Imagine a living being older than dinosaurs, more resistant than a bullet of mold, and who sleeps quietly in the back of the lakes for billions of years. These are cyanobacteria. Nothing spectacular with the naked eye. However, as soon as the water warms up a little too much, they wake up … and transform our beaches into toxic traps.
It only takes a few ingredients: a shallow body of water, nutrients from agricultural fertilizers or treatment plants, good sunshine, and a temperature of more than 15 ° C. In a few days, it’s the explosion. The water is colored in green, filaments are raised on the surface, mosses form … and all that without warning.
While vacationers set up, nature is launching an invisible operation. And sometimes this operation can be transformed into silent chemical attack.
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Cyanobacteria, poison at the end of the wavelets
Some cyanobacteria are not content to be invasive. They make toxins. The word may seem strong, however it is right. These cyanotoxinesas they are called, can attack liver, nerves or skindepending on the species.
A child who drinks the cup? He can end up in the emergency room with vomiting, fever or violent cramps. A dog that jumps in the water and then licks out? He risks poisoning, and in some cases, death in a few hours. Even without direct contact, evaporated water or spray can cause eye irritation or coughing in sensitive people.
Each summer, cases are reported, especially in Occitania. Doctors see children covered with red plates, feverish walkers or dogs that have not been able to save.
Rigorous checks … but a race against the watch
In the Occitanie region, the ARS has decided to take matters into its own hands. Once the summer has arrived, the agents regularly pass inspecting water bodies: color change, suspicious odor, strange foam … The slightest index is enough to trigger samples, then sent to the laboratory.
We then use a very precise French standardcalled XP T90-330. It measures the amount of cyanobacteria in water. If some species exceed the alert thresholds, swimming is immediately prohibited. Not out of excessive prudence, no. Because some toxins are active Even in very low doses.
From June 2026, only COFRAC accredited laboratories will be able to validate cyanotoxin analyzes. A way to guarantee the reliability of the measures, and to avoid hazardous interpretations. Because a beach closure is not trivial. You have to be able to explain it clearly to users.
The case of Geneva : the snowy swimming
On July 15, 2025, the Plaine beach in Dardagny was closed. Do not panic, no mermaid, just information fallen: Worldinichinia SP rate greater than 20000 cells by milliliter. This species, although not very toxic, still was enough to bring down the curtain. We take no risk, especially with children.
Then, a few days later, rebelote: the beaches of avengeron and pregny-chambesy were prohibited following pollution with wastewater. Nothing confirmed on cyanobacteria this time, but the context was conducive. Warm waters, nutrients in water … The ideal cocktail.
The reopening was not improvised. Samples, analyzes, counter-analyzes. The swimming only resumed on July 22 and 23. And today, Geneva continues to send its technicians to a jersey … with a test tube in hand. On certain beaches, the samples are weekly.
In New Aquitaine, a vigilance that rises in temperature
New Aquitaine is not to be outdone. Region countless bodies of waterwith quiet rivers and more and more long summers, it checks all the boxes in the field favorable to cyanobacteria. Each summer, the Limousin swimming lakes, the artificial deductions from Lot-et-Garonne or the wetlands of Charente-Maritime are under close surveillance.
One of the most publicized cases occurred at Lake Vassivière, where swimming had to be suspended for several days following a flambé of microcystines detected in early August 2024. Analyzes revealed a concentration greater than 10 µg/Lwell beyond the health recommendations. Picnics canceled, deserted beaches, and a direct impact on local tourism.
Faced with this recurring threat, the region has strengthened its analysis network. Partnerships were established between ARS Nouvelle-Aquitaine, the departments and the municipalities. Objective : take earlier, react faster, inform better. Certain communities such as those around the lake of Saint-Pardoux or Lake Hourtin have even invested in light panels with variable messages to alert users in real time.
What changes the game here is experience. Mayors of small lake municipalities now know how to read in water as in a biology manual. A green color a little too sustained, an abnormal smell … and the protocol activates. It is no longer a surprise, it has become a reflex.
An ecosystem that suffocates in silence
What we see on the surface is a small part of the problem. When they die, cyanobacteria flow and break down. This process consumes oxygen of water. Result : School of fish, disappearance of larvae, collapse of the aquatic food chain.
In some ponds or retained of water, we speak of “dead zones”. More a noise, the more a stir. The water is there, motionless, sterile. And even when the cyanobacterium does not produce toxin, its proliferation imbalances the ecosystem.
On the agriculture side, it is also a thorn in the foot. Water loaded with microcystins cannot irrigate a risk -free field. As for fish farming or mussel crops, they must take expensive breaks, while the water quality returns to normal.
What everyone can do at their level
Avoid proliferation often starts out of the water. Less fertilizer, less polluting discharges, better sanitation of wastewater: so many actions that limit nutrients in rivers and lakes.
And then there are good reflexes. Green water, trouble or foam? We avoid entering it. Does your dog come back from the lake with funny filaments glued to the hair? Shower it right away, and monitor his reactions.
Finally, always consult the alert bulletins. It is not reserved for professional swimmers. In a few clicks, you will know if your favorite lake is still a haven of freshness … or a toxic oven.
Some figures on cynobacteria:
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Indicator
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Approximate
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Temperature promoting proliferation
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> 15 °C
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WHO Alert threshold for swimming
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20,000 cellules/ml (Woronichinia SP)
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Life of toxins in water
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From several days to several weeks
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|
Cost of a complete analysis
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Between 400 and 800 euros
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Case of poisoning in Occitania (summer)
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Several dozen in humans and animals
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Sources :
- https://www.occitanie.ars.sante.fr/les-cyanobacteries-0
- https://www.anses.fr/fr/content/les-cyanobacteries-en-questions
- https://www.ge.ch/plages-lieux-baignade-eaux-vives-geneve/reconnaitre-cyanobacteries-que-faire
Image: Lake Hourtin and Carcans
