Ecological impacts of chemicals such as metal mixtures in the field are predicted by one of two approaches: component-based and whole-mixture approaches. Knowledge about how to relate results of these approaches to field effect levels is limited. We aimed to examine the ability of two methods to discriminate field effect levels: (1) a component-based approach that calculates a cumulative criterion unit (CCU) equal to the sum of the ratios of four trace-metal concentrations (Cd, Cu, Zn, and Pb) to the corresponding US EPA hardness-adjusted water quality criterion, and (2) a whole-mixture approach that employs three ambient toxicity levels based on acute and chronic toxicity tests with cladocerans. To assess field effect levels, we used data from benthic macroinvertebrate surveys conducted at 26 sites, including 13 metal-contaminated sites, in four river basins across Japan. Based on decreases in mayfly richness and abundances of two metal-sensitive mayfly families, we defined four field effect levels 0–3: (0) no significant effect; (1) a significant decrease in ephemerellid and/or heptageniid mayfly abundance; (2) a significant decrease in mayfly richness; and (3) no mayflies or a few baetid mayfly species present. Sites with CCU values of ∼5–10 were assigned to either field effect level 1 or 2, while a site with a CCU value of 27 was assigned to field effect level 3. Sites with significant chronic toxicity to cladocerans (ambient toxicity level 1) were assigned to field effect level 1 or 2, whereas those with significant acute and chronic toxicity (ambient toxicity level 2) were assigned to field effect level 2 or 3. CCU values and ambient toxicity levels were both significantly correlated with field effect levels. These results provide encouraging evidence that field effect levels such as losses of species richness can be approximately discriminated by CCU values and ambient toxicity levels.
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