Nitrogen loadings and macroalgal (Ulva sp.) Mass Accumulation in Brittany (France)

Abstract The most common manifestation of eutrophication along the coasts of Brittany (France) is the proliferation of free-floating green macroalgae of the genus Ulva on some beaches during spring and summer. In the most eutrophicated site, the Bay of Saint-Brieuc, a monitoring program has been running since 1986 until 1992 (1987 excepted) to estimate during spring and summer the nitrogen and phosphorus river loadings, the growth rate of macroalgae, their N- and P contents and the annual maximum biomass present on the site. Every year, algae get increasingly N-depleted during the vernal rapid growth phase and stay N-starved (1.5% of dry weight) all summer long. Algal phosphorus content, on the contrary, remains roughly constant at a rather high level (0.4% of dry weight). This physiological argument for nitrogen control on algal growth is confirmed by independent results at the ecosystem level. Year-to-year fluctuations of the observed annual maximum biomass at the site have been successfully correlated ...