Lake Geneva: When citizens participate in preserving the ecosystem

By providing a secure and efficient workspace, the LéXPLORE platform makes it possible to collect data that was previously inaccessible. It is now possible to measure the entire water column, particularly the surface zone, in all climatic conditions and even at night. This is essential for studying extreme events.

Several teams of researchers can work simultaneously, which encourages interdisciplinary approaches. In addition, the platform makes it possible to bring the laboratory directly onto the lake for innovative analyses.

Thanks to the many submersible sensors installed on LéXPLORE, a large amount of physico-chemical data has been collected at high temporal and spatial frequency since 2019.

These data perfectly complement the long-term data collected by the CIPEL since 1963, which show trends in climate change. The high-frequency data, meanwhile, enable processes to be modelled on an appropriate timescale, in order to predict how the lake will evolve under different scenarios.

This greatly improves our understanding of the impact of climate change on Lake Geneva. And a better understanding means better protection for Lake Geneva.

The main impacts of climate change observed in Lake Geneva include an increase in water temperature, favouring a particular type of phytoplankton and fish at the expense of others, and influencing the phenology of species (e.g. fish spawning). According to CIPEL measurements, the average temperature of surface water has risen by around 0.05°C per year. This increase in temperature reduces the frequency of complete lake mixing during milder winters and extends the period of lake stratification (separation of the lake into two layers) in spring and autumn.

Complete mixing brings nutrients to the surface and oxygen to the depths.

The absence of complete mixing since 2012 has altered the availability of nutrients and oxygen, disrupting the food chain.

Climate change is also having an impact on the Lake Geneva catchment, with changes in the seasonal dynamics of precipitation, flow rates and therefore external inputs. For example, an increase in extreme events can lead to untreated discharges from wastewater treatment plants (WWTPs) into the lake during periods of intense rainfall, or on the contrary, limit the supply of nutrients from the catchment area during periods of drought.

In the Lémanscope project (www.lemanscope.org), more than 600 volunteer co-researchers are measuring the colour and transparency of the lake to gain a better understanding of seasonal variations in water quality throughout the lake. In two months, more than 700 measurements have already been collected, which would have been impossible without the co-researchers. This data will be used to calibrate satellite data in order to develop a model for monitoring water quality in Lake Geneva. In this way, the co-researchers are making a direct contribution to research on Lake Geneva by collecting scientific observations that are relevant for monitoring environmental changes.

This participatory project is a collaboration between the Association pour la Sauvegarde du Léman, EPFL, Eawag and Unil. Its aim is also to raise public awareness of the issues surrounding the health of the lake and the importance of preserving its quality. Citizens can learn more about water quality and how their lake functions. In conclusion, the active participation of the public and the dialogue between researchers and the public foster a deeper understanding of the issues relating to the health of Lake Geneva, thereby encouraging concrete action to preserve it in the long term.

It is important to stress that Lake Geneva is facing several simultaneous threats, in addition to climate change. Continued population growth in the catchment area is leading to an increase in phosphate and pollutant inputs, exerting greater pressure on the ecosystem. The proliferation of invasive species, in particular the Quagga mussel, is altering the lake's food network and is already having economic repercussions, as they are clogging up the drains. In addition, pollution from micropollutants and microplastics, originating from the whole catchment area, is concentrated in the lake and can have harmful effects on the health of the ecosystem and human populations.

Tertiary treatment to limit micropollutants in the large WWTPs (e.g. at Vidy) will undoubtedly have beneficial effects on lake water quality.

Other actions to mitigate these impacts could include the further reduction of phosphorus pollution to limit the proliferation of algae, which leads to deoxygenation of the lake, however with the risk of reducing fish populations.

Integrated management of water resources, such as the creation of sponge towns and the increase in separate water networks, could help to protect Lake Geneva.

The challenge is to maintain good water and ecosystem quality so that Lake Geneva continues to provide many essential services for the society of the future.