Oxygen depletion in Lake Geneva

Low oxygen concentrations remain a global concern for the ecological health of lakes. High nutrient inputs and climate-induced changes in stratification and mixing are anthropogenic threats which largely impact aquatic oxygen budgets and overall ecosystem health. In this thesis, the relevant processes for hypolimnetic oxygen depletion were investigated on different temporal and spatial scales in the deep perialpine Lake Geneva and the corresponding total oxygen budget was estimated. The short-term variability of sediment oxygen uptake (SOU) and its dependency on the bottom boundary layer currents were investigated using microprofile measurements. Sediment core analyses for reduced substances profiles allowed distinguishing between SOU caused by both oxic respiration and the flux of reduced substances out of the sediment. Long-term monitoring data were used to estimate the relative importance of SOU for the total oxygen depletion in the lake. Finally, one-dimensional numerical models were used to reproduce lake temperature and oxygen concentrations and to assess the impact of changing environments on the oxygen budget of the deep-water. The results of the microprofile measurements led to a new parameterization of turbulent diffusion close to the sediment and enabled a similarity scaling of diffusivity as well as oxygen close to the sediment. However, the comparison of microprofile measurements at different lake depths showed that SOU decreased consistently with depth from ~1 g m-2 d-1 at 40 m to ~0.2 g m-2 d-1 at 133 m independently from the small-scale variability due to hydrodynamic forcing. Similar vertical structures of SOU and total oxygen depletion have been found in other Swiss lakes. The decrease of SOU with depth was attributed to the greater amount of easily degradable organic matter available in the upper layers. The comparison between SOU and the reduced substances flux revealed that oxic respiration is by far the dominant pathway of organic matter mineralization. While the long-term monitoring data did not show a decreasing trend in either the areal hypolimnetic mineralization rate (1.34 g m-2 d-1) or the extent of hypoxia, a strong relationship between deep mixing in winter hypoxic conditions was found. Hence, deep-water oxygen concentrations were predominantly controlled by resupply during the unstratified period in winter. To assess the long-term changes of winter mixing in Lake Geneva, the one-dimensional model SIMSTRAT was used to reproduce lake temperature and stratification between 1981 and 2012 and was run afterwards under atmospheric conditions representative for the years 2045â2076 and 2070â2101, according to the IPPC scenario A1B. The simulations predicted (i) a decrease in winter mixing depth from an average of ~172 m to only ~127 m at the end of this century, and (ii) complete homogenization of temperature and oxygen in winter will decrease by ~50%. Hence, changes in mixing may have stronger impact than eutrophication on the deep-water oxygen. A simple oxygen model coupled to SIMSTRAT predicted an increase in hypoxic conditions in the deep part of Lake Geneva by ~25%. Additionally, a detailed oxygen model was developed based on the observational findings of this dissertation which takes the spatial variability of oxygen depletion and its dependency on lake turbulence into account. This model can be generalized to understand and predict climate-induced changes of future oxygen concentrations in other deep la

Sediment oxygen uptake in Lake Geneva

Between 1960 and 1980, the trophic state of Lake Geneva (situated between France and Switzerland) has changed from oligotrophic to eutrophic. Since then, the areal hypolimnetic oxygen demand of 1.31 gm-2 d-1 is now the highest measured in Switzerland and low oxygen contents of less than 4 mg l-1 regularly occur in the deepest parts (300m deep). Oxygen depletion is to a large amount driven by sediment oxygen uptake (SOU). In a large and complex lake like Lake Geneva, SOU can vary significantly temporally and spatially and is strongly dependent on the diffusive boundary layer (DBL) thickness which in turn varies with bottom boundary layer (BBL) currents

Optimizing the parameterization of deep mixing and internal seiches in one-dimensional hydrodynamic models: a case study with Simstrat v1.3

This paper presents an improvement of a one-dimensional lake hydrodynamic model (Simstrat) to characterize the vertical thermal structure of deep lakes. Using physically based arguments, we refine the transfer of wind energy to basin-scale internal waves (BSIWs). We consider the properties of the basin, the characteristics of the wind time series and the stability of the water column to filter and thereby optimize the magnitude of wind energy transferred to BSIWs. We show that this filtering procedure can significantly improve the accuracy of modelled temperatures, especially in the deep water of lakes such as Lake Geneva, for which the root mean square error between observed and simulated temperatures was reduced by up to 40 %. The modification, tested on four different lakes, increases model accuracy and contributes to a significantly better reproduction of seasonal deep convective mixing, a fundamental parameter for biogeochemical processes such as oxygen depletion. It also improves modelling over long time series for the purpose of climate change studies

Effects of climate change on deep-water oxygen and winter mixing in a deep lake (Lake Geneva)

Oxygen is the most important dissolved gas for lake ecosystems. Because low oxygen concentrations are an ongoing problem in many parts of the oceans and numerous lakes, oxygen depletion processes have been intensively studied over the last decades and were mainly attributed to high nutrient loads. Recently, climate-induced changes in stratification and mixing behavior were recognized as additional thread to hypolimnetic oxygen budgets in lakes and reservoirs [Matzinger et al., 2007; Zhang et al., 2015]. Observational data of Lake Geneva, a deep perialpine lake situated between France and Switzerland showed no decreasing trend in hypoxia over the last 43 years, despite an impressive reduction in nutrient input during this period. Instead, hypoxic conditions were predominantly controlled by deep mixing end of winter and in turn by winter temperatures. To test the sensitivity of Lake Geneva on future climate change and changes in water transparency, we simulated the hydrodynamics and temperature of Lake Geneva under varying conditions for atmospheric temperature and water clarity performed with the one-dimensional model SIMSTRAT [Goudsmit, 2002]. The results show, that the stratification in lakes is only weakly affected by changes in light absorption due to varying water quality. For conditions expected for the end of the century, a decrease in the annual mean deep convective mixing of up to 45 m is predicted. Also complete mixing events over the whole lake are less likely to occur. A change in the hypolimnetic oxygen concentration of up to 20% can thus be expected in the future. These results show, that changes in deep mixing have an equally strong impact as eutrophication on the deep-water oxygen development of oligomictic lakes and have to be considered in the prediction of the future development of lakes

Impact of deepwater mixing on hypoxia in Lake Geneva

Lake Geneva is recovering from its eutrophic past but no significant trend in the areal hypolimnetic oxygen depletion rate has been observed over the last 30 years. Due to the large depth of 309 m, the lake is not mixed completely every winter which can lead to severe hypoxic condition in the deeper layer. An analysis of field data measured between 1970-2012 shows, that the severity of hypoxia is strongly related to mixing depth and in turn on the mean Schmidt-Stability during winter. We used the one dimensional k-eps model Simstrat (Goudsmith, 2002) to predict the change in deep-water mixing with increasing temperature. To improve the simulation of deep-water mixing, a new method of calculating the amount of wind energy transferred into internal waves is presented. The model was validated with the period 1981-2013 and different scenarios from the predicted temperature change from the Swiss Climate Change Scenario CH2011 (2011) were used to simulate the period 2040-2085. The results show a significant decrease in deep-water mixing with increasing temperature and also a strong impact on the lake’s oxygen budget. References CH2011 (2011), Swiss Climate Change Scenarios CH2011, 88 pp., C2SM, Zurich, Switzerland, ISBN: 978-3-033–03065-7. Goudsmith (2002) et al, Application of k-ϵ turbulence models to enclosed basins: The role of internal seiches, Journal of Geophysical Research 107(C12) 3230