Composição da matéria orgânica no gradiente estuarino da Baia de Sepetiba/RJ

Abstract

This study was developed in Sepetiba Bay, a complex estuarine system with high economic, social and environment importance to the Rio de Janeiro State. The main objective of this study was to determine the composition of organic matter in an estuarine gradient, involving the river compartment, consisting of the Guandu River and Channel, Guarda River and São Francisco Channel; a salt gradient in the São Francisco Channel characterizing the mixing zone and the estuarine region represented by Sepetiba Bay itself. This approach was applied in order to determine the main sources and processes acting in the Sepetiba Bay. To assess the composition of organic matter elemental (C/N), isotopic (δ 13 C and δ 15 N) and molecular (lignin, sterols and hydrocarbons) markers were used in sediment and suspended particulate matter samples. In this last compartment was only possible analysis of elemental and isotopic composition of organic matter. The trace metals were also determined to verify its correlation with organic matter or with existing processes in the Sepetiba Bay. The results of C/N ratio, δ 13 C and δ 15 N indicate that in the riverine compartment dominates fluvial terrigenous organic matter from higher plants debris. In the salt gradient was observed mixture of terrestrial and marine sources and, in Sepetiba Bay was observed the predominance of bacteria and phytoplankton, corroborated by the ch-a data and the highest concentrations of sterols (β-sit., cholestanol, cholestanone). The hydrocarbon showed high concentrations in the study area, ranging from 16 to 488 μgg -1 , recorded for coastal environment with high anthropogenic activity. These high concentrations associated with the distribution of n-alkanes and the presence of unresolved complex mixture, indicate an intense introduction of petrogenic hydrocarbons. The coprostanol, a fecal sterol associated with the release of untreated sewage, was detected in 81% of the stations, with concentrations ranging from undetectable levels to 4.0μgg -1 , and the highest values recorded in the Guandu River and Channel and after its mouth, in Sepetiba Bay. The lignin phenols showed low concentrations in the study area, with values between 0.05 and 0.72 mg.100 mg -1 O.C. with syringyl group predominance, indicating that Sepetiba Bay is a depositional system and the transport by watershed is not significant. The principal component analysis (PCA) showed 30.1% of the variability explained on axis 1, which grouped the samples from the rivers characterized as allochthonous terrigenous source. Moreover, samples from the estuary were characterized by processes of autochthonous production and degradation of organic matter. Conversely, the ACP when considering the axis 2, had 51% of the total variability explained, suggesting that the samples located in the mixing zone was influenced by the process of dredging of the Canal of San Francisco and the entry of industrial effluent.