Effects of metal mixtures on benthic macroinvertebrate communities in the field

Concerns about the toxic effects of chemical mixtures have led to regulatory organizations considering how best to address exposures to complex mixtures in the environment. The ubiquitous nature of metals means that they are always present in the environment, even if only at very low levels. It is appropriate to consider whether the mixtures of commonly regulated metals in the environment are likely to cause adverse effects on ecosystems if the environmental quality standards (EQSs) for all the individual metals are complied with. The total risk from four metals (copper, lead, nickel, and zinc) was evaluated in terms of the potential effects on freshwater benthic macroinvertebrate communities from the United Kingdom. The total risk was expressed as the sum of the individual risk characterization ratios for each metal (∑RCR). The ecological data are expressed relative to predicted reference conditions to provide an ecological quality ratio, which indicates whether the local community has been affected by any stressors by comparison to unaffected reference conditions. Very high metal exposures, expressed as the ∑RCR value, were found to be associated with reduced ecosystem diversity. However, a 10% reduction in community diversity relative to the predicted unaffected reference conditions is expected to occur only at ∑RCR values of greater than 8 ∑RCR units. This indicates that in "real world" situations, where a suite of inorganic and organic pollutants may be present, if the EQS for each of the individual metals is complied with (in this case, a ∑RCR value no higher than 4), there will likely not be any observable impact on benthic invertebrate community diversity despite the presence of these metals and other contaminants.