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Some of France鈥檚 beaches and lakes have been closed to the public after recording concentrated levels of cyanobacteria, sometimes known as 鈥榖lue-green鈥� algae.

Cyanobacteria are microorganisms naturally present in aquatic environments (fresh and saltwater) that sometimes create cyanotoxins, which can be dangerous to humans and animals. 

They often spread more quickly in hot weather, and can be identified by:

• Thick green or blue-green algae on the surface of the water

• Red or green-tinted bodies of water 

• A nauseating odour

In France, cyanobacteria typically proliferate between May and October in calm, nutrient-rich waters such as lakes, ponds and rivers.

Affected sites that have recently closed include the plage de Mouteou Pipiou (Landes) and Lac de Marcachaud (Lot-et-Garonne). However, locals are advised to before visiting sites as the situation evolves. 

Testing is regularly carried out to monitor water quality as the bacteria can spread 鈥渕assively and rapidly, sometimes in the space of just a few days,鈥� states the French Agency for Food, Environmental and Occupational Health & Safety (ANSES).

One of the six beaches on the lac du Der (Marne) was closed last week (July 9-11) while water samples were being analysed. The beach has since reopened to the public, with the lake鈥檚 tourist office announcing that the water quality alert was a 鈥渇alse alarm.鈥� 

Note that traces of cyanobacteria do not always lead to the closure of a site, as is the case with the Lac de Saint-Ferr茅ol (Haute-Garonne). The Ville de Revel published a statement last week (July 7) saying 鈥渧ery low levels of cyanobacteria were detected,鈥� meaning swimmers are still permitted to enter the lake as long as they: 

• Avoid swallowing the water

• Stay away from algae on the water鈥檚 surface or stones

• Supervise young children

• Take a shower after exiting the water

Pets (mainly dogs) should not enter the lake. 

Contamination risks 

Cases of cyanobacteria-related poisonings are more likely to affect animals than humans, states ANSES. 

In 2024, the city of Lille banned dogs from entering water in the Citadelle area after a pet dog died from ingesting cyanobacteria, while similar cases were linked to dog deaths in Bordeaux in 2023. 

Read also: Alert after several dogs found to have been poisoned in south-west France 

However, humans can be exposed to the toxins in various ways, for example, by drinking untreated water or eating fish from contaminated sites. 

The Fishing Federation of Bas-Rhin alerted their social media followers to the presence of cyanobacteria in the Marne-Rhine canal.

鈥淔ishing is not banned, but we would urge you to exercise extreme caution, particularly with your pets,鈥� the federation wrote. 

鈥淭hese microscopic cells are capable of releasing toxins into the water that are potentially harmful to human health. They do not kill fish, but they can make their flesh unfit for consumption.鈥�

The Facebook group shares regular updates regarding beaches and lakes where high levels of cyanobacteria have been identified. 

Possible symptoms in case of poisoning 

No human deaths due to cyanotoxins have been recorded in France. However, inhaling or accidentally ingesting cyanobacteria can be fatal.

ANSES reports that the symptoms most commonly experienced by humans include gastrointestinal issues, fevers and skin irritations. 

Ingestion or inhalation of cyanobacteria can also lead to liver problems and neurotoxicity, causing trembling, tingling, and in extreme cases, paralysis.

Symptoms can appear within 鈥渁 few minutes鈥� or even 鈥渁 few hours鈥� for skin-related and neurological reactions, while liver toxins can take effect after 鈥渟everal hours.鈥�

If you notice any of these symptoms following exposure to contaminated water, it is recommended to take a shower and consult a medical professional. 

Read also: Rise in number of drownings in France due to heatwave