Return to the waters of Lake Geneva in the summer of 2021
The water temperature at the surface of the lake did not exceed 21.1°C during the summer of 2021, and the average temperature between the surface and 10 m depth did not exceed 20.3°C.
surface to 10m depth did not exceed 20.3°C. It is only in the summer of 2014 that such low
to find such low summer temperatures.
However, the bottom of the lake has never been warmer since the beginning of the CIPEL records in 1957.
The water temperature at the bottom of the lake reached 6.03°C on 15 September 2021 and has been rising since
winter 2012, when the lake water was last fully stirred. Since the winter of 2012, the water temperature at the bottom of the lake has increased by an average of
Since the winter of 2012, the water temperature at the bottom of the lake has risen by an average of 0.1 °C per year.
The lake lacks oxygen at depth. The dissolved oxygen concentration at the bottom of the lake was
between 1.7 and 2.8 mg O2 per litre this summer. Since winter 2017, it has not exceeded 4 mg/L (the value above which there is little or no oxygen).
above which there is little or no negative effect on aquatic life).
Chlorophyll biomass was relatively low this summer compared to the last 10 summers, despite a very large Uroglena bloom in early September.
This was despite a very large bloom of the alga Uroglena that occurred in early September.
Finally, the transparency of the surface waters is within the average of the last 10 summers. This
summer, Secchi's depth was between 3 and 7.7 m.