Inventaire des Ulves en Bretagne année 1998 - rapport de synthèse

The 1998 survey of green tides on the Brittany coast was performed in the same way as that of 1997 in order to allow comparison. 11 includes an inventory ofail the concerned sites, and a precise assessment ofthirteen ofthem, the most loaded. On these ones, nitrogen fluxes were surveyed during the critical period, and the green seaweed stock on the beach and in the first waves was quantified. The climatic conditions were opposite between the two years, very dry in spring and summer 1997, and wet in 1998. The irradiance contrast was inverse, and less pronounced. The nitrate contents and mostly the flows were more important in 1998. They generated nitrogen fluxes about 3.3 times those of 1997. The total number of sites concerned in 1998 is clearly higher than in 1997. The algal biomass on the beach and in the next waves amounts 21 000 metric tons in 1998, versus 19 900 t in 997. The large increase of the nitrogen flux seems to be expressed by only a very small extra biomass. The relative index of transformation yield of nitrogen into U/va in the various sites are 1ess than those of 1997. In the bays of La Fresnaye, Lannion, and Douarnenez, the index remains high. The bays of Lancieux and l'Arguenon must be considered as a unique site, due to the easy communication at low tide; the same for the different parts of the bay of Douarnenez. An important stock of U/va was discovered in deeper parts of that bay (-2 to -18 m). It seems that such "offshore" stocks are present in other sites. 11 probably represents a way of spread of marine eutrophication in Brittany, as the increase of the number of concerned sites. (Not controlled OCR)