April 2021: Lake surface warming and its consequences

SPRING: Warm-up and restart of aquatic life (April-June)

From spring onwards, CIPEL's monitoring program will return to a fortnightly schedule, in order to keep a close eye on warming and the renewed development of phytoplankton on the lake's surface. Phytoplankton is the plant plankton that develops each year in the form of microalgae suspended in the lake waters.

Why go from 1 withdrawal/month in winter to 2 withdrawals/month in spring?

More frequent sampling is essential to measure the impact of global warming. This has shown that the lake's surface waters are warming earlier and earlier each year. It would also appear that phytoplankton development is starting up again earlier and earlier each year.

Comments of April 20, 2021

The water temperature at a depth of 1 m in the center of the lake was 9.5°C (+2°C compared with March 16). This warming has the effect of separating the lake waters in two. A thin layer of warmer (and therefore lighter) water lies above a large mass of colder (and therefore heavier) water. This stable situation, known as "thermal stratification", has the effect of confining phytoplankton to the thin surface layer and encouraging their development.

A phytoplankton species typical of Lake Geneva in springtime

There are numerous phytoplankton species in the lake waters. One in particular thrives in spring. Tabellaria floculosa belongs to a group called diatoms. It measures 0.1 mm and has the particularity of being enveloped in a skeleton made from silica, called a frustule.

Impact on water transparency

As you may have noticed, the lake is less transparent in early spring than in winter! This is because phytoplankton growth reduces the transparency of the lake water. The more phytoplankton, the less transparent the water.