Bioavailability of Sediment-Bound Heavy Metals along the Coast of Eastern Africa and Evaluation of Metal Concentrations along the Trophic Hierarchy.

Abstract

Heavy metals pollution of near shore marine ecosystems is a growing problem in most of developing countries. Unfortunately, total concentrations of metals in sediments have been shown not to be fully worth relying upon as a good predictor of the bioavailability and toxic effects of metals to the biota. Furthermore, besides anthropogenic sources, natural processes such as weathering of parent materials can contribute significantly towards total metal concentrations in the sediments. Generally metal fractions of natural origin are relatively less accessible to the biota compared to metals from anthropogenic sources. Therefore, in order to accurately evaluate the possible biological effects of trace metals in sediment, additional information on mobilization capacity and bioavailability are required. The results presented in this study showed that levels of total metals in sediments from locations along the coast of Eastern Africa were in some cases comparable to heavily polluted areas around the world. But further analysis revealed that heavy metal content in this lowly industrialized region was actually coming from natural sources. The study also showed that sediment-bound metals (especially at rural locations) were generally not easily available to the local biota. Interlocations comparisons of metal concentrations in the biota showed higher values around polluted urban areas compared to rural locations. In the case of marine plants, the study showed that the labile metal fraction was a better predictor of bioavailability, while in for animal species both labile and EDTA extractable fractions were found to best predict bioavailable metals to this group of organisms. The study approach which involved analysis of metal concentrations in the biota, in sediments and different types of sediment metal extraction procedures and the calculation of metal enrichment factors in sediments, proved to be able to delineate metals of natural and anthropogenic origin.