LANDSCAPE ECOTOXICOLOGY: LINKING CATCHMENT LITHOLOGY TO THE ECOLOGICAL STATE OF STREAMS IN THE CENTRAL ROCKY MOUNTAINS OF COLORADO

Recent geochemical surveys of mountain streams in Colorado suggest that up to 25% are characterized by elevated trace-metal concentrations. These degraded streams are in basins where historical mining has occurred and in unmined basins that contain hydrothermally altered rocks. Federal land management priorities included remediation of watersheds on federal lands to improve fisheries habitat. Few geochemical baseline data area available for different rock types to evaluate the effects of variations in lithology on ecological communities, thus it is difficult to define reasonable expectations for remediation. Remedial targets are often selected to restore fisheries without regard of geochemical background concentrations that might influence success of remediation. To develop geochemical baselines that are ecologically relevant, we investigated the influence of catchment lithology on water and sediment toxicity to aquatic communities in the central Colorado Rocky Mountains. Sixty-two catchments (n = 84 samples) underlain by single rock types were sampled for benthic communities, metal concentrations in water and sediment, and water quality parameters that influence trace-metal bioavailability to aquatic organisms. Results show that rock type (i.e., sedimentary, metamorphic, igneous) and age (i.e., Cenozoic, Mesozoic/Paleozoic, Precambrian) are less important variables than hydrothermal alteration and the presence of pyrite in determining aqueous and sediment toxicity to aquatic organisms. Unmined basins underlain by Tertiary tuffs and metapelitic rocks and rocks altered to quartz-sericite-pyrite have metals concentrations in water and/or sediment that consistently exceeded federal water quality criterion or sediment quality guidelines. Aqueous and sediment toxicity explain an equal amount of variance in benthic community metrics. Sediment quality guidelines (i.e., Probable Effect Concentrations) were protective of benthic communities whereas aqueous exposure to metals caused impairment of benthic communities at metal concentrations below federal water quality criterion. These data suggest that some altered rocks likely impaired fisheries prior to mining and that remediation based on existing water quality criterion values may fail to recover fisheries.