Excessive proliferation of UIva sp. (Chlorophyta) during spring and summer was responsible for increasing injuries to recreational use of several beaches of Britanny (France) during the last decade. These worldwide mass blooms are well known in eutrophicated lagoons (Venice, Tunis... ), but look paradoxical along open beaches with large tidal range and weak terrestrial nutrients inputs. Numerical modelling of the coupled physical and biological aspects clearly explains the Ulva mass blooms by the dynamical trapping of water masses in bays where tidal residual drift vanishes, and shows the main responsability of the increase of nitrogen inputs (of agricultural sources) in triggering the recent magnification of a natural phenomenon.