Ecosystem quality and natural heritage preservation: the case of the littoral eutrophication 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 insufficient to curb this phenomenon. However, the resorption policy of the nitrogenized surpluses arriving in the bay in order to fight against the aestival green tides could, in the long term, reduce this food resource and force geese to find 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 converting 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 feeding grounds for young fish 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.