Eutrophisation littorale et conservation de l'avifaune aquatique : le cas de la Bernache cravant (Branta bernicla bernicla) hivernant en baie de Saint-Brieuc

ecosystem quality and natural heritage preservation: the case of the littoral eutrophica- tion and the wintering of Brent Geese Branta b. bernicla in the bay of Saint-Brieuc (France). — the national nature reserve of saint-Brieuc Bay, located in the north of Brittany, is a site of international importance for the wintering of the Black-bellied Brent goose Branta bernicla bernicla although its preferred food, eelgrass, is completely absent. local monitoring points out that the green seaweeds Ulva armocicana represents 90% of the food eaten by the Brent geese. these algae are responsible for the spectacular green tides which greatly impact this bay each summer. the 400 odd tons consumed each winter by the Brent geese are insuf- fcient to curb this phenomenon. however, the resorption policy of the nitrogenized surpluses arriving in the bay in order to fght against the aestival green tides could, in the long term, reduce this food resource and force geese to fnd a substitution food which, locally, could be a grass growing on salt-marshes (Puccinellia maritima) or winter cereals growing in arable lands bordering the bay. to avoid possible crop, the national nature reserve could have to increase the surface of Puccinellia maritima meadows by mowing or by con- verting to pasture on the climacic areas with Halimione portulacoides as is the practice on the atlantic coast, in aiguillon Bay. this will have to be done with caution, the areas with sea-purslanes being important feed- ing grounds for young fsh during the high tides. it is quite singular to see that, in this bay, a wintering area of international interest for a water bird developed thanks to serious environmental perturbation and that this ornithological interest could be disrupted by current curative policies of water quality restoration, should no compensatory measures come to balance for the hoped reduction of the green tide.