THE 4 SEASONS OF LEMAN

How much does the lake warm or cool over the year? How does its oxygen level change? And how does its transparency and the development of algae fluctuate? To better understand its functioning, follow the evolution of the lake with the seasons.

Lake Geneva in summer

Separation into 3 superimposed water layers and algal development

In summer, the lake is divided into 3 superimposed water layers: the epilimnion (warm), the metalimnion (warm), the hypolimnion (cold).

The algae suspended in the water continue to grow in the epilimnion and metalimnion where they have sufficient light and nutrients (mainly nitrogen and phosphorus). The more algae there are, the less transparent the surface water becomes.

Algae are important to the functioning of the lake as they form the basis of the food chain. Most of them are not a problem, but some species are problematic: those that clog fishing nets, and toxic cyanobacteria.

Limnological Bulletin - Summer 2022

The temperature of the waters of Lake Geneva in summer

In summer, the lake is divided into 3 layers of water. The epilimnion, located between the surface and about 10 m depth, is the layer which is almost homogeneous in temperature. Below, the metalimnion is the layer where the temperature decreases rapidly with increasing depth. It is located between 10 m and 30 m depth. Further down, the hypolimnion is the layer of cold water almost homogeneous in temperature located between the bottom of the metalimnion and the bottom of the lake. The metalimnion acts as a barrier and prevents the waters of the epilimnion and hypolimnion from mixing.

The measuring station

Lake Geneva is regularly monitored at the SHL2 measuring station located at the deepest point of the lake (309 m).

The measurement campaigns are carried out by the Centre alpin de recherche sur les réseaux trophiques et les écosystèmes limniques (CARRTEL, Université Savoie Mont Blanc, INRAE, Thonon-les-Bains, France). The water temperature is measured continuously between the surface and the bottom of the lake with a probe. The quantities of oxygen and algae in suspension in the water (phytoplankton) are measured from water samples taken at different depths between the surface and the bottom of the lake. The transparency of the surface water is measured with a Secchi disk.

Data for 2021 and previous years have been validated and are available for download on the OLA Lakes Observatory website. Data for 2022 and 2023 are provisional.