MP2 Toxicity of Mixtures
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() Joint-Toxic Responses to Cadmium and Phenanthrene Mixtures in Benthic Invertebrates is Variable and Species Specific.

Gust, K1, Fleeger, J1, Marlborough, S1, 1 Department of Biological Sciences, Louisiana State University, Baton Rouge, LA, US

ABSTRACT- Bioassays were conducted employing Cd and phenanthrene (Phen) as model toxicants to investigate mechanisms mediating joint toxicity of complex mixtures of sediment-bound metals and hydrocarbons. The fresh water bulk-deposit feeding annelid Iylodrilus templetoni exhibited antagonistic lethal joint toxicity. Sublethal concentrations of Phen in combination with Cd increased the 10-d LC50 of Cd for I. templetoni by 40% (95% C.I. 38-42%). Uptake rates of Cd into I. templetoni tissues slowed in the presence of Phen and equilibrium concentrations of Cd in tissues were half that of individuals exposed to Cd alone. A bioenergenic-kinetic model incorporating experimentally derived data for sediment-ingestion rate, assimilation efficiency, rate of aqueous uptake, and elimination rate accurately predicted Cd body burden reductions and suggested that these reductions resulted from reduced sediment-feeding rates caused by Phen. In contrast, synergistic lethal toxicity was found in the estuarine meiobenthic copepod Schizopera knabeni. The 4-d LC50 for equi-toxic equivalents of Cd and Phen was reduced by 70% (95% C.I. 62-78%) compared to toxic unit addition. Similarly, the freshwater amphipod Hyalella azteca exhibited synergistic lethal toxicity to combined Cd and Phen. Sublethal concentrations of Phen reduced the 10-d LC50 of Cd in H. azteca by nearly 50% (95% C.I. 37-59%). However, no synergistic lethal toxicity and no difference in Cd body burden were observed in H. azteca exposed to combined contaminants in aqueous solution. These results suggest the synergisitic toxicity found in H. azteca is sediment mediated. Combined Cd and Phen may modify bioavailability or alter H. azteca's sediment processing activity enhancing bioaccumulation. Benthic invertebrates' association with sediment may influence joint toxicity of Cd and Phen and therefore standard aqueous tests including WET tests and single compound bioassays may miss potentially hazardous mixture effects found in sediment environments. Our results suggest Cd and Phen mixtures may cause synergistic or antagonistic toxicity depending on the species investigated and the exposure medium used. Therefore, risk assessment protocols for environments containing metal-PAH mixtures must include methods to detect interactive effects between contaminants to optimize environmental protection.

Key words: PAHs, Sediment Toxicity, Metals, Benthic Invertebrates

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