an atypical proliferation in the bay of Saint-Brieuc in 2023

an atypical proliferation in the bay of Saint-Brieuc in 2023
an atypical proliferation in the bay of Saint-Brieuc in 2023

The proliferation of green algae, likely to impact the health of humans and animals, is closely monitored by state services. On the scale of Côtes-d’Armor, the phenomenon was not more significant in 2023 than in previous years. But the bay of Saint-Brieuc was particularly affected.

The Prefecture of Côtes-d’Armor has just established the results for the year 2023 concerning the proliferation of green algae in the department.

A year described as “atypical” because of a “late start of strandings, with the exception of the bay of Saint-Brieuc which concentrates 90% of stranded green algae surfaces in May” specifies the Prefecture.

A scenario which is explained by the presence “of a high stock of green algae at the end of 2023 at the bottom of Saint-Brieuc bay, which the turbulent weather conditions of winter did not, however, allow them to disperse”.

If the situation in the bay of Saint-Brieuc stands out, the average level of green algae strandings in the department is no more alarming than in previous years.

The stranding areas are within the multi-year average according to monitoring established by the Center for the Study and Valorization of Algae (CEVA), which regularly flies over the sites studied to measure the extent of the phenomenon.

The latest readings even indicate that the surface covered by green algae for the month of May is 40% lower than the average level for the years 2002-2023.

With just over 15,000 tonnes of green algae collected in Côtes-d’Armor, “the year 2023 is in the low range observed in recent years” assures the Prefecture.


Collecting green algae in Hillion in the bay of Saint-Brieuc

© Thomas Bregardis – Maxppp

The dangerousness of green algae is a problem studied by the Regional Health Agency: in 2022, new H2S sensors had been installed in 11 bays in Côtes-d’Armor and Finistère affected by the phenomenon.

These instruments make it possible to measure the level of hydrogen sulfide, a very toxic gas released by these aquatic plants, every quarter of an hour.

On the indication of the High Council of Public Health, if the values ​​exceed the threshold of 1ppm in the air, a precise protocol is triggered:

  • information to local authorities and populations
  • greater collection of algae on coastlines and beaches
  • a strengthening of bans on access to sites.

In 2023, the health surveillance campaign was marked by six exceedances of the alert threshold, in the town of Hillion.

For this year 2024, the monitoring system in Côtes-d’Armor will include nine sensors distributed across the three “green algae bays” of the department. A new machine installed along the GR34 in Hillion will complete the arsenal deployed, no later than June 1st.

Present at sea in its natural state, green algae began to multiply more than 40 years ago on certain Breton beaches, where they are nourished by nitrates spread by farmers in the fields and the surpluses of which are transported by coastal rivers.

A plan to combat their proliferation has been deployed since 2010 in Brittany.

The toxicity of decomposing green algae has been questioned for many years by environmental associations. These plants are believed to be the cause of the deaths of several people and animals in the region.


In 2009 in Saint-Michel-en Grève (Côtes-d’Armor), the rider and owner of the horse, who had lost consciousness, was saved thanks to witnesses, but the animal died suddenly.

© D. Ademas / AFP

In 2009, Thierry Morfaitse collapsed after breathing for hours the hydrogen sulphide released by the decomposing green algae he was carrying.

The same year, a horse died and its rider was poisoned by the fumes of these aquatic plants. Justice considered the State responsible in this case.



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