BOAT-COMPARATOR Guide

Green Tides in Brittany: Where to Avoid on a Boat This Summer

Every summer, the bays of Saint-Brieuc and Lannion fill with stranded green algae that can turn dangerous as it decomposes. What it means for a boat rental in Brittany, and where to sail without a second thought.

Every summer since the late 1960s, the same bays on Brittany's north coast fill with thick mats of stranded green algae. The phenomenon is confined to a handful of specific spots — it doesn't call the rest of the Breton coast into question for a boat outing — but it's worth knowing where it happens and why before picking an anchorage.

A localised phenomenon, tied to intensive farming

The "green tides" are actually ulva, opportunistic algae that bloom when the water carries too much nitrate — runoff from agriculture across Brittany's watersheds. A study on how the Brittany and Florida coasts cope with green tides estimates that roughly 100,000 m³ of ulva is collected off Breton beaches every year, a figure that gives a sense of the scale (Charlier, Morand & Finkl, How Brittany and Florida Coasts Cope with Green Tides, International Journal of Environmental Studies, 2008, see the study on Google Scholar). Long-term satellite monitoring confirms the phenomenon has clustered on the same beaches in northern Côtes-d'Armor for decades, year after year (Schreyers, van Emmerik, Biermann & Le Lay, Spotting Green Tides over Brittany from Space: Three Decades of Monitoring with Landsat Imagery, Remote Sensing, 2021, see the study on Google Scholar).

Which bays are affected

Two areas account for most of the strandings: the Bay of Saint-Brieuc (the beaches at Hillion, Morieux, Sables-d'Or) and the Bay of Saint-Michel-en-Grève, between Lannion and Perros-Guirec, hit every summer since the late 1960s. A network of seventeen monitoring points tracks strandings from 15 May to 15 October, and the Côtes-d'Armor prefecture can close a beach to the public when concentrations pass health thresholds. The rest of the Breton coast — the Gulf of Morbihan, the Glénan islands, Belle-Île, the Brest roadstead, the Crozon peninsula, the Bay of Douarnenez, Roscoff — isn't affected by this chronic phenomenon.

The real risk: decomposition, not swimming at sea

The danger isn't sailing over water that contains ulva, but lingering next to a pile of algae decomposing on the sand: as it rots, it releases hydrogen sulfide, a toxic gas whose rotten-egg smell usually gives warning before concentrations turn dangerous. The best-known case remains the death of a horse and the collapse of its rider in 2009 on a beach in the Bay of Saint-Brieuc, an event that led to France's first anti-green-tide action plan in 2010. For a boat crew, the practical risk comes down to avoiding landing on or anchoring right next to a pile of stranded, decomposing algae — especially late in summer, once the oldest piles have had time to ferment.

What it means for a boat outing

• Avoid anchoring right against the beach in flagged areas — pulling back just a few dozen metres offshore solves most of the problem.
• Check the local or prefectural order before landing on a Côtes-d'Armor beach in peak season; closures are posted on site and relayed by the town concerned.
• Pick clear, open water for a swim off the boat rather than the nearest stretch of sand — the same habit that matters for water quality after heavy rain, covered in our guide on swimming water quality after rain.
• Head to another part of Brittany during a stranding peak: the phenomenon touches neither southern Finistère nor the Morbihan.

Where to sail without giving it a thought

Almost the entire Breton coastline sits outside the affected zone. Boat-Comparator compares Boataround, SamBoat and Click&Boat live for the Brest roadstead and the Crozon peninsula, the Bay of Douarnenez, the Glénan islands, Lorient and Groix, La Trinité-sur-Mer and the Gulf of Morbihan — all well clear of the affected bays. For an outing near Perros-Guirec regardless, our guide to the Pink Granite Coast covers which coves to favour, away from the Bay of Saint-Michel-en-Grève.

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