Assessment of human effects through phosphorus partitioning in sediments from two catchment basins around Guanabara Bay, SE Brazil

Geochemical analyses of sediment samples collected along the fluvial systems of the Guapi-Macacu River and the Mangue Channel hydrographic basins, which flow into a distinct portion of the Guanabara Bay, show the presence of different anthropogenic impacts in the area. The Guanabara Bay is located inside the metropolitan area of Rio de Janeiro, which is one of the most urbanized and industrialized regions in Brazil. Consequently, this estuarine system is subjected to intense degradation processes due to the eutrophication of the hydrographic systems around the bay. This study aims to investigate the influence of the seasonal variations of environmental factors on the fractionation of phosphorus in the hydrographic basins of the Guapi-Macacu River and the Mangue Channel. The geochemical features of the water and sediments were analyzed, and the interactions between these features, the physicochemical parameters, and the sediment properties of the two basins were identified. Phosphorus fractionation revealed that the various forms of phosphorus displayed distinct behaviors. In the Mangue Channel Basin, the main phosphorus fraction detected during the winter and summer was associated with iron, indicating a reductive environment rich in organic matter (i.e., a polluted environment). In the basin of the Guapi-Macacu River during the winter, the residual phase (P-Re) was predominantly detected, whereas the metallic phase (P-Me) prevailed during the summer. The fractions associated with the physicochemical parameters did not vary significantly in this region, which implies oxidized and natural conditions.