From April to September, the stranding of green algae on sandy bays in Brittany was 20 to 25% lower than average. Last winter’s storms dispersed the algae from the most exposed bays and delayed the resumption of proliferation in the spring.
Reassuring news. The proliferation of green algae was below average in Brittany this summer, after a stormy winter favorable to the dispersal of ulva, we learned this Wednesday, September 11 from the Center for the Study and Valorization of Algae (Ceva).
Over the period from April to September, the stranding of green algae on the sandy bays of Brittany is at a “level which should be around 20 to 25% lower than the average” according to “the first provisional assessments”, indicated Sylvain Ballu, head of the green tide monitoring project at Ceva.
Since 2002, Ceva has been carrying out a flight every month between April and October, at low tide and with a high tidal coefficient, as part of the monitoring of green tides. Present in the sea in their natural state, green algae proliferate in the bays of Brittany, where they are nourished by nitrates spread by farmers in the fields and the surpluses of which are carried by coastal rivers.
The unprecedented areas of green algae recorded in October 2023 (611 hectares compared to 286 ha on average) raised fears of a strong resumption of proliferation in spring 2024. But last winter’s storms dispersed the algae from the most exposed bays, significantly delaying the resumption of proliferation in spring. Only the bay of Saint-Brieuc escaped this “destocking” phenomenon.
A potentially deadly gas emitted when they decompose
Influenced by weather conditions (rainfall, sunshine, water temperature), algae proliferation gradually caught up during the season, with differences depending on the bay.
In September, the green algae strandings extended over a surface area slightly smaller than the annual average for this season, according to initial analyses by Sylvain Ballu.
These algae become dangerous when they decompose because they emit hydrogen sulfide (H2S), a gas that can be fatal in high doses. In early September, the health alert threshold was exceeded in Guissény (Finistère), where a beach was temporarily closed due to the concentration of H2S in the air.
The situation has improved over the last twenty years, with most rivers seeing their nitrogen concentration drop sharply, but “we are still in the turbulence zone, in the danger zone”, underlines the head of the green tide monitoring project at Ceva.
“We are not immune to a year like 2021, which was a catastrophic year with areas that were much higher than the multi-year average,” the engineer pointed out.
