Photo taken on August 9, 2009 in Saint-Michel-en-Greve, western France, shows a green algae. Part of the coastline has been declared off-limits after a horse collapsed and almost instantly died when he slipped on a patch of rotting algae. Photo courtesy of AFP. Mounds of rotting seaweed clogging beaches across northwestern France are emitting a toxic and potentially lethal gas, test results released by the government showed on Thursday. Tests were ordered on the foul-smelling algae, which green groups blame on nitrates fertilisers used by local farmers, after a horse apparently died from inhaling fumes on a beach in Saint Michel de Greve in Brittany. Results showed the seaweed in Saint Michel was giving off dangerous levels of hydrogen sulphide (H2S), sometimes referred to as “sewer gas” because it is produced by the breakdown of putrified waste material. “Measurements carried out on site … showed in several places that the gas released by sediment containing the decomposing algae could be dangerous,” said France’s national institute for environmental threats, INERIS. Several points on the beach tested positive for hydrogen sulphide at a concentration of 1,000 parts per million, a level that “can be deadly in a few minutes,” the report said. …

Seaweed on French beaches emitting lethal fumes: study via Democratic Underground