On March 13, 2025, the Administrative Court of Rennes mandated the Brittany prefecture to take necessary measures to mitigate ecological damage caused by nitrate pollution along the Breton coastline.

The court also awarded 5,000 euros to the environmental association Eau et Rivières de Bretagne for moral damages, highlighting the inadequacy of current measures to combat green algae wash-ups.

In its ruling, the court required the prefect to implement effective control tools within ten months to manage actions against the persistent issue of green algae on the Breton coast.

Despite the association's challenges, the state claims compliance with its obligations regarding water pollution, having incorporated new actions into its seventh regional action program for 2024.

While Brittany, particularly the Côtes-d’Armor region, is most affected, similar green algae issues are emerging in other areas, including Nouvelle-Aquitaine, where significant deposits have been reported.

Nicolas Forray, secretary general of Eau et Rivières de Bretagne, expressed hope that the prefecture would respond decisively after previous requests for additional measures went unanswered.

The green algae problem has persisted for nearly fifty years, raising environmental and public health concerns due to incidents involving fatalities and animal deaths.

Since 2010, France has implemented seven regional action programs aimed at addressing this issue, but their effectiveness on water quality remains uncertain, as noted in a 2021 report from the Court of Auditors.

The legal actions initiated by the association included a challenge against the prefect's refusal to implement stronger pollution control measures after previous complaints went unaddressed.

The court acknowledged the ongoing increase in the extent and duration of green algae blooms, indicating that current governmental measures are inadequate for sustainable management.

A 2021 report from the Court of Auditors revealed that over 90% of green algae proliferation in Brittany is linked to agricultural practices, particularly the increased use of nitrogen fertilizers since the 1960s.

Since 1971, significant quantities of green algae have washed up on Breton beaches, producing hydrogen sulfide gas upon decomposition, which can be lethal in high concentrations.