Les "marées vertes" en Bretagne, la responsabilité du nitrate

Since 25 years, an increasing number of beaches and coves along the Britanny coast are invaded from spring until autumn by proliferating green macroalgae (free ulvae, fixed enteromorphae). This typical case of eutrophication, which has been well studied in the bays of Saint-Brieuc, Lannion, Douarnenez and Brest, could be explained by the conjunction of a natural containment of shallow water masses with a recent enrichment of them by nitrate river loadings. In the naturally confined sites, field measurements of summer biomass showed a good correlation with the spring nitrate river loadings ; furthermore, the summer rarefaction of these loadings explains the fall of the nitrogen content of the ulvae, which induces the summer stop of the algal growth. The mathematical models built by Ifremer show that the only way to lower the ulva biomass on the beaches is to reduce the agricultural nitrate leaching. In the most significant sites, it would then be necessary to bring the nitrate concentration in the rivers from the present 40 mg/L back to 10 mg/L or less, what constitutes a true challenge for the Breton society. One century ago, this concentration probably did not exceed 3 or 4 mg/L...