John Wiley & Sons, Ltd.

Mixture toxicity of nickel and zinc to Daphnia magna is non‐interactive at low effect sizes, but becomes synergistic at high effect sizes

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To incorporate metal mixture toxicity effects into risk assessment procedures, more information is needed about combined and interactive effects of metal mixtures during chronic exposure. Here, the authors investigated the toxicity of binary Ni‐Zn mixtures in two independent full‐factorial experiments using standard chronic (21‐day) Daphnia magna reproduction toxicity tests. Global statistical analysis (i.e. when considering all investigated mixture treatments simultaneously) showed non‐interactive effects according to the concentration addition (CA) model and significant synergistic effects according to the independent action (IA) model. However, treatment‐specific statistical analysis revealed that both occurrence and type of interactive effect were dependent on the effect size at which Ni and Zn were combined in the mixture. Only non‐interactive or weakly antagonistic effects occurred in mixture treatments in which each of the individual metals produced only weak adverse effects on their own (i.e. 20% or less reduction of reproductive performance). On the other side of the spectrum, synergistic mixture effects occurred in all mixture treatments where both metals already caused a greater than 20% (for IA) and 40% (for CA) effect on reproduction on their own. Since low effect sizes are the most relevant ones in most regulatory frameworks, the authors' data suggest that the CA and IA mixture toxicity models can both serve as conservative models for predicting effects of Ni‐Zn mixtures. The present study highlights the importance of investigating metal mixture toxicity at low effect sizes and warns against extrapolating conclusions about metal mixture interactions from high to low effect sizes. This article is protected by copyright. All rights reserved

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