Nutrient and suspended matter discharge by tributaries into the Berre Lagoon (France): The contribution of flood events to the matter budget

The Berre Lagoon receives freshwater from three rivers and a hydroelectric plant. The latter has become the main tributary since the beginning of its functioning in 1966. In a recent juridical context of the plantʹs discharge restriction, this study focussed on rivers’ implication in the lagoonʹs eutrophication, with a strategy particularly adapted to floods. Rivers carry an annual mean load of 27,100 tons of suspended matter (SM) into the lagoon, 275 tons N–NO3, 36 tons P–PO4, 680 tons total N and 130 tons total P. Depending on the river, floods are responsible for up to 99% of SM annual input, 33% nitrate, 53% phosphate, 44% total nitrogen, and 72% total phosphorus. Respecting the quotas given, the power plant remains the main tributary of the lagoon: at least doubling the annual natural discharge of SM and nutrients. Nevertheless, global discharges are lower than those measured 20 to 30 years ago were due to the restrictions applied to the power plant, to dryer conditions and better treatment of sewage waters along the catchment basin.