The simulation results paint a clear picture: if the climate warms by more than 2° C, many of the lakes at mid altitudes, such as Lac de Joux or Klöntalersee, risk losing their ice cover in the course of the 21st century. Less ice means increased vertical exchange between surface and bottom waters in winter. This counteracts the build-up of stable stratification in winter and therefore shortens its duration. In summer, on the other hand, the duration of stratification is prolonged, raising the risk for oxygen deficiency in deep waters. Furthermore, longer periods of stratification in summer have shown to favour algal growth of toxic cyanobacteria.
At mid altitudes, many lakes risk shifting from a dimictic mixing regime with bottom waters mixing twice per year to a monomictic regime with only one such event per year. Such regime shifts have fundamental consequences for the storage of heat in lakes and for the distribution of oxygen and nutrients. The habitats of many aquatic species could change considerably as they are warmed up from above and oxygen becomes scarcer deeper down. "But by limiting climate warming, we can maintain lake ice and prevent mixing regime shifts for most lakes." says Råman Vinnå.
Mountain lakes and large lakes in the midlands retain their mixing regime
High-altitude lakes, such as Lake St. Moritz at 1768 meters, on the other hand, will remain dimictic under all climate scenarios, at least in the 21st century, according to the model study. Worth to note, lake water warms up more and duration of ice cover and stratification shortens faster at higher compared to lower elevations. Nevertheless, a tipping point is not likely to be reached in the current century. Ice will thus continue to form in winter on lakes at high elevation and biannual vertical mixing events are maintained. The large lakes at the Swiss Plateau, such as Lake Zurich or Lake Geneva, which already have a monomictic regime, are likely to retain their mixing cycles according to this model study.
“Lakes in Switzerland will undergo climate related changes during the 21st century. What we show here is that by abiding to stringent measures, most effects can be limited and only a small number of lakes will pass a tipping point and experience drastic changes in how they function” says Råman Vinnå.