Changes in Nutrient Loads (N, P and Si) in the Sao Francisco Estuary after the Construction of Dams

The aim of this study was to investigate the post-dam conditions of the loads and yields of dissolved inorganic nitrogen (DIN), orthophosphate (DIP, silicate (DSi) and total phosphorous (TP) in the Lower Sao Francisco river-estuary (NE-Brazil) after the river was regulated to a constant flow by the dams. Loads and yields of monthly measurements performed from November 2000 to March 2002 at a gauging station downstream of the dams (80 km from the coast) showed 4.1 x 10(3) t/yr and 0.006 t/km(2)/yr of DIN, 0.2 x 10(3) t/yr and 0.002 t/km(2)/yr of DIP, and 448 x 10(3) t/yr and 0.71 t/km(2)/yr of DSi, respectively. Over the last 15 years, DIN loads reduced by 94 % and DSi by 31%. The river turned into an oligotrophic system with primary production limited by nitrogen and nutrient yields being among the lowest of Brazilian coastal rivers.