Composition and behaviour of heavy metals in suspended sediments in a tropical estuarine system

Guanabara Bay is one of the largest embayment in the Brazilian coast, being subjected to a high anthropogenic impact. The goal of this work is to describe the content and dynamics of heavy metals in the suspended sediments of this tropical coastal embayment, and to evaluate exchanges between the bay and the ocean. With this purpose, information on heavy metal concentrations in suspended sediments, and profiles of temperature, salinity and currents were acquired at an anchor station in the central channel of the bay, and coupled with the tidal cycle. The vertical profiles of Al and Fe presented the lowest values in the surface layer and the highest concentrations at the bottom due to their binding to the particulate matter. Conversely, Mn concentrations were lower at the bottom, where it was influenced by the low water temperatures of the salt wedge. Zn and V showed affinity with Fe and Mn oxyhydroxides. Pb, Cd and Hg were subjected to dilution, from mixing with particles that present low metal concentrations. The metal pollution index showed a clear relationship with the tidal stage, with higher values occurring in the ebb tide. The water column was stratified during the survey due to presence of oceanic cold water mass and high freshwaters discharge. As consequence, the residual transport of metals in the surface was toward to the coastal area and near the bottom was toward to the inner bay for the majority of the metals.