Climate change could leave some Swiss lakes with higher amounts of algae and toxic cyanobacteria, according to a recent study by the Swiss Federal Institute of Aquatic Science and Technology (Eawag).
A simulation done by scientists at Eawag shows that as climate change alters water temperature, ice cover and mixing of many Swiss lakes could be upset.
Lakes are subject to strong seasonal cycles. Temperature differences between bottom and surface layers of water cause seasonal mixing events. These mixing events influence many chemical and ecological processes. Oxygen rich and nutrient poor water on the surface mixes with oxygen-depleted and nutrient-rich waters at the bottom. Lake ecosystems and the entire food web from plankton to fish are adapted to seasonal differences between surface and bottom water temperatures and mixing events.
Mid-altitude lakes are especially at risk to climate change. If they lose their winter ice cover they will no longer fully mix twice a year. Such a change would have fundamental consequences for the functioning of lake ecosystems.
“How strongly the circulation of lakes reacts to climate change depends in particular on their altitude and size. Lakes at mid altitudes are very sensitive,” says Love Råman Vinnå at the Surface Waters Research Department.
If the climate warms by more than 2° C, many of the lakes at mid altitudes, such as Lac de Joux (1,004m) and Klöntalersee (848m), risk losing their winter ice cover over the course of the 21st century. Less ice will increase mixing between surface and bottom waters in winter so the stable stratification of warm and cold water layers in winter will not last for as long. In summer, the stratification will be prolonged, raising the risk of oxygen deficiency in deep waters, which favours algal growth of toxic cyanobacteria.
In July 2020, beaches around Lake Neuchâtel were closed after six dogs died of suspected cyanobacteria poisoning. Cyanotoxins produced by cyanobacteria are toxic to animals and humans.
At mid altitudes, many lakes risk shifting from a twice-yearly mixing regime to a single annual mix. Such regime shifts have fundamental consequences for the storage of heat in lakes and for the distribution of oxygen and nutrients.
High-altitude lakes, such as Lake St. Moritz at 1,768 m, should maintain twice-yearly mixing, at least in the 21st century, according to the model.
Large lakes on the Swiss Plateau, such as Lake Zurich or Lake Geneva, which already mix once a year, are likely to retain their mixing cycles. according to the study.