April 2021: Lake surface warming and its consequences
SPRING: Warming up and restart of aquatic life (April-June)
From spring onwards, the CIPEL's monitoring programme will become fortnightly again in order to follow the warming and the resumption of the development of phytoplankton 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 change 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 seems that the start of phytoplankton growth is also occurring earlier and earlier each year.
Comments of 20 April 2021
The water temperature at 1 m depth in the centre of the lake was 9.5 °C (+2 °C compared to 16 March). This warming has the effect of separating the water in the lake into two layers. 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 its development.
A typical phytoplankton species in Lake Geneva in spring
There are many phytoplankton species in the lake waters. One of them thrives 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 a frustule.
Impact 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.