The Méjean Lagoon (France) is a confined, shallow system, 0.7 m in depth, with a surface area of 747 ha. Its sediments have a fine granulometry and are evenly distributed. The bathymetry and hydrodynamic behaviour of the basin create two zones with restricted exchanges between them. The western part (60% of the total surface area) is rich in dissolved phosphate (> mg l-1 in summer) and receives the majority of the phosphate (P) inputs from the watershed. The heavy macroalgal population consists of Gracilaria throughout the year and Ulva in summer and autumn. Phytoplankton levels sometimes reach 100 µg l-1 of chlorophyll a. The sediment-water phosphate exchange varies with environmental conditions and macroalgal growth. P mobility was studied on the basis of these characteristics. Algal biomass and water characteristics were measured at 15-day intervals throughout 1993. A seasonal study was conducted in 1994 to investigate which P fractions in the sediment were involved in the exchanges. The springtime rise in temperature reactivated decomposition of the macroalgal biomass that had accumulated in autumn. The redox potential (EH) fell as a result of this biological activity, leading to a decrease in the inorganic P fractions. This P release accounts for the higher P water concentrations observed in the lagoon in summer. The mobility of P fractions shows that the P stored in the sediments plays an active role in the dynamics of the overlying water. Seasonal variations in these fractions explain the patterns of P storage and mobilization.