The waters of Lake Geneva in summer

Separation into 3 superimposed water layers and algal growth

In summer, the lake is divided into three layers of water. The epilimnion is the almost temperature-homogenous layer between the surface and about 10 m depth. The metalimnion is the layer between about 10 m and 30 m depth, in which the temperature decreases rapidly with increasing depth. The hypolimnion is the almost temperature-homogeneous layer between the bottom of the metalimnion and the bottom of the lake. The metalimnion acts as a barrier and prevents the waters of the epilimnion and hypolimnion from mixing.

Suspended algae usually grow less than in spring because there are fewer nutrients in the epilimnion. The surface waters are therefore usually more transparent. However, some species can grow in the metalimnion, where there is enough light to carry out photosynthesis. Most of these algae are not a problem. They form the basis of the food chain. But some filamentous algae are problematic: those that clog fishing nets, and toxic cyanobacteria.

Monday 9 August 2021:

  • The surface waters have warmed slightly compared to Wednesday 21 July 2021. The surface temperature is 20.8°C (+0.2°C). The waters below the surface have warmed considerably. The average temperature between the surface and 10 m depth is 19.9 °C (+3.9 °C). Overall, the surface temperature and the average temperature between the surface and 10 m depth are lower than in the summer of 2020.
  • The bottom temperature is stable compared to 21 July 2021 (6.02°C). It has increased by 0.08°C compared to August 2020.
  • The amount of suspended algae has decreased slightly compared to 21 July 2021. The average chlorophyll a concentration between the surface and 10 m depth is 3.2 µg/L (-0.1 µg/L). The transparency of the surface water has improved significantly. Secchi depth is 8 m (+5 m). No signs of filamentous algae blooms that clog fishing nets or toxic cyanobacteria.