Algues Vertes

Green algae and animal health

Assessment of the toxicity of hydrogen sulfide (H2S) in animals

Every summer for more than 30 years, massive quantities of green algae have been washing up on parts of the French coastline. Once washed up on beaches, these algae deposits decompose and produce large quantities of gases, especially hydrogen sulfide (H2S), potentially exposing walkers, local residents, workers required to collect these algae and animals in the vicinity, to risks. In July 2011, the discovery of wild animal carcasses in the Gouessant estuary led the French Ministry of Agriculture to make a formal request to ANSES to establish the cause of death and the possible existence of a link between H2S fumes at the site and the deaths of these animals. Below are details of the Agency's work on this subject.

Episodes of massive proliferation of macro-algae on the coasts and beaches of Europe have been reported in the scientific literature since as early as 1905 on the Irish coast. This phenomenon seems to have increased over the past forty years. The proliferation of these algae is mainly linked to the presence in the water of nitrates generated by human activities (particularly agriculture), and the shape of the coastline (many bays).

Once washed up on the beaches, these massive deposits of algae decompose. During the decomposition process, large quantities of gases are released, including hydrogen sulfide (H2S) which may cause disagreeable smells and health problems for walkers, local residents and animals in the vicinity. 

In summer 2011, wild animal carcasses (thirty-six wild boars, three coypu and one badger) were discovered on the beach at Morieux and on the banks of the Gouessant estuary (Côtes-d'Armor département). Laboratory tests were performed on the dead animals and the environment. 

In view of the results of these tests, ANSES received a formal request from the Directorate General for Food for an opinion on the toxicity of hydrogen sulfide in animals and the possible link between the presence of H2S at the site and the animals’ death. 

The Agency’s work

To analyse the available evidence, ANSES set up an emergency collective expert assessment group bringing together specialists in toxicology, wildlife, animal diseases, laboratory techniques and the environment.

In view of all the available data, the Agency considered that poisoning by hydrogen sulfide (H2S) was the most probable hypothesis, although it was unable to confirm whether this was the only factor contributing to the mass mortality. ANSES recommended further work and stressed that the health issues associated with the decomposition of green algae and the production of H2S in the mudflats also point to the need to investigate gas emissions from estuary bottoms and some river beds.