April 2021: Lake surface warming and its consequences

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

From spring onwards, the CIPEL monitoring program becomes bi-monthly again in order to follow the warming and the restart of the phytoplankton development on the surface of the lake. Phytoplankton is the plant plankton that develops each year in the form of microalgae suspended in the lake waters.

Why go from 1 sample/month in winter to 2 samples/month in spring?

More frequent sampling is needed to measure the impact of global warming. This has shown that the surface waters of the lake are warming earlier and earlier each year. It also appears that the restart of phytoplankton development is also occurring earlier and earlier each year.

Comments of April 20, 2021

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

A typical phytoplankton species of Lake Geneva in spring

There are many phytoplankton species in the lake waters. One of them grows particularly in the spring. Tabellaria floculosa belongs to a group called the diatoms. It measures 0.1 mm and has the particularity of being enveloped by a skeleton made from silica called frustule.

Consequence on water transparency

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