Alerte pollution - Association pour la Sauvegarde du Léman

Pollution alert

Here you can report pollution in the Lake Geneva catchment area and find out about the probable causes of certain types of pollution.

What to do if you notice pollution, a three-step checklist:

1. Contact the fire brigade if there is evidence of pollution. Be careful, however, not to see pollution everywhere. The phenomena presented in the FAQ are the result of natural dynamics in a body of water.

2. Report the pollution to the ASL using the form below.

3. Take photos and send them to the ASL for follow-up.

Frequently asked questions

High fish mortality (e.g. perch)

At the end of spring, the perch spawning season comes to an end. It is common to see a high mortality rate at this time, mainly among young males. This phenomenon can become more widespread when the perch population is large (as in 2022). Mortality can be particularly high when the large numbers of young males are weakened by the lack of food and when the water temperature is below 20°C. These two aspects represent ideal conditions for the development of the perch rhabdovirus. In general, mortality stops when the water warms up more and the perch are able to replenish their reserves.

Fluorescent flow in a sewer

As part of tests to detect leaks in pipe systems, fluorescent products such as fluorescein are used as tracers. This product, which is totally harmless to the environment, colours the water fluorescent green. This can give the impression of impressive pollution. If in doubt, simply phone the fire brigade to make sure there is no danger to the environment.

Yellow-green slicks on the surface of the water

Spring is synonymous with the release of pollen for anemogamous plants (which reproduce using the wind). Birches, grasses and hazelnut trees are among these species and are well represented on the shores of Lake Geneva. As pollen dispersal by wind is fairly random, some of it ends up in Lake Geneva, where it collects in a slick due to the action of waves and currents. Although quite impressive, this is a totally natural phenomenon that poses no danger to the environment!

Green or brown algae blooms

In summer, when the temperature of Lake Geneva rises, filamentous microalgae can develop strongly, forming colonies visible to the naked eye. These clusters of algae can then gather at the surface. These are not cyanobacteria, but just a natural phenomenon in any body of water. To distinguish algae from cyanobacteria, one method is to stick your hand in the water to pick up what's left on the surface. If what's left in your hand is filamentous, it's algae, whereas if there are just a few small pieces stuck to your fingers, it's cyanobacteria. Please note that this detection method is not infallible and it is important to wash your hands thoroughly with soap and warm water after the test. If you suspect cyanobacteria, it is advisable to contact the fire brigade to have the species analysed. Some species, particularly those of the genus PlanktotrixThese can be dangerous for humans when present in excessive quantities. The summer of 2021, for example, saw a major proliferation of brown algae. Uroglena sp. which is harmless to humans but has given the water in Lake Geneva a brownish colour and a strong smell of rotting fish.