Lake Geneva and the challenges of climate change and environmental pressures: results of the CIPEL Scientific Report 2024
The International Commission for the Protection of the Waters of Lake Geneva (CIPEL) publishes its annual scientific report, based on 2023 data, highlighting growing challenges for the preservation of the lake's ecosystem.
Record temperatures, incomplete mixing and oxygen-depleted deep waters
Surface temperatures reached unprecedented levels, with an annual average of 13.6°C, 1.3°C above the 30-year average. Deep waters show a similar trend, with an increase of 1.1°C in 11 years, i.e. an unprecedented 6.2°C. These trends confirm the direct impact of climate change on Lake Geneva.
In 2023, for the eleventh year running, the winter mixing of Lake Geneva, essential for supplying the deeper zones with oxygen and redistributing nutrients in the water column, remained incomplete, reaching a depth of only 130 m. This situation, exacerbated by high winter temperatures, led to worrying oxygen levels in the deeper zones of the lake. This situation, exacerbated by high winter temperatures, led to worrying oxygen levels in the deeper areas of the lake (2.0 mg/L annual average, well below the quality threshold of 4 mg/L set by the Swiss Water Protection Ordinance).
Stabilizing phosphorus concentrations and weakening biodiversity in the face of a changing Lake Geneva
The average annual phosphorus concentration has stabilized at around 16.9 µg/L. The specific composition of phytoplankton indicates an improvement in water quality, reflecting a positive trend.
However, the lake's biological dynamics continue to reveal the effects of global warming and environmental pressures. Phytoplankton has peaked early, while zooplankton populations, essential to the food chain, continue to decline, particularly in herbivorous cladocerans.
Since its arrival in Lake Geneva in 2015, the quagga mussel has changed the dynamics of mollusc larvae. Unlike the zebra mussels already established, it has introduced an earlier reproductive cycle, starting as early as January and first observed in 2016. However, by 2023, this behavior seems to have diminished, with larvae only visible during the summer. Recent surveys have also revealed that quagga mussels can colonize depths of up to 250 meters, with maximum concentration between 10 and 50 meters, where temperature and oxygen conditions are most favorable.
On the fish side, the reproductive cycles of whitefish and perch show seasonal variations directly linked to high water temperatures. These phenomena underline the growing vulnerability of the Lake Geneva ecosystem to climate change.
Micropollutants and microplastics: the need for vigilance
The 2023 monitoring campaign confirms a general downward trend in micropollutant concentrations, thanks to combined efforts in the agricultural and wastewater treatment (domestic and industrial) sectors. However, drug residues such as ibuprofen are still detected in some coastal areas. A study on plastic pollution also revealed high concentrations, with an average of 7,600 microplastic particles per square meter of beach, mainly synthetic fibers. Best practices have been sent to municipalities in the Lake Geneva watershed, to prevent this pollution at source.
Better knowledge for better action
Three specific studies enrich this report, exploring in particular the role of phosphorus, the ecotoxicological effects of the waters of Lake Geneva, and, as mentioned, the impact of plastic pollution. These studies will help guide measures to preserve this unique ecosystem.
In the face of climate change, Lake Geneva is an essential strategic resource. This report confirms the need for concerted action to guarantee the quality of its waters and the resilience of its ecosystem. The report provides a valuable scientific basis to guide authorities, researchers and local stakeholders in their protection efforts.