The São Francisco Estuary, Brazil

Abstract

This is a first account of the physical and biogeochemical characteristics of the tropical São Francisco (SF) estuarine system, East Brazil, western South Atlantic. The estuary (Lat. 10°36′S Long. 36° 23′W) is fed by the humid to semiarid SF river basin (AB = 634 × 10³ km², L = 2700 km), the second largest of Brazil's territory. Since the 1950s, SF has evolved into a unique system almost solely impacted by a cascade of dams, which now control 98% of the basin and reduced discharge to the estuary by 35%. The recent Xingó dam, operating since 1994 at 180 km from the coast, regulated the formerly unimodal seasonal discharge (range 800 to 8000 m³ s⁻¹) to a constant flow of around 2000 m³ s⁻¹. The formerly turbid river waters have become transparent and oligotrophic due to drastic material retention by the dams. The young Xingó reservoir exerted significant changes in the relative composition of inorganic and organic dissolved and particulate constituents being delivered to the estuarine mixing zone and thus also to the composition and sustenance of phytoplankton biomass and production. The more or less constant river flow eliminated the former seasonal migration pattern of the estuarine mixing zone and its lower saline portion (S>5 to <15) is now largely positioned over its pro-delta shoals. Relict muddy deposits beyond the river mouth, eroded and resuspended by intense wave mixing, have become the main source of suspended matter to the mixing zone and maintain the coastal plume more turbid than the river. These processes seem to control the behavior of several dissolved inorganic constituents, except for dissolved silicate, which behaves conservatively proportional to the mixing of fresh and marine waters. The extremely low chlorophyll a concentrations indicate that nutrient uptake by primary production along the mixing zone is minor. The coastal plume generally disperses southwestwards at an oblique angle to the coast. Its oligotrophic conditions are maintained by both the low material yields of the basin and efficient flushing by the oceanic South Equatorial Current (SEC), which impinges directly upon the coast.