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). Water temperatures at 1 m depth in the center of the lake were 12.3 °C and 12.1 °C, respectively, on May 3 and 19.

The rebound 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 May 3 at about 15 m depth in the center 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 develop deeper where there are still nutrients and where there is enough light. This is why in the first meters, the waters are sometimes more transparent in May than in April, as it is the case this year.

Photos: Clogging of a fishing net by Mougeotia gracillima

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