Using invertebrate and microbial communities to assess the condition of the hyporheic zone of a river subject to 80 years of contamination by chlorobenzenes

For over 80 years, chlorobenzenes were discharged into the Sebasticook River, Maine, from a woollen mill. Environmental conditions were assessed using invertebrate and bacterial techniques that were applied to river bed sediments at three contaminated and two reference sites. Invertebrate densities and species richness did not differ markedly among the impacted sites, one reference site, and data in the literature from clean waters. Paradoxically, the highest diversity and densities of invertebrates and their eggs occurred at the most contaminated site. Insect representation was low compared with other hyporheic zones. Although chlorobenzene concentrations were much greater than published limits for freshwater life, certain species (e.g., mayflies, caddisflies, and midges) were associated with high concentrations. The majority of variance in the faunal and microbial data was attributable to redox potential, ammonium levels, and downwelling, rather than to chlorobenzene. Genetic fingerprinting revealed a unique microbial community at the site most heavily contaminated with chlorobenzenes, but a high degree of similarity among the other two mill sites and the reference sites (although the latter proved subsequently to be contaminated with ketones and methyl chloride). There were no differences in taxonomic richness among sites.