International Commission for the Protection of the Waters of Lake Geneva
Back on the waters of Lake Geneva in summer 2021
The water temperature at the lake's surface did not exceed 21.1°C during the summer of 2021, and the average temperature
from the surface to a depth of 10 m did not exceed 20.3°C. You have to go back to the summer of 2014
find such low summer temperatures.
However, the lake bottom is the warmest it has been since CIPEL began recording data in 1957.
The water temperature at the bottom of the lake reached 6.03°C on September 15, 2021, and has been rising ever since
winter 2012, the date of the last complete mixing of the lake waters. Since winter 2012, the temperature of the water
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 liter 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 impact on aquatic life).
Chlorophyll biomass was relatively low this summer compared with the last 10 summers, despite the Uroglena algae bloom in early September.
Lastly, surface water transparency was in line with the average for the last 10 summers. This
summer, Secchi's depth ranged from 3 to 7.7 m.