Mixtures of metals and polynuclear aromatic hydrocarbons elicit complex, nonadditive toxicological interactions in meiobenthic copepods.

The acute toxicity of metal-polynuclear aromatic hydrocarbon (PAH) mixtures (i.e., Cd, Hg, Pb, fluoranthene, and phenanthrene) associated with sediments was assessed in two benthic copepods. Schizopera knabeni was exposed to sediment amended with single contaminants and mixtures. Adult S. knabeni were highly tolerant of single-contaminant exposures to phenanthrene, Cd, Hg, and Pb as well as a mixture of Cd, Hg, and Pb. Binary experiments revealed that although phenanthrene was synergistic with Cd and Hg, the phenanthrene-Cd synergism was much stronger (2.8 times more lethal than predicted). When a mixture of Cd, Hg, and Pb was combined with phenanthrene, a synergistic response was observed, eliciting 1.5 times greater lethality than predicted. A Cd-phenanthrene synergism in S. knabeni was also observed in aqueous exposures, suggesting that the interaction was related to a pharmacological insult rather than a sediment-related exposure effect. An antagonism between Cd, Hg, and Pb was indicated, and this antagonism may have moderated the Cd-phenanthrene synergism in mixtures containing Cd, Hg, Pb, and phenanthrene. Experiments with Amphiascoides atopus revealed that phenanthrene and fluoranthene were each synergistic with Cd in aqueous exposures. Our studies suggest that interactive toxicity among metal-PAH mixtures may be common among benthic copepods and that strong synergistic effects observed in binary mixtures may be moderated in more diverse contaminant mixtures. However, the strength of the observed synergisms raises concerns that established sediment quality criteria may not be protective for organisms jointly exposed to PAH and metals, especially Cd-PAH mixtures.