Green tides retreat on some beaches in Brittany, but now reach the mudflats

Some Breton beaches finally seem to be breathing again after decades of green tides. But behind this improvement lies a dramatic shift in the phenomenon: Ulva blooms are now invading the mudflats, hard-to-reach areas where mechanical collection becomes nearly impossible.

Beaches finally show signs of improvement after decades under surveillance

On certain stretches of the coastline, the landscape is truly changing. At La Lieue-de-Grève and in the Bay of La Forêt, the amounts collected have noticeably decreased between 2014 and 2024. Douarnenez has remained overall steadier. The Court of Auditors also notes a favorable or stabilized trend in areas across several historic bays.

This relative improvement might look like a victory. Yet, taken from above, the map becomes far less reassuring. In 2024, Breton beaches and mudflats accounted for 2,091 hectares of strandings, or 93% of the 2,237 hectares recorded nationwide. Brittany thus still concentrates the overwhelming majority of the French phenomenon.

While the sands green up less, Ulva blooms aggressively invade mudflats

The shift is spectacular. In 2024, 84.1 % of the Breton surfaces affected lay on mudflats: 1,759 hectares out of 2,091. Since 2008, these marshy zones have seen stranding areas increase by 17%. In other words, the problem isn’t really disappearing. It is partly changing its scenery.

The Morbihan is at the front line, as it accounts for 58 % of the mudflat surfaces affected. The Gulf of Morbihan alone had 675 hectares affected in 2024. These environments are far more difficult to manage than a beach: loose soils, difficult access, and heavy equipment ill-suited for operations make mechanical interventions delicate.

Even more troubling, researchers are beginning to understand how Ulva persists there. CEVA studies conducted in the Cantizac mudflat identified winter micro-propagules capable of contributing to the restart of the next green tide. A kind of microscopic reserve, invisible during winter, but ready to restart.

Millions invested, but the real fuel of green tides remains present

Public funding has nonetheless risen substantially. The third combat plan, set for 2022-2027, amounts to €128.6 million, up from €56.1 million for the first. Preventive expenses per agricultural area have also increased from €42.70 to €80.80 per hectare per year.

But Ulva blooms respond less to budgets than to the chemistry of the water. The Court recalls that reducing nitrates is the key lever to significantly curb their proliferation. In 2024, the average Q90 of Breton stations reached 32.69 mg/L, while the annual average concentration stood at 25 mg/L.

Now, the battle is fought far from the beaches that tourists know

The problem is therefore deeply terrestrial. Nitrates reach the coastline via rivers after crossing watershed basins. The average nitrogen pressure has fallen since 2015, but the retreat has slowed in recent years. For the Court of Auditors, achieving a durable reduction of green tides therefore requires acting upstream as much as on the visible strandings.

The judiciary has also increased pressure. On March 13, 2025, the Rennes Administrative Court ruled that the government’s measures are insufficient and gave it ten months to take the necessary steps to effectively reduce pollution of Breton waters by agricultural nitrates.

This evolution ultimately changes the way we view green tides. A beach less covered by Ulva does not necessarily mean that the ecosystem is healed. When the bulk of the strandings moves to less visible and harder-to-access mudflats, the real question becomes almost unsettling: how to combat what we can no longer simply rake up?