Seasonal bulletin for Lake Geneva: Spring 2025

CIPEL has just published the results of six measurement campaigns carried out by INRAE Thonon between March and May 2025 at point SHL2, just below the deepest point of Lake Geneva (309 m).

These observations, compared with the 1991-2020 reference period, highlight several notable changes in the lake's physico-chemical parameters:

🌡️ Water temperature

- Surface water (0-10 m) : Average temperature 11.1°C, +2.1°C compared with the reference period.

- Deep waters (250-309 m): Average temperature 6.5°C, an increase of +0.9°C.

👉 These trends confirm a warming trend in the lake at all depths, with potential implications for thermal stratification and vertical exchange dynamics.

🫧 Dissolved oxygen

- Deep waters (250-309 m): Oxygen concentrations remain below the minimum requirement of 4 mg-L-¹ set by the Swiss Water Protection Ordinance.

👉 The persistence of hypoxia at depth represents a risk for benthic biodiversity and the functioning of aquatic ecosystems. The complete mixing of the waters that allows oxygenation of the depths and which takes place during sufficiently cold and windy winters, has not occurred for 13 consecutive winters due to climate change.

🌿 Chlorophyll a (phytoplankton biomass indicator)

Reminder: "Chlorophyll a" is the main green pigment in microalgae (phytoplankton), and is used to measure the amount of microscopic life in the lake.

- March 2025 (0-10 m): Concentrations similar to reference average.

- March 2025 (0-30 m): Slight increase (+0.8 µg-L-¹).

- April and May 2025 (0-30 m): Significant decrease (-1.0 µg-L-¹ in April; -1.3 µg-L-¹ in May) compared with the reference period.

👉 This dynamic reflects increased seasonal variability, with an early peak followed by a marked decline in late spring.

👀 Water transparency

Transparency measured at 16.1 m at the end of May is the highest value since 2009 ⚠️

👉 This record is directly correlated to the low phytoplankton biomass observed in April and May.

 

CIPEL continues to monitor the lake in order to anticipate future developments and support sustainable management.

More information in the spring limnological bulletin: direct link