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