Biomasses d'ulves et flux de nutriments en baie de Douarnenez

In spring and summer time, green tides outbreaks yearly occur on sandy beaches of the central and southern parts of the bay of Douarnenez. During four years the nitrate loadings of incoming streams were measured during the growth season of the green algae. Every summer the stock of stranded algae as well as those in the very shallow water were quantified. Three streams from the eight investigated provide the most important part of nitrogen loadings. These ones tend to diminish in the course of spring and summer. They are very different from year to year. The observed biomass of algae is also very different according to years (from 120 to 3 400 t), and even from day to day according to the wind direction. However, the southern part of the bay is more frequently affected. Surprisingly no link was found between nitrogen loadings and observed biomass of Ulva on beaches. This is caused by the coastline straightness which makes the algal mobility easier, and probably also the existence of a deeper stock of Ulva (-2 to ¿18 m below LWST), recently discovered. Its great size (till 10 000 t) and its mobility suggest an important role in the green algal phenomenon. This so-called offshore stock, is probably one way of eutrophication enhancement. From this point of view, the whole bay must be considered as a functional unit. Scientifically speaking, the recovery of a good nitrogen quality of water and the control of green tides in the bay of Douarnenez should not be split up geographically. Moreover the algal quantification requires to assess simultaneously both the littoral and offshore stocks.