May 2021: Lake surface warming and its consequences

The surface waters of Lake Geneva continue to warm up (about +2°C compared to the April reading). The water temperature at a depth of 1 m in the centre of the lake was 12.3 °C and 12.1 °C on 3 and 19 May respectively.

The resurgence of phytoplankton this spring continues with the rapid development of a filamentous species well known to fishermen, as it interferes with their activity by clogging fishing nets. Mougeotia gracillima was measured in relatively large quantities on 3 May at a depth of 15 m in the centre of the lake.

As phytoplankton develop in the spring, the surface waters of Lake Geneva become depleted of nutrients. Some species (such as M. gracillima) then grow deeper where there are still nutrients and where there is sufficient light. This is why the water in the first few metres is sometimes more transparent in May than in April, as is the case this year.

Photos: Clogging of a fishing net by Mougeotia gracillima

Read more - Subscribe to the newsletter - – Health status of April 2021