High temperatures
Lake surface temperatures reached record levels in 2023, with an annual average of 13.6°C, 1.3°C above the 30-year average.
Deep water temperatures also rose, reaching an unprecedented 6.2°C.
Incomplete winter mixing
For the eleventh year running, the lake's winter mixing, crucial for oxygenating deep waters and redistributing nutrients, remained incomplete, reaching a depth of only 130 meters. This led to worrying oxygen levels in the deeper zones, well below the quality threshold set by the Swiss Water Protection Ordinance.
Water quality trends
The average annual phosphorus concentration has stabilized at around 16.9 µg/L, indicating an improvement in water quality.
Phytoplankton peaked early, while zooplankton, essential to the food chain, continued to decline, particularly in herbivorous cladocerans.
The quagga mussel, present since 2015, has modified the dynamics of mollusc larvae, with an earlier reproductive cycle.
Impact on wildlife
Whitefish and perch reproductive cycles show seasonal variations linked to high water temperatures, underlining the vulnerability of the ecosystem.
A general downward trend in micropollutant concentrations has been observed, thanks to efforts in the agricultural and wastewater treatment sectors. However, drug residues such as ibuprofen are still detected in some coastal areas.
A study reveals high concentrations of microplastics, with an average of 7,600 particles per square meter of beach, mainly synthetic fibers. Recommendations have been issued to local authorities to prevent this pollution at source.
The report underlines the need for concerted action to guarantee the quality of the waters of Lake Geneva and the resilience of its ecosystem in the face of climate change. The specific studies carried out enrich the knowledge needed to guide measures to protect and preserve this unique ecosystem. This report serves as a scientific basis to guide authorities, researchers and local stakeholders in their efforts to protect Lake Geneva.
