Green Tides in Brittany caused by nitrate

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For the past 25 years, an increasing number of beaches and coves along the Breton coast have been invaded, from spring to autumn, by a proliferation of green macro algae – or chlorophytas (free ulvas, attached enteromorphas). This typical example of eutrophication was studied in the bays of St. Brieuc, Lannion, Douarnenez and in the Brest Harbour, and attributed to both the natural confinement of shallow waters in these areas and the recent increase in nitrate in these waters. In naturally confined sites, studies linked summer biomass levels to the amount of nitrates conveyed by rivers in the spring.

The decrease in these amounts in the summer also explains the drop in nitrogen content in the ulvas and the absence of algae growth during this period. IFREMER mathematical models show that the only way to reduce ulva biomass on beaches is to reduce the amount of agricultural nitrate that is carried to the area. In the most sensitive areas, nitrate levels in rivers need to be reduced from 40 mg/l to less than 10 mg/l, which represents a real challenge for society. A century ago, these levels did not exceed 3 or 4 mg/l.

Alain Ménesguen, IFREMER (French Research Institute For Exploitation of the Sea), Juin 2003

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