) Aral Sea after 15 years of recovering: physical and biogeochemical outcomes of the lake-wide conservation experiment.
(withdrawn)
Alexander Izhitskiy and Georgiy Kirillin
14:41–14:47
|
EGU22-7815
|
Highlight
|
Presentation form not yet defined
|
Georgiy Kirillin, Liu Liu, Asiya Murakaeva, Hauke Dämpfling, and Hans-Peter Grossart
Natural waters in urban and suburban areas experience growing stresses from active water use and recreational activities, such as leisure boating, fishing, and swimming. Among other factors, leisure swimmers are considered to have a minor effect on the large-scale dynamics of water bodies. Certain physical processes are however very sensitive to disturbances of boundary layers - a thin diffusive layer at the lake surface and the upper layer of sediment. During hot summers, a high concentration of swimmers in small lakes and ponds may disrupt the boundary layers, intensifying the vertical heat and mass exchange and producing localized outbursts of methane into the atmosphere and(or) release of the dissolved nutrients from sediment to the water column. We performed a series of experiments in the suburban area of Berlin, Germany to estimate the potential swimmer effects on the vertical heat and mass transport. Monitoring of physical and water quality parameters in small (~2 km2) Lake Mellensee revealed a consistent  increase of turbidity and decrease of transparency in the vicinity of the beach actively visited by weekend swimmers from Berlin and surroundings. The measured concentrations of the dissolved methane and the methane fluxes at the lake surface, while indicating potential increase due to localized swimming activities, were heterogeneous and depended strongly on variations in sediment composition and on wind conditions. To quantify the effects on the lake-atmosphere fluxes, we performed estimations of the surface temperature disturbance by swimmers in a large mesocosm facility using UAV-based multispectral and infrared cameras. A manifold increase of the surface heat flux derived from the root-mean-square temperature fluctuations results from the diffusive layer disruption and implies a proportional increase of the dissolved gas release.

How to cite: Kirillin, G., Liu, L., Murakaeva, A., Dämpfling, H., and Grossart, H.-P.: How do recreational swimmers affect lake-atmosphere and lake-sediment transport?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7815, https://doi.org/10.5194/egusphere-egu22-7815, 2022.

Coffee break
Chairpersons: Georgiy Kirillin, Damien Bouffard
15:10–15:16
|
EGU22-8050
|
ECS
|
Presentation form not yet defined
A multispectral remote sensing monitoring of the temporal and spatial extent of whiting events in Lake Geneva.
(withdrawn)
Gaël Many, Michele Ferrari, Nicolas Escoffier, Pascal Perolo, Isabel Herr, and Marie-Elodie Perga
15:16–15:22
|
EGU22-8305
|
On-site presentation
|
Elena Shevnina

The summertime evaporation over a large shallow lake located in the Schirmacher oasis, Dronning Maud Land, Antarctica. Lake Zub/Priyadarshini is the second largest lake in the oasis,  its maximum depth is  6 m. The lake is among the warmest lakes, and it is free of ice during almost two summer months. The summertime evaporation over the water table of the lake was estimated after the eddy covariance (EC) method, the bulk aerodynamic method and Dalton type empirical equations. We used special meteorological and hydrological measurements collected during the field experiment carried out in 2018 in addition to the standard observations at the nearest meteorological site. 

The EC method was considered as the most accurate given a reference for other estimates of evaporation over the lake water surface. We estimated the evaporation over the ice free lake surface as 114 mm in the period from 1 January to 7 February 2018 (38 days) after the direct EC method. The average daily evaporation is estimated to be 3.0 mm day-1 in January 2018. The largest changes in the daily evaporation were driven by the synoptic-scale atmospheric processes rather than local katabatic winds. 

The bulk aerodynamic method suggests the average daily evaporation to be 2.0 mm day-1 , and it is over 30 % less than the EC method. This method is much better in producing the day-to-day variations in evaporation compared to the Dalton type semi-empirical equations, which underestimated the evaporation over the lake open water table for over 40–72 %. We also suggested a linear empirical relationship to evaluate the summertime evaporation of Lake Zub/Priyadarshini from the observations at the nearest meteorological site and surface water temperature. After this method, the evaporation over the period of the experiment is 120 mm, and it is only 5 % larger than the result according to the EC method. We also estimated the daily evaporation from the ERA5 reanalysis, which suggested the average daily evaporation during austral summer (December – February) 2017–2018 to be 0.6 mm day-1. It is only one fifth of the evaporation estimated with the direct EC method. 

