Ecotoxicology of Stable Pollutants in African Marine Ecosystems

Molluscs (particularly bivalves) and fish are usually proposed as good bioindicators for environmental contamination of marine ecosystems. Different steps can be recognized when assessing such contamination. The first approach consists of comparing stable residue concentrations determined in various environments or different periods. In this chapter, this is illustrated for heavy metals in bivalves from Africa. For some of them, species differences could be detected making comparison difficult: such data can only be used for comparing orders of magnitude. More detailed discussions need the data to be first corrected (normalized) as a function of the main factor influencing accumulation, which means that the transfer, excretion and accumulation mechanisms have first to be identified and understood. Such factors vary, however, with the type of residues, mainly in their liposolubility, and also with the biota concerned: age, lipid content, sex, weight, etc. The importance of this step, as well as the difficulty of coming to clear conclusions, are illustrated for heavy metals, with emphasis on the non-essential ones—mainly mercury—in bivalves. The special case of mercury contamination of the sardine in the Mediterranean Sea is also presented. Finally, the importance of speciation and detoxification as well as the "biomass effect" of phytoplankton are discussed.