RELATIONS BETWEEN BENTHIC COMMUNITY STRUCTURE AND METALS CONCENTRATIONS IN AQUATIC MACROINVERTEBRATES - CLARK-FORK MONTANA

We sampled macroinvertebrate communities at six sites on the upper Cla rk Fork River, Montana, to determine relations between macroinvertebra te community structure and metals in invertebrates and the best benthi c community metrics to use for ranking sites based on the relative sev erity of the effects of metals, Concentrations (mu g/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). Con centrations of As, Cd, Cu, Pb, and total metals were significantly cor related with at least one benthic metric. Copper (r = 0.88-0.94) and t otal metals (r = 0.90-0.97) provided the most highly significant corre lations, Based on longitudinal site comparisons of metals in invertebr ates, benthic community structure, and differences between proportiona lly 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 t axon, and density. The two sites with the highest accumulations of inv ertebrate metals also demonstrated the greatest relative degree of imp act based on these parameters. The most meaningful combinations of met rics 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 o ther factors such as organic enrichment, We recommend that benthic com munity structure and metals in invertebrates collected from riffle hab itats be used to determine relative impacts in metals-contaminated riv er systems, owing to their close relation to metal availability and tr ansfer to higher trophic levels.