The poster shows the results which were obtained together with Timo Vihma and Tuomas Naakka (Finnish Meteorological Institute, Helsinki, Finland), Miguel Potes (Institute Earth Science, Evora, Portugal), Pankaj Ramji Dhote and Praveen Kumar Thakur (Indian Institute of Remote Sensing, Dehradun, India). The study was funded by the Academy of Finland (contract number 304345) and the COST Snow Action ES1404. The measurement campaigns were supported by the Finnish Antarctic Research Program, the Russian Antarctic Expedition, and the Indian Antarctic expedition.

How to cite: Shevnina, E.: Summertime evaporation over glacial lakes in the Schirmacher oasis, East Antarctica., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8305, https://doi.org/10.5194/egusphere-egu22-8305, 2022.

15:22–15:28
|
EGU22-8356
|
ECS
|
Virtual presentation
Taru Olsson, Anna Luomaranta, Kirsti Jylhä, and Henri Nyman

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.

15:28–15:34
|
EGU22-8439
|
ECS
|
Virtual presentation
Daria Gladskikh, Evgeny Mortikov, and Victor Stepanenko

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.

15:34–15:40
|
EGU22-9026
|
Presentation form not yet defined
|
Stan Benjamin, Tatiana Smirnova, Eric James, Eric Anderson, Ayumi Fujisaki-Manome, and John Kelley

Application of lake models coupled within earth-system prediction models, especially for short-term predictions from days to weeks, requires accurate initialization of lake temperatures.   Here, we describe a lake initialization method by cycling within an hourly updated weather prediction model to constrain lake temperature evolution.   We compare these simulated lake temperature values with other estimates from satellite and in situ and interpolated-SST data sets for a multi-month period in 2021.   The lake cycling initialization, now applied to two operational US NOAA weather models, was found to decrease errors in lake temperature from as much as 5-10K (using interpolated-SST data) to about 1-2 K (comparing with available in situ and satellite observations. 

How to cite: Benjamin, S., Smirnova, T., James, E., Anderson, E., Fujisaki-Manome, A., and Kelley, J.: Inland lake temperature initialization via cycling with atmospheric data assimilation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9026, https://doi.org/10.5194/egusphere-egu22-9026, 2022.

15:40–15:46
|
EGU22-9182
|
ECS
|
Presentation form not yet defined
Sergei Smirnov, Alexander Smirnovsky, Sergey Bogdanov, Roman Zdorovennov, Nikolay Palshin, Tatiana Efremova, Arkady Terzhevik, and Galina Zdorovennova

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.

15:46–15:52
|
EGU22-9521
|
Presentation form not yet defined
Direct measurements of sea currents on the Mangistau shelf of the Caspian Sea 
(withdrawn)
Peter Zavialov and Abilgazi Kurbaniyazov
15:52–15:58
|
EGU22-10100
|
ECS
|
Virtual presentation
François Mettra, Rafael Sebastian Reiss, Ulrich Lemmin, and David Andrew Barry

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.

15:58–16:04
|
EGU22-10108
|
ECS
|
Highlight
|
Virtual presentation
Ihab Alfadhel, Isabel Reche, Enrique P. Sánchez-Cañete, Ignacio Peralta, Sergio David Aguirre-García, Jesús Abril-Gago, Andrew S. Kowalski, Francisco Domingo, and Penélope Serrano-Ortiz

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.

16:04–16:10
|
EGU22-10778
|
ECS
|
On-site presentation
|
Habiba Kallel, Murray Mackay, Antoine Thiboult, Daniel Nadeau, and François Anctil

