Eutrophication of coastal waters: the case of Brittany’s green tides

By Nicolas ROLLO

L’Atlas Bleu / Protecting

eutrophisation, zone côtière, océan global
The intensification of agricultural methods used during the second half of the 20th century significantly altered inputs from continental hydrosystems. The most notable effect of the environmental disturbances associated with these terrigenous inputs is undoubtedly the eutrophication of coastal waters, due to an excessive supply of nutrients. In 2008, it was estimated that green tides affected more than 400 coastal water bodies worldwide. This phenomenon may occur in two major forms: coloured water and green tides.

In France, although strandings of green algae (Ulva) certainly occur elsewhere than Brittany, the media coverage they attract has led them to become associated with the Breton coastline. This association is such that the Brittany Region responded to the campaign carried out by France Nature Environment in 2011 by saying that ‘its image was damaged’, even though it was not explicitly mentioned in the offending posters. It should be said that since the early 1990s, Brittany has very often been singled out concerning deterioration in water quality, which has also resulted in the implementation of various wide-ranging intervention programs. Despite the fact that the situation is improving as a result of the actions taken, it is still a problem.

Poster of the campaign of the collective of associations France Nature Environnement, published in 2011

carte, algue verte, bretagne, france

After having spread well beyond the historic sites of the bay of Saint-Brieuc and bay of Lannion in the 1990s, the green algae blooms seem to have peaked in 2009. The extent of the problem meant that the government had to take emergency measures integrating the implementation of the ‘green algae plan’. Specifically, the goal of this plan was to reduce by 40% the nitrogen fluxes that fed into the eight most affected bays, primarily located on the northern and western coastlines of the peninsula. Besides the imbalances caused within ecosystems, the eutrophication of coastal waters can have significant economic impacts. In the case of Brittany, it was estimated that it cost more than one million euros to collect and put to good use the 2009 green algae alone. The losses incurred, especially in terms of tourist activities, also need to be taken into account.

The mixed results of the actions taken can be partly explained by the inertia of the environment. As a result, given the slow response time of hydrosystems to changes in practices, the measures undertaken must be carried out over the long term in order to truly restore Breton aquatic environments.

Nicolas ROLLO, Enseignant-chercheur à l’Université de Nantes

Nicolas ROLLO, « Eutrophisation des eaux côtières : le cas des marées vertes en Bretagne », L’atlas Bleu, Revue cartographique des mers et des littoraux. Mis en ligne le 11 janvier 2020,

(version digitale adaptée d’après l’article paru dans L’Atlas Permanent de la Mer et du Littoral n°7 « Risques littoraux et maritimes ». Ed. LETG-Nantes, 2015. pp.54-55)


DOI : 10.35109/atlasbleu-eng.10012