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 spring

Warming and renewed algal growth

In spring, the lake's water quality measurement campaigns become fortnightly once again, in order to monitor the warming of the water and the resumption of algae growth in suspension in the water.

Winter mixing has resulted in the remobilization to the surface of the nutrients required for algal growth (mainly nitrogen and phosphorus). Spring warming, combined with increased solar radiation, allows algae growth to resume in the nutrient-rich surface layer. The resumption of algae growth is accompanied by a drop in surface water transparency.

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.

Springtime water temperatures on Lake Geneva

Rising air temperatures and longer daylight hours cause surface waters to warm up to depths of around 30 m.

The measuring station

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

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). Water temperature is measured continuously between the surface and the bottom of the lake using a probe. The quantities of oxygen and algae suspended in the water (phytoplankton) are measured from water samples taken at various depths between the surface and the lake bottom. Surface water transparency is measured with a Secchi disk.

Data for 2021 and previous years have been validated and can be downloaded from the OLA Lake Observatory website. Data for 2022 and 2023 are provisional.