Freshwater reservoirs modify the regional climate through mass, energy, and momentum exchanges with the atmosphere. Recent studies have shown that hydropower reservoirs tend to evaporate more than the land they have flooded, hence reducing water availability for other uses, at a level that should with local climate conditions, however. Knowing that evaporation is a key component of the water balance and that very few studies have focused on evaporation from northern reservoirs, which are ice covered several months per year, there is a real need for models that can provide reliable estimates of this water vapor flux. This project focuses on the modeling of evaporation from an 85-km2 hydropower reservoir located in the boreal biome of eastern Canada (50.7°N, 63.2°W), with a mean depth of 60 m and an elongated shape. To support this modeling effort, two flux towers (one on the shore and one on a raft) and a vertical chain of thermistors were deployed. Exchanges between the water surface and the atmosphere are simulated with the Canadian Small Lake Model (CSLM), a 1-D physical-based surface scheme designed to be coupled with a numerical weather prediction model. The model also simulates the thermal regime of the water body, including ice formation. Considering the irregular shape of the reservoir as well as its depth, a new model parameterization was adopted that improved simulations (albedo parameterization, leakage parameter, mixed layer maximum depth...). Turbulent fluxes were successfully predicted during the open water period. Comparison between observed and modeled time series showed a good agreement specifically for sensible heat fluxes.  Deviations mostly occur before freeze-up (October to November) and around ice off (April to May) with a tendency of overestimating latent heat fluxes when its observed magnitude is small (ice period). Thermal mixing as well as mixed layer deepening were well estimated. Thermal mixing, as well as mixed layer deepening, were well estimated. Near-surface water temperature confirmed the ability of the CSLM to simulate the near-surface seasonal cycle. However, in early fall, an overestimation of the water temperature induced an overestimation of the heat fluxes leading to early depletion of the energy storage that led to an early modeled freeze-up.

How to cite: Kallel, H., Mackay, M., Thiboult, A., Nadeau, D., and Anctil, F.: Validation of a 1-D lake model for modeling evaporation from an elongated and deep boreal reservoir, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10778, https://doi.org/10.5194/egusphere-egu22-10778, 2022.

16:10–16:16
|
EGU22-11157
|
ECS
|
Presentation form not yet defined
Henri Schauer, Stefan Schlaffer, Emanuel Büechi, and Wouter Dorigo

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.

16:16–16:22
|
EGU22-11727
|
Highlight
|
Presentation form not yet defined
Wouter Kranenburg, Meinard Tiessen, Meinte Blaas, and Nathalie van Veen

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.

16:22–16:28
|
EGU22-12027
|
ECS
|
Virtual presentation
Mehrshad Foroughan, Ulrich Lemmin, and David Andrew Barry

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.

16:28–16:34
|
EGU22-12536
|
ECS
|
Virtual presentation
Rafael S. Reiss, Ulrich Lemmin, and D. Andrew Barry

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.

16:34–16:40
|
EGU22-13350
|
Presentation form not yet defined
|
Sina Shahabi-Ghahfarokhi, Mahboubeh Rahmati-Abkenar, Leonie Jaeger, Sarah Josefson, Anna Apler, Henric Djerf, Changxun Yu, Mats Åström, and Marcelo Ketzer

The eutrophication of the deep central Baltic Sea, Baltic Proper, has caused deoxygenation severe consequences for benthic life. To overcome such limitations and spreads of anoxic basins, proposed techniques such as re-oxygenation of anoxic bottom waters are proposed. However, in the case of the Baltic Proper, the effects of oxygenation in the short term are unknown. Therefore, this research focuses on understanding the geochemistry changes in bottom waters' and sediments during the transition from anoxic/hypoxic to oxic conditions. Six sediment cores were retrieved from the northern and southern Baltic Proper (triplicate cores for each location), where bottom waters have dissolved O2 concentrations of ≈0 mg/L. The bottom waters were exchanged with oxygenated Baltic Sea surface waters for 96 hours. The pH, electroconductivity (EC), and metal concentrations in the exchanged water were measured over time in 12 and 24 hours intervals in the experiment cores. The results indicate that the pH of both sites didn't show any significant change in the exchanged bottom waters during the experiment. However, the EC of the bottom waters on average reduced from 15 to 9 µs/m in both sites. In terms of soluble metals/metalloids, As, Ba, Co, Mn, Mo, Rb, Sr, and U were detected in higher concentrations than unoxygenated bottom water from southern Baltic Proper. Manganese and Sr showed the highest released concentrations in both sites in terms of concentration. This study indicates that undesired release of metals from sediment to the water column may occur during re-oxygenation of Baltic Sea bottom waters. The next stage of this research will focus on the metal transfer from the surface sediments to the bottom waters over the 4-day experiments. This will be done via a sequential chemical analysis scheme, in which surface sediment samples from the experiment will be compared to reference cores collected at the same time as the experimental cores were collected.

How to cite: Shahabi-Ghahfarokhi, S., Rahmati-Abkenar, M., Jaeger, L., Josefson, S., Apler, A., Djerf, H., Yu, C., Åström, M., and Ketzer, M.: The effects of re-oxygenation of central Baltic Sea sediment; a laboratory study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13350, https://doi.org/10.5194/egusphere-egu22-13350, 2022.