“I usually let my daughter put a finger in the water, but today I forbade her,” Hadya says, taking her child by the hand as they walk around the lake in Évry-Courcouronnes (Essonne). “It’s just a precaution… Since I don’t know what it is, I prefer not to let her touch it.”
More than a quarter of the surface of the body of water is covered with filamentous algae, around the Dame du Lac. The image is striking. Since the arrival of the high temperatures, several bodies of water in the department, but also in Seine-et-Marne, have been covered with a thick layer of this type.
What worries this father from Évry-Courcouronnes is actually harmless to health. “These algae are not dangerous,” assures Nadège Bernard, the director of the water cycle for the Grand Paris Sud urban area. “There is no danger. If there was, we would have already seen a significant fish mortality. We have not seen anything like this to date.”
No intervention planned, but samples taken
But then, how can we explain their sudden appearance? “The high temperatures have encouraged the development of algae,” continues the director. “The water has warmed up and the oxygen level has dropped. It’s a sort of vicious circle that has been set up.”
But, given the absence of danger, the agglomeration services exclude for the moment any intervention. “The algae continue to develop, no curative operation would be effective”, explains Grand Paris Sud. Despite everything, the site remains under increased surveillance.
This Monday, samples were taken to check the pH of the water, its temperature, its conductivity and its oxygen saturation. “These are fairly lengthy analyses,” warns the director of the water cycle. The results are expected in about two weeks. They will allow us to characterize with certainty the exact nature of the algae.”
Although it is particularly visible around the Évry-Courcouronnes lake, the phenomenon affects many other sites. “They are not necessarily the same algae, but in the Grand Paris Sud area, we have identified them in Lieusaint, Combs-la-Ville, Savigny-le-Temple and Moissy-Cramayel for Seine-et-Marne and in Bondoufle, Coudray-Montceaux and Ris-Orangis in Essonne,” Nadège Bernard adds. “Depending on the case, they are duckweed, filamentous algae like in Évry or pondweed.” (an aquatic plant). »
To limit the phenomenon, do not give bread to the ducks
Elsewhere in the department, the presence of filamentous algae has also been observed in the waters of the Saclay ponds. Hervé Cardinal, director of technical services for the Intercommunal Syndicate for the Sanitation of the Bièvre Valley, explains the phenomenon in detail: “The algae produce nutrients which, in winter, fall to the bottom of the water. The following summer, when the high temperatures arrive, the algae will reappear. If you add sediment to the sun, heat and nutrients, you have the winning cocktail!”
Hervé Cardinal reminds us that, to limit the phenomenon, we must in particular avoid feeding ducks with bread. “First of all, it’s not good for them… Then, everything that is not consumed remains on the surface of the water. And this bread is nutrients that are added to those already present. It is obviously not the only cause of algae but it contributes to it. These bad habits contribute to the degradation of the physicochemical quality of the water.”
In a very rare event, the Blanchette lake in Massy will be emptied this summer to be completely cleaned. “It’s a very expensive operation,” emphasizes Hervé Cardinal. “Just like mowing, which consists of mowing the plants present in the water.” This is one of the reasons why the mowing of the Évry-Courcouronnes lake is not yet scheduled. “In some cases, the algae disappear on their own,” concludes Nadège Bernard. Given the size of the lake, mowing would cost nearly 100,000 euros if not more… Given that there are no health risks, it is preferable to wait before launching the operation.”
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