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THE 4 SEASONS OF LEMAN

To what extent does the lake warm up or cool down during the year? How does its oxygen level change? And how does its transparency and the development of algae fluctuate? To better understand how it works, follow the evolution of the lake through the seasons.

Lake Geneva in autumn

Cooling and end of algal growth

In autumn, the lake remains divided into three superimposed layers of water: the epilimnion (warm), the metalimnion (warm) and the hypolimnion (cold). The drop in air temperature and the shorter daylight hours cause the epilimnion to cool and mix with the metalimnion.

The summer development of the algae in suspension in the water has led to a decrease in the content of nutrients (mainly nitrogen and phosphorus) necessary for their development in the epilimnion. As the metalimnion is richer in nutrients than the epilimnion, the autumn cooling remobilises nutrients to the surface, and this remobilisation allows the algae to continue to grow. The more algae there are, the less transparent the surface water becomes.

Algae are important for 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 - Autumn 2021

The temperature of the waters of Lake Geneva in autumn

In autumn, the lake remains divided into three superimposed layers of water. A surface layer that is almost homogeneous in temperature (epilimnion), below this a layer where the temperature decreases rapidly as the depth increases (metalimnion), and deeper down a layer that is 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. The autumn cooling is characterised by a thickening of the epilimnion to a depth of about 50 m.

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