• In Rennes and the surrounding towns, the Vilaine is green in color.

  • This color is indicative of the concentration of cyanobacteria, also called blue algae.

  • In Cesson-Sévigné, the town has invited associations sailing on the watercourse to limit contact with water.

It displays a hardly engaging green color. Rather accustomed to brown, the Vilaine has for a few days presented green tones that do not bode well. With the arrival of the first heat, the origin of this color is beyond doubt. Cyanobacteria are back. These microalgae regularly pollute the surface waters of Rennes and the surrounding area, forcing municipalities to ban swimming. In the ponds of Apigné, Vern-sur-Seiche or even Guerlédan, we are used to seeing these small blue algae tint the surface.

This week, La Vilaine has seen its color change.

The alert was given to Acigné on Sunday.

Two days later, the town of Cesson-Sévigné saw the stream display a green tint.

"We noticed it visually on Tuesday, it was all green," explains Ronan Le Breton, head of the sports department of the town located east of Rennes.

The instructions were then given to associations sailing on the Vilaine to “limit contact” with the water.

“We adapt the canoe and kayak sessions.

We avoid going into the sports river and we stay in calm waters in order to limit desalting, ”he explains.

The risk is primarily related to the ingestion of water, which can lead to nausea or even diarrhea.

Dogs dead after swimming

As of Tuesday, the municipality of Cesson-Sévigné sent an e-mail to all associations likely to navigate on the river. These recommendations do not have the force of prohibition. For bathing to be prescribed, municipalities must rely on the readings carried out by the Regional Health Agency. Canoe and kayak clubs are used to it and adapt their sessions. “We avoid swimming games with children. We take precautions, ”explains a supervisor. Always annoying for a whitewater sport. This appearance could only be temporary. "In Acigné, they already have nothing," says Ronan Le Breton.

In Rennes too, the Vilaine is green. “I am on the water and I can tell you that we can see it well. The water is green and cloudy ”, assures Fabien Aubanel, trainer of the Régates Rennaises. Less affected than the canoe and kayak clubs, its rowing association remains vigilant to “limit contact with water”. Before reassuring. "In the fifteen years that I have been there, I have never had a problem". These cyanobacteria can be dangerous for animals. They have already caused the death of several dogs, especially in 2017 in the department of Maine-et-Loire. The animals had been drinking or swimming in the Loire. Cyanobacteria generally appear during high temperatures in low flow rivers under the combined effect of light and phosphorus.

In order to limit any risk of contamination, the ARS recommends avoiding prolonged contact with water or ingesting it.

She recommends taking a neat shower after the activity, especially when entering the water and wearing waterproof protective clothing.

It is recommended to consult a doctor in case of side effects.


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  • Pollution

  • Water

  • Canoe

  • Rowing

  • Kayak

  • Reindeer

  • Algae

  • Bathing

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