Current data on the toxicity of metals to aquatic organisms show effect levels over many orders of magnitude of total metal concentrations. Thus it appears that the bioavailability of metals is not related to this parameter. Only a fraction of the metal present is related to the amount “seen” by the organism. From solution, toxicity is a function of the concentration of the free metal ion and some hydrolyzed species. From sediments, metal availability is not well understood, although recent work has indicated that metal bound in specific sediment compartments or phases may be the toxic form.

Bioavailability can be predicted from knowledge of the speciation of metal in an aqueous system. Currently, two approaches are used to study speciation in solution and sediment: fractionation and titration.

This paper examines the relation between water quality and the availability of metals to organisms. The chemistry and measurement of metal species should be used as the basis for reviewing bioassay results. Further research needs to be conducted to elucidate the portion of sediment-bound metal which is bioavailable. Problems in the interpretation of bioassay results for natural waters arise from the addition of metal in excess of the capacity of organic ligands for complexation and in not allowing reactions to attain equilibrium.