in Lake Geneva, the mixing of the waters has stopped

Climate change is not only having an impact on the health of the oceans but also on that of large lakes such as Lake Geneva, between France and Switzerland. It is 73 km long and you have to imagine it as being made up of two superimposed layers of water. One on the surface, warmer and more oxygenated, and another below, colder and rich in nutrients. Normally, in winter, with the drop in temperatures and the wind, the surface waters that have been warming up in the summer cool down. They become denser and are therefore dragged to the bottom with their oxygen. At the same time, the lower water layer rises to the surface with the nutrients, which enriches the food chain. This is the ideal scenario, but this year again, the complete mixing of Lake Geneva has not taken place because it was very hot last summer and not very cold this winter, which means that the waters of surface have not cooled sufficiently to allow thorough mixing.

Measurements made by the International Commission for the Protection of the Waters of Lake Geneva (Cipel) indicate that the waters only mixed on the first 120 meters deep, while the bottom of the lake is 300 m. This is not new because the lake is not completely stirred every year, but here the problem is that the phenomenon is repeated for the 11th consecutive year, which compromises oxygenation and the survival of animals. at the bottom of the lake.

The situation is still reversible

For Marie-Elodie Perga, professor of limnology at the University of Lausanne, the situation is worrying but the lake is neither dead nor dying. Firstly because the lack of oxygen really concerns the great depths and then because long periods without mixing have already occurred in the past, in particular for 13 years between 1986 and 1999. Finally the ecosystem has regained a balance.

It is true that with global warming, unfortunately, mixing will become less frequent, so oxygenation problems will increase. But it is possible to act by limiting the consumption of oxygen by the algae and for this, there is a trick: it is necessary to limit the discharges of phosphates. Monitoring diffuse pollution in Lake Geneva is therefore a good way to act for its future.