May 2021: Lake surface warming and its consequences

The surface waters of Lake Geneva continue to warm (around +2°C compared with the April reading). Water temperatures at a depth of 1 m in the center of the lake were 12.3°C and 12.1°C respectively, on May 3 and 19.

This spring's phytoplankton revival 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 a depth of around 15 m in the center of the lake.

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

Photos: Clogging of a fishing net by Mougeotia gracillima

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