As of the beginning of May, the dynamic of green algae strandings is at two speeds, in Côtes-d’Armor, according to the latest surveys carried out by the Center for the Study and Valorization of Algae (Ceva). The surface covered by green algae is around 40% lower than the average level for the years 2002-2023 for the month of May. But the bay of Saint-Brieuc stands out for its precocity and is 10% above this same average, concentrating 90% of the green algae surfaces observed in the department.
This difference is explained, on the one hand, by the stock present in 2023 but also by the large surface area of the bay of Saint-Brieuc, the bottom of which is rather sheltered from the rough weather conditions of winter.
In 2023, algae concentrated in the bay of Saint-Brieuc
In 2023, 15,000 tonnes of green algae were collected in the department. The alert threshold was crossed six times and the proliferation extended until late in the year. Without reaching records, this record is, however, not rosy.
“The start was later with a lower proliferation in April than the average recorded between 2002 and 2022 (2002 marking the start of aerial monitoring of the phenomenon), reports Sylvain Ballu, monitoring project manager at Ceva. On the other hand, we reached the highest level ever observed for the month of October in the bay of Saint-Brieuc. »
This, moreover, attracts all the attention since 81% of the green algae present in the Côtes-d’Armor were found there. “Even in the bay, we observe very different dynamics with a major concern at the level of Anse d’Yffiniac and Morieux, when Binic, Pordic and Saint-Quay-Portrieux are more spared,” underlines the prefect of Côtes- d’Armor, Stéphane Rouvé.
First collections in mid-May
At the same time, the arrival of green algae in the bay of Saint-Michel-en-Grève was later and less significant (- 30%). And, on the Fresnaye Bay side, there was no green algae observed. “We could see the presence of brown algae,” notes Sylvain Ballu. This is a good sign that other algae is growing since it protects against the return of green algae. »
The results observed at the end of 2023 also make it possible, in part, to predict the extent of the phenomenon for 2024. “Experience shows that when we still have a large stock in October, the stranding starts again earlier the following year », remarks the Ceva scientist. And this year is no exception. The first collections began on May 15 in the bay of Saint-Brieuc, when, in the bay of La Fresnaye and that of Saint-Michel-en-Grève, the presence of green algae was still very limited.
So what should we expect for the coming summer? “To make a decision for July and August, we will have to wait to see the evolution of river flows,” says Sylvain Ballu. If they continue to decline, this should limit proliferation. » On the other hand, the scientist expects strong growth, during the months of May and June, on all sites.
