Changes in nutrient loads (N, P and Si) in the São Francisco estuary after the construction of dams

The aim of this study was to investigate the post-d am conditions of the loads and yields of dissolved inorganic nitrogen (DIN), orthophosphate (DIP, silicate (DSi) and total phosphorous (TP) in the Lower Sao Franci sco riverestuary (NE-Brazil) after the river was regulated t o 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 (8 0 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, D IN loads reduced by 94 % and DSi by 31%. The river turned into an oligotrophic system with p rimary production limited by nitrogen and nutrient yields being among the lowest of Brazilian coastal rivers.