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 autumn

Cooling and end of algal development

In autumn, the lake remains divided into 3 superimposed water layers: the epilimnion (warm), the metalimnion (warm), the hypolimnion (cold). The decrease of the air temperature and the shortening of the day length lead to a cooling of the epilimnion which mixes with the metalimnion.

The summer development of the algae in suspension in the water has led to a decrease in the content of nutrients necessary for their development (mainly nitrogen and phosphorus) in the epilimnion. Since the metalimnion is richer in nutrients than the epilimnion, the fall cooling remobilizes nutrients to the surface, and this remobilization allows the algae to continue to grow. The more algae, the less transparent the surface water.

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.

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

Limnological Bulletin - Fall 2022

The temperature of the waters of Lake Geneva in autumn

In autumn, the lake remains divided into 3 superimposed water layers. A surface layer almost homogeneous in temperature (epilimnion), underneath a layer where the temperature decreases rapidly when the depth increases (metalimnion), and deeper a layer relatively homogeneous in temperature and cold (hypolimnion). The decrease in air temperature and the shortening of the daylight hours lead to a cooling of the epilimnion which mixes with the metalimnion. Fall cooling is characterized by a thickening of the epilimnion to about 50 m depth.