Episodes of massive proliferation of macro-algae on European coasts and beaches have been reported in the scientific literature as early as 1905 (Irish coasts). For the past forty years, this phenomenon has continued to grow. The proliferation of these algae is mainly linked to the presence of nitrates, brought by human activities (agriculture in particular), in the waters and to the topographic configuration of the coast (bays).
Once stranded on the beaches, these massive deposits of algae lead, through their putrefaction, to significant releases of gases, in particular hydrogen sulphide (H2S) which can be the cause of olfactory nuisances and health problems for walkers, residents and passing animals.
In summer 2011, carcasses of wild animals (thirty-six wild boars, three nutria and a badger) were discovered on the beach at Morieux and on the banks of the Gouessant estuary (Côtes-D’armor [22]). Laboratory tests were carried out on the dead animals as well as in the surrounding environment.
In view of the analysis results, ANSES received a request from the Directorate General for Food for an opinion on the toxicity of hydrogen sulphide in animals and the possible link between the presence of ‘H2S on the site and the death of animals.
The Agency’s work
To analyze the available evidence, ANSES urgently set up a collective expert review group bringing together specialists in toxicology, wildlife, animal pathology, laboratory techniques and the environment.
In view of all the data available, the Agency considered that the hypothesis of poisoning by hydrogen sulphide (H2S) was the most probable, without however being able to affirm that it was the only factor contributing to the massive mortality. ANSES recommended additional work and stressed that the health issues associated with the decomposition of green algae and the production of H2S in the mudflats should also lead to the exploration of gaseous emissions from the bottoms of estuaries and the beds of certain rivers.