'I couldn't breathe': the sinister spread of France's killer seaweed

https://www.theguardian.com/news/2026/may/12/i-couldnt-breathe-the-sinister-spread-of-frances-killer-seaweed

‘I couldn’t breathe’: the sinister spread of France’s killer seaweed

After a series of deaths on the beaches of Brittany, one bereaved family set out to prove the foul-smelling bloom was to blame

By Marta Zaraska
Tue 12 May 2026 00.00 ED

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When she arrived, Rosy noticed that the dog was behaving bizarrely: she refused to walk, then collapsed under a bush. Her fur stank of rotten eggs, of overflowing sewers. Rosy knew where that smell came from: the mudflats roughly three miles from the family home in Brittany, where seaweed had been accumulating and putrefying. The soggy, decomposing seaweed stretched for miles along the shore, sometimes as much as five feet thick, killing other plants and suffocating fish and small birds.

The Brittany coastline is famed for its green hills, rugged cliffs and miles of sandy beaches. But over the past few decades, in places, the sand has begun to disappear beneath a carpet of green goo. At certain times of year, when Ulva armoricana, a type of seaweed, blooms, banks of green mass form on the beaches, releasing hydrogen sulphide, a foul-smelling, potentially harmful gas. In recent years, red and yellow warning signs have appeared on stretches of the coastline. Occasionally, beaches are closed to the public. Over the spring and summer months, tractors work their way along the coast, raking up thousands of tonnes of seaweed and carting it away: it’s an unending task that has to be done quickly, before the seaweed starts to rot.

Still it spreads: a stain on the landscape, killing biodiversity and breeding anger, frustration and shame. Alix Levain, a social anthropologist at the French National Centre for Scientific Research, calls the seaweed a “monster”. Since 1989, at least one man has died while clearing it from the beaches. In one incident in 2011, 36 wild boar were found dead in the sludge. The media began to talk about the “killer seaweed”.

What lies behind this explosion of seaweed are the high levels of nitrates in the water, which come from industrial farming’s intensive use of synthetic fertilisers and nitrogen-rich animal feed. Brittany is the agricultural heartland of France. On just 5% of the country’s surface, it crowds more than half of its pig population.

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