Decryption. What are cyanobacteria and why do they prevent you from swimming during a heatwave?

It's the double whammy of climate change: while France is suffocating , several freshwater swimming spots have had to be closed due to the proliferation of cyanobacteria caused by rising temperatures. On Monday, as the mercury exceeded 30 degrees around Lake Bourget in Savoie, swimmers seeking fresh air were disappointed to learn that all water activities – swimming, canoeing and paddleboarding – were prohibited due to the presence of potentially toxic bacteria .

This is far from an isolated case: in Ille-et-Vilaine, on the Mayenne (Maine-et-Loire), in Aveyron (Pareloup and Villefranche-de-Panat lakes), or even in Nièvre (Baye pond) or the Ardennes (Sedan lake), swimming bans have multiplied in recent days. The cause is photosynthetic microorganisms called cyanobacteria.

Those present in lakes, ponds and other waterways are of two types: the first are planktonic and float in suspension in the water – they are sometimes called blue algae, even though their color can also range from green to red – and the second, which are generally found at the bottom, attached to rocks, are called benthic, described Catherine Quiblier, researcher at the National Museum of Natural History.

With each heatwave, they swarm in bodies of water, thanks to the rise in temperatures: their optimal development rate is around 25 to 30°C, explains the scientist. The temperature of Lake Bourget on Monday was 28°C. For planktonic organisms, the resurgence is favored by "eutrophication, that is to say, the enrichment of water bodies with nitrogen and phosphorus" coming mainly from intensive agriculture and poor wastewater management.

"And for benthic organisms, (...) it's the prolonged low water periods, that is to say the fact that we have droughts occurring earlier and longer" and their consequences on the volume or flow of water bodies that are the cause. But "in any case, it's certain that global warming is a factor in the resurgence" of cyanobacteria observed since the 1990s, assures the researcher.

"As soon as water temperatures warm up, like they are right now during a heatwave," proliferation "explodes." And this year is particularly risky, the scientist emphasizes. "We've had alternating periods of drought and rain, which have leached the soil, bringing nitrogen and phosphorus" into lakes and rivers. "And so when the next heatwave arrives, cyanobacteria have all the food they need to grow, and all the heat they need. All the conditions are right."

Harmless when present in small numbers, some cyanobacteria can, when allowed to multiply, release toxins into the water that can be dangerous or even fatal to living beings. The most common effects include nausea, vomiting, dizziness, skin irritation, abdominal pain, and diarrhea. "This risk is particularly high in the event of accidental ingestion while swimming or playing in the water," warns the Auvergne-Rhône-Alpes Regional Health Agency (ARS) on its website.

Prolonged exposure can also eventually cause cancerous tumors, says Catherine Quiblier. But some cyanobacteria are also neurotoxic, with proven effects on the mortality of some animals. "They attack the nervous system, which can lead to respiratory paralysis. In dogs, it can be rapid, with convulsions occurring within 10 to 15 minutes," explains the scientist. A dog died at Lake Bourget on Sunday.

And for humans, "there have been cases of suspected deaths" due to cyanobacteria, the last in 2021 when a couple, their baby, and their dog were found dead near a cyanobacteria-contaminated river in California with no other apparent cause of death, "even if they could not be formally confirmed," notes Catherine Quiblier. A study published in 2017 in the journal Archives of Toxicology had identified only 6 cases of human deaths worldwide linked to cyanobacteria contamination since 1960.