Why Lake Geneva Is Not Monitored for Blue-Green Algae

Cyanobacteria are not systematically monitored in Lake Geneva or in other bodies of water in the Canton of Vaud. There is simply no method available to monitor their presence on a daily basis and across the entire area, said Florence Dapples, head of the Water Protection Division at the cantonal Department of the Environment, to the Keystone-SDA news agency.

These microorganisms occur naturally in lakes and rivers and multiply particularly rapidly in warm weather. Of the thousands of known species, an estimated 40 produce toxins. If ingested in large quantities, these toxins can cause poisoning in humans and be fatal to pets. Among other incidents, six dogs made headlines in 2020 after they died following a swim at the mouth of the Areuse River where it flows into Lake Neuchâtel.

Clearly visible

Cyanobacteria form carpet-like mats on the bottom of bodies of water. Through photosynthesis, they later rise to the surface as cotton-like, floating masses. Other species turn the water blue-green, resembling streaks of color—which is why they are also called blue-green algae. These highly visible changes are usually recognizable to the general public, explains Dapples.

Since wind and rain can alter or disperse these accumulations within a short time, the canton believes that ongoing monitoring of all swimming areas would be too costly. Instead, for the past three years, the canton of Vaud has been providing information about the risks before the start of each swimming season. Municipalities receive informational materials and conspicuous yellow warning signs for swimming areas. However, swimming waters continue to be tested regularly for fecal bacteria, but not for cyanobacteria.