The results of new analyzes concerning the presence of cyanobacteria in the waters of Lake Lamothe, at the domain of Hostens (Gironde), fell on Thursday evening. “The first partial results indicate the presence of toxigenic cyanobacteria, raising fears of the presence in the future results of dangerous toxins” indicated this Friday the departmental council of Gironde, owner of the domain, in a press release.
Even if these results must be consolidated during the day, “as a precautionary measure, the department has decided to close the Hostens swimming pool from this Friday, September 1, until receipt of the additional analyzes. »
Conflicting results
These analyzes follow the sudden death of a dog who had bathed in the waters of the lake on August 17. A second dog suddenly fell ill after bathing on August 23, but was saved after being rushed to a vet. Analyzes carried out by the departmental branch of the ARS (Regional Health Agency), on August 19 and 28, did not however show the presence of cyanobacteria in the water of Lake Hostens.
The owners of the deceased dog ordered on their side, on August 24, an independent analysis at the Aqua Gestion laboratory, located in Haute-Vienne, and specialized in the phenomena of cyanobacteria. Surprise, his results showed a strong development of toxinogenic cyanobacteria, up to forty times the threshold. Faced with these contradictory results, the department therefore took the decision to carry out additional analyzes on Thursday, which it had sent to Aqua Gestion, and to a second approved independent laboratory, the LDA33 (Departmental Analysis Laboratory).
These new results therefore confirm the analyzes carried out by Aqua Gestion. Joined by 20 minutes this Friday, the head of Aqua Gestion, Philippe Combrouze, said he was “relieved. The department “made the right decision, because it was no longer possible to keep the bathing open” he believes.
Planktonic and benthic analyzes
But how to explain that the ARS found nothing in the two analyzes it carried out in the waters of the lake? Philippe Combrouze assures that “it’s normal” questioning the modus operandi of the departmental branch of the ARS. “She only analyzed planktonic samples, while on my side I sampled both planktonic and benthic, with the two samples that the dog’s owner sent me. »
Explanations. “Cyanobacteria, you have planktonic ones that live and grow in the body of water, and you have benthic ones that grow on a substrate [de la vase, du sable, des galets…], specifies Philippe Combrouze. These are two categories that do not intersect, they do not live in the same ecological niche. But you have to look for both, because it’s the benthic cyanobacteria that cause animal mortality, we’re not going to miss it. »
When there are brutal dog or wildlife mortalities, “in 95% of cases it is linked to a neurotoxin which, most of the time, is produced by benthic cyanobacteria, and in particular by a genus which is calling Oscillatoria. And it’s good Oscillatoria that I found in the benthic sampling of Lake Hostens, which reinforces the explanatory diagram of this dog mortality. »
“Drawing lessons to implement new provisions”
Contacted by 20 minutes, the ARS did not wish to react, referring us to the departmental council, “manager of the site. “The ARS cannot take a position until it has the consolidated results, but we have taken the decision to close swimming in agreement with it” we are told at the department.
While the head of Aqua Gestion would have liked this closure to take place earlier, he also recognizes that as long as samples had not been taken by approved laboratories, it was difficult to proceed with the closure of the lake. Because there is a way to take. “And a single sample can be a problem in terms of quantification, but not in terms of quality. These cyanobacteria were present, it was all the same the main information. »
This specialist assures “not to seek controversy” and assures “to have total confidence in the ARS” with whom he “has been working for more than thirty years”, but wonders about the modus operandi of the ARS in Gironde with regard to the analyzes of cyanobacteria. “Each departmental directorate of ARS works in a different way, it is not for me to explain why. But normally, benthic sampling is systematically carried out. In Vienne, a swimming pool has been closed for six months due to a proliferation of benthic cyanobacteria. “Questions that do indeed deserve clarification. And for Philippe Combrouze, it will be necessary that “lessons are learned to implement new provisions next year. »
“An increase in the proliferation of cyanobacteria from year to year”
The head of Aqua Gestion is also concerned about “the increase in the proliferation of cyanobacteria from year to year, which causes more and more animal mortality. He says he is “convinced that, within a timeframe that I do not know, there will be an impact on humans. »
The proliferation of cyanobacteria is linked to the presence of phosphorus. “If there were no excess phosphorus in the water, there would be no development of cyanobacteria. We must therefore wonder about the sources of phosphorus, which generally comes from the discharge of our wastewater and our agriculture. On the other hand, “the development of cyanobacteria has nothing to do with the heat, we also have them in the middle of winter. »