Joint toxic action of binary metal mixtures of copper, manganese and nickel to Paronychiurus kimi (Collembola)

• Mixture toxicity of Cu, Mn and Ni on reproduction of P. kimi was evaluated.
• Toxic unit (TU) approach was used to assess the joint toxicity of binary mixtures.
• Mixture toxicity varied with effect level on which the mixture is being judged.
• The additive interaction between metals was fundamental to predict mixture toxicity.

The joint toxic effects of binary metal mixtures of copper (Cu), manganese (Mn) and nickel (Ni) on reproduction of Paronhchiurus kimi (Lee) was evaluated using a toxic unit (TU) approach by judging additivity across a range of effect levels (10–90%). For all metal mixtures, the joint toxic effects of metal mixtures on reproduction of P. kimi decreased in a TU-dependent manner. The joint toxic effects of metal mixtures also changed from less than additive to more than additive at an effect level lower than or equal to 50%, while a more than additive toxic effects were apparent at higher effect levels. These results indicate that the joint toxicity of metal mixtures is substantially different from that of individual metals based on additivity. Moreover, the close relationship of toxicity to effect level suggests that it is necessary to encompass a whole range of effect levels rather than a specific effect level when judging mixture toxicity. In conclusion, the less than additive toxicity at low effect levels suggests that the additivity assumption is sufficiently conservative to warrant predicting joint toxicity of metal mixtures, which may give an additional margin of safety when setting soil quality standards for ecological risk assessment.