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 autumn

Cooling and end of algal growth

In autumn, the lake remains divided into 3 superimposed water layers: epilimnion (warm), metalimnion (lukewarm), hypolimnion (cold). As air temperature drops and daylight hours shorten, the epilimnion cools and mixes with the metalimnion.

The summer growth of algae in suspension in the water has led to a drop in the nutrient content of the epilimnion (mainly nitrogen and phosphorus). As the metalimnion is richer in nutrients than the epilimnion, autumn cooling remobilizes nutrients to the surface, enabling the algae to continue growing. 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.

When algae die, they sediment towards the bottom. Their decomposition during sedimentation consumes oxygen and lowers the oxygen content in the hypolimnion.

Lake Geneva water temperature in autumn

In autumn, the lake remains divided into 3 superimposed water layers. A surface layer that is almost homogeneous in temperature (epilimnion), below this a layer where temperature decreases rapidly as depth increases (metalimnion), and deeper down a layer that is relatively homogeneous in temperature and cold (hypolimnion). As air temperature drops and daylight hours shorten, the epilimnion cools and mixes with the metalimnion. Autumn cooling is characterized by a thickening of the epilimnion to a depth of around 50 m.