Previously, toxicity studies have mainly focused on the responses of organisms to single toxicants; however the importance of studying mixtures of toxicants is now being recognised, along with the importance of speciation as a modifier of toxic effect. The aim of this study was to improve our understanding of the toxic response of the compost worm Eisenia veneta to cadmium, copper and zinc by integrating understanding of speciation effects into existing mixture models. Adult earthworm tests were carried out in four soils with different pH and organic matter contents, spiked with varying single and mixed doses of Cu/Zn and Cd/Zn. The reproduction rate of the earthworms after a four week exposure was studied. The three metals had different levels of toxicity in different soils. There were clear differences in the responses of the earthworms to the toxic metals in the different soils, and the extent of metal mixture interaction found depended on the form of metal (total metal, free ion (M2+) or diffusive gradient in thin film (DGT) effective concentrations) used to express the dose to the organisms. Mixture interactions were different when expressed in different metal forms and in the four soils. Detailed analysis of the different chemical species provides further insight into how the soil environment instigates the observed mixture effects. The results from this study support the need to consider bioavailability and mixture effects in toxicity studies
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