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But why are cyanobacteria the pet peeve of the bodies of water in Puy-de-Dôme (and elsewhere)?

But why are cyanobacteria (re)talking about them – badly?! – each summer ? The rest of the year, these microorganisms spend quiet days at the bottom of waters of all kinds, in this natural environment that they share with other microalgae, without disturbing anyone.

Hobbies. Swimming again authorized in the Aubusson d’Auvergne lake (Puy-de-Dôme) from Wednesday August 3.

But when the heat wave arrives – or even the heat wave – they play spoilsports in bodies of water, because they then proliferate at high speed. Unlike other algae, which do not tolerate heat well, they are in paradise when the temperature reaches 25°C.

Risks of dermatitis and gastoenteritis

The problem is that these cyanobacteria can produce toxic molecules, mainly microcystin and anatoxin, which risk causing dermatitis and gastroenteritis (if ingested) in humans.
If we have never deplored in France cases of human mortality, cattle, dogs, deer died after having swallowed, while drinking in rivers, balls of toxic cyanobacteria.

A three-step health monitoring protocol

Caution is therefore essential, and the health authorities monitor bathing water carefully.
Benjamin Legrand, research engineer at Athos environment (the Clermont design office specializing in aquatic environments mandated by the Regional Health Agency in Puy-de-Dôme) explains that “the health monitoring of these waters is different, since this summer, pursuant to a decision taken by the National Agency for Food, Environmental and Occupational Health Safety (ANSES) in 2020”.

Samples and assays

They now take place in three stages: first a chlorophyll assay; if this reveals a biomass exceeding ten micrograms per liter, a microscopic count is carried out to see if there may be potentially toxic cyanobacteria. If this biomass exceeds the threshold of 1 cubic millimeter per litre, a cyanotoxin analysis is carried out by looking for different molecules. And, above the concentration threshold set by regulation, the decision is taken to prohibit swimming in the bodies of water concerned, as a precautionary measure.
Samples are taken every week and their analysis takes a few days.

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No immediate solution but long-term management

The managers of the sites concerned – and the swimmers – have no other solution than to take their troubles patiently. Indeed, there is currently no way to solve the problem immediately: it is necessary to wait for the temperature to drop for the population of cyanobacteria to decrease.

While there is no immediate solution to the proliferation of cyanobacteria, site management must be done over the long term and this is what several communities have begun to do by creating wetlands, revegetating banks… If the facilities that cool the atmosphere are useful, it is also the whole environment that must be managed to starve the cyanobacteria, which feed mainly on nitrogen and phosphorus. However, this comes mainly from human activity in the catchment area of ​​the lakes (individual and collective sanitation, certain agricultural activities, industry, certain angling techniques, etc.).

An effective wetland in Aydat

At Lake Aydat, where a wetland was created ten years ago as part of the river contract by the Syndicat mixte des vallées de la Veyre et de l’Auzon, there were no problems related to cyanobacteria since 2018. Developments and measures have also been implemented on other sites, in particular at the Cournon lake (vegetation of the banks, turbine, ban on fishing, etc.). If these measures improve the situation, they do not always measure up to major climatic hazards, such as the current temperature peaks.

Aydat Lake. Photo Renaud Baldassin

Seven bodies of water in Puy-de-Dôme prohibited for swimming – Swimming is currently prohibited, due to cyanobacteria, at the Hermines lake in Super Besse, at the Ribeyre campsite lake in Saint-Victor-la-Rivière, at the Philippe pond in Saint-Gervais-d ‘Auvergne, in the lakes of Cournon, Cunlhat and La Tour-d’Auvergne. The body of water of Ambert is also without bathing.

Laurence Couperier


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