Relations between Benthic Community Structure and Metals Concentrations in Aquatic Macroinvertebrates: Clark Fork River, Montana

ABSTRACT

We sampled macroinvertebrate communities at six sites on the upper Clark Fork River, Montana, to determine relations between macroinvertebrate community structure and metals in invertebrates and the best benthic community metrics to use for ranking sites based on the relative severity of the effects of metals. Concentrations (μg/g) of six metals in invertebrates were determined: Al (range = 591–4193), As (2.7–34.1), Cd (0.13–8.38), Cu (26–1382), Pb (0.54–67.1), and Zn (212–1665). Concentrations of As, Cd, Cu, Pb, and total metals were significantly correlated with at least one benthic metric. Copper (r = 0.88–0.94) and total metals (r = 0.90–0.97) provided the most highly significant correlations. Based on longitudinal site comparisons of metals in invertebrates, benthic community structure, and differences between proportionally scaled ranks, five benthic metrics provided the best indicators of relative impact: taxa richness, Ephemeroptera-Plecoptera-Trichoptera (EPT) richness, chironomid richness, percentage of the most dominant taxon, and density. The two sites with the highest accumulations of invertebrate metals also demonstrated the greatest relative degree of impact based on these parameters. The most meaningful combinations of metrics indicate that the benthic community at the most upstream site is being severely impacted by metals. Two sites demonstrated little or no negative impact, and three sites demonstrated low or moderate levels of negative impacts, which may be due to a combination of metals and other factors such as organic enrichment. We recommend that benthic community structure and metals in invertebrates collected from riffle habitats be used to determine relative impacts in metals-contaminated river systems, owing to their close relation to metal availability and transfer to higher trophic levels.