Every summer, the Breton coastline becomes a battleground between nature and human activity, where a seemingly harmless green algae can turn deadly. Beachgoers often encounter these algae mats, which appear as lush, emerald patches on the sand. However, as the algae begin to decompose, they release hydrogen sulfide, a toxic gas that can cause death in humans or animals within minutes. This phenomenon is not isolated—it affects 93% of Brittany, where the accumulation of these algae has become a recurring and dangerous issue. The cause is linked to agricultural practices, particularly the runoff of nitrates from farms into local waterways, which feed the algae's growth. The danger occurs when the algae form thick layers and begin to decay beneath a surface crust. In this oxygen-deprived environment, the algae break down and release hydrogen sulfide, a gas known for its strong odor and high toxicity. While the gas is most commonly associated with the deaths of dogs and wild boars that come into contact with the decaying algae, it can also cause severe health issues in humans, including respiratory distress and even cardiac arrest. To mitigate this risk, emergency collection operations are conducted every summer. For example, in early August 2026, a large-scale cleanup was organized on the beach of Ris in Douarnenez, where the volume of algae required a swift response. This phenomenon is not new; it has been a recurring issue in certain Breton bays, especially those that are shallow and well-exposed to sunlight. These conditions, combined with high temperatures and an abundance of nutrients—particularly nitrogen—create an ideal environment for a specific type of algae called Ulva armoricana. The nitrogen comes primarily from agricultural runoff, including livestock manure and chemical fertilizers. Rainwater carries these nitrates into rivers and eventually into coastal bays, acting as a kind of fertilizer for the algae. As a result, Brittany accounts for 93% of all green algae wash-ups in France, making it the region most affected by this environmental issue. Despite efforts to reduce the impact of this problem, the situation remains complex. While some data suggest a decrease in the amount of algae collected—such as the 8,400 tons collected in Côtes-d’Armor in 2025, compared to 25,000 tons in 2010—this does not necessarily reflect improved water quality. In 2026, the Court of Audit criticized the lack of progress in reducing nitrogen inputs into affected bays, despite increased resources. Legal actions have also been taken, with the Rennes administrative court in 2023 acknowledging ecological damage linked to the state’s failure to control algae growth. As 2026 draws to a close, stricter regulations are expected to be implemented in environmentally sensitive areas, with local authorities tasked with developing more effective regional strategies.