Breton lakes: should we be afraid of cyanobacteria? – Brittany

Breton lakes: should we be afraid of cyanobacteria? – Brittany
Breton lakes: should we be afraid of cyanobacteria? – Brittany

Almost everywhere in Brittany, during the summer, freshwater bathing areas were closed due to the presence of toxigenic cyanobacteria.

Cyanobacteria are micro-organisms present on Earth for two to three billion years and which develop in water when conditions are favorable: “We need heat, light and nutrients (nitrogen and phosphorus)” , says Frédéric Pitois, head of the Limnology design office in Rennes and specialist in plankton ecology. “These micro-algae are naturally present in the aquatic environment, they are part of phytoplankton. But the more there are human activities, the more there is eutrophication and the more they will develop”, underlines Marie-Agnès Pilard, sanitary studies engineer and head of the leisure waters division within ARS Bretagne. which monitors about thirty bathing areas.

Less and less poisoning

There are about 5,000 species of cyanobacteria, including about 300 in fresh water. “Depending on the genus of the cyanobacterium, it has the capacity, or not, to produce toxins. When toxins are present, there are problems for swimming,” explains Marie-Agnès Pilard. These toxins can cause health problems in humans and animals (digestive, neurological or skin disorders). But “we have to put things into perspective, assures Frédéric Pitois. In France, there are a few cases of poisoning, occasionally, but there are fewer and fewer. Since 2021, a new monitoring system has been put in place with lowered alert thresholds. The principle is simple: “When the concentration of toxins exceeds the guide values, we ask for the closure to swimming”, underlines Marie-Agnès Pilard.

A question of threshold

Is this summer’s drought synonymous with the multiplication of cyanobacteria and therefore bathing bans? Not necessarily ! “This year, we had a lot of light but few nutrients, precisely because of the drought because the nutrients are brought by the rivers. For the moment, it is too early to draw conclusions because the cyanobacteria grow until it starts to get cold. But we haven’t seen any particularly higher quantities than in other years,” says Frédéric Pitois. For the specialist, the number of sites closed to swimming is not synonymous with an “abnormal situation”: “The criteria are much stricter than before. For example, for microsistis [NDLR, la plus fréquente des toxines], the threshold went from 13 to 0.3 micrograms/litre; the bathing prohibition threshold has been divided by 40.”

Earlier appearances of cyanobacteria

Nevertheless, global warming is visible and poses other problems: “The appearance of cyanobacteria is much earlier in the season, due to warmer temperatures earlier,” remarks Frédéric Pitois. Since cyanobacteria are therefore present longer, can we imagine long-term effects on humans? “The frequent presence of small concentrations in the background can cause chronic problems including inflammation, allergies, and even for certain toxins, the promotion of tumours. These are the kinds of effects that led to the reduction of the bathing ban thresholds in 2021, ”continues the specialist. Another phenomenon: tropical cyanobacteria which regularly arrive in Brittany, transported in the feathers of migratory birds. They acclimatize and become local in a few years. “It would be worrying if all cyanobacteria were toxic. This is not the case. What is more worrying are the migrations of pathogens: birds are also capable of carrying viruses, parasites, etc. »


The article is in French

Tags: Breton lakes afraid cyanobacteria Brittany

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