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Evaluation of Exposure-Effects Relationships of Metals in the Benthic Macroinvertebrate Community in the Upper Clark Fork River, Montana

Previously published studies conducted on the Upper Clark Fork River (UCFR) suggest adverse effects due to metals-enriched sediments found in the depositional areas that comprise approximately 4% of the riverbed. While these studies measure exposure concentrations from depositional areas, effects measurements (e.g., benthic invertebrate abundance and diversity measures and tissue residues) have been predominantly obtained from coarse substrate riffle areas. Comparing exposure data from one habitat to effects data from another is problematic. An integrated approach was conducted to assess effects data by sampling benthic macroinvertebrates for community composition measurements, tissue residues, and sediment contaminant concentrations. Thirteen co-located sampling sites along the UCFR were sampled in both depositional and riffle habitats in 1996. Metals concentrations in bulk sediments and benthic invertebrate tissues decreased with distance from metals sources in the headwaters, but sediment porewater concentrations did not. Although there were some significant relationships between bulk sediment metals concentrations and tissue metals residues, BMI community metrics did not appear to vary along this gradient, or with exposure concentrations. This is consistent with an evaluation of sediment toxicity tests performed using sediments from Warm Springs Ponds, as well as the UCFR, which do not suggest that metals are of potential concern to benthic macroinvertebrates.