THE 4 SEASONS OF LEMAN

To what extent does the lake warm up or cool down over the year? How does its oxygen content change? And how does its transparency and algae growth fluctuate? To better understand how it works, follow the lake's evolution through the seasons.

Lake Geneva in summer

Separation into 3 superimposed water layers and algal growth

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

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

Algae are important to the functioning of the lake, forming the basis of the food chain. Most of them pose no problem, but some species are problematic: those that clog fishing nets, and toxic cyanobacteria.

Limnological bulletin - Summer 2022

Lake Geneva water temperature in summer

In summer, the lake divides into 3 superimposed water layers. The epilimnion, located between the surface and a depth of around 10 m, is the layer that is virtually homogeneous in temperature. Below this, the metalimnion is the layer where temperature decreases rapidly with depth. It is located at depths of between 10 m and 30 m. At greater depths, the hypolimnion is the layer of almost temperature-homogeneous cold water between the bottom of the metalimnion and the lake bed. The metalimnion acts as a barrier, preventing the epilimnion and hypolimnion waters from mixing.