Bioavailability of Heavy Metals in Guanabara Bay, Rio de Janeiro (Brazil)

Guanabara Bay represents a coastal environment propitious to the study of environmental quality. Toxicity of coastal ecosystems is not only determined by metal concentrations of their sediments, but also by how these elements are partitioned between the various geochemical phases present within them. Recent studies have used selection extraction techniques and the determination of bacterial activity to identify the bioavaliability of metals for the trophic chain. In this study bioavailability is examined using detailed selective extraction, total element, organic carbon and bacterial activity analyses of surface sediments. Samples were collected in the bay, from 12 stations that were considered important in terms of pollution during rainy and dry seasons. Analytical results indicate high seasonal and spatial variations and heavy rain is an important parameter with respect to increasing superficial sediment runoff from the city, where this material acts as a source for Pb, Cr and Ni. Naval activities also reveal unquestionable sources of Cu, and the highest concentrations of this element were found in Niterói Harbour during both seasons. The reducible phase is shown to be an important "sink" for the majority of metals, whereas the soluble and residual phases bind lower concentrations. Exoenzyme and organic matter contents show poor correlation, highlighting the latter as a determinant parameter with respect to the geochemistry of the bay.