The proliferation of green algae began in Brittany in the 1970s. This complex phenomenon depends on numerous parameters (sunshine, temperature, rainfall, tides, storms, sea currents, etc.) that cannot be anticipated with certainty, and whose extent cannot be predicted.
For example, heavy rainfall in the spring can encourage the transfer of nutrients into rivers and the sea, resulting in a green tide in areas where algae had not proliferated in the drier previous year. Conversely, heavy winter storms can disperse the algae that remained in the autumn and delay their development the following year. The same is true when the end of winter is cold and sunny. This was the case in 2018, resulting in a significant delay in the growth of algae and particularly late strandings.
It is therefore impossible to predict whether a beach affected one summer will be affected again the following year. However, data does allow us to identify the sites that are most exposed to seaweed growth. There are eight bays where green tides have occurred every year since 1998, varying greatly in scale: the bays of Saint-Brieuc, Douarnenez, la Forêt and la Fresnaye, la Grève de Saint Michel and the coves of Locquirec, l’Horn-Guillec and Guisseny. It should be noted, however, that in recent years the volume of seaweed stranded has fallen overall. This phenomenon is particularly noticeable in the Lieue de Grève bay, near Lannion.
(Source: Schéma Directeur d’Aménagement et de Gestion des Eaux du Bassin Loire-Bretagne 2010-2015)