Landscape ecotoxicology: Linking catchment-scale geology to trace-metal bioavailability and benthic macroinvertebrate population and community responses

Natural geologic processes cause alterations in rock chemistry which make them candidates for mineral extraction and likely affected water quality prior to mining. The extent to which mining has increased the transport of trace-metals to aquatic ecosystems above and beyond natural weathering is largely unknown. We conducted a regional-scale study to quantify the relationship between rock type and water chemistry in streams from Central Colorado. We developed a model capable of predicting toxicity due to trace-metal mixtures, Chronic Criterion Accumulation Ratio (CCAR), which is derived from the Biotic Ligand Model (BLM) and incorporates current theory about the interactions between aqueous constituents that affect metal toxicity and accumulation in fish and aquatic invertebrates. We evaluated if CCAR could differentiate the bioavailability of metals produced to streams by various rock types and if it could predict the responses of resident aquatic communities exposed to bio-available metal. We also compared the predictive capacities of CCAR to a traditional toxic unit model, Cumulative Chronic Units (CCU).; Hydrothermal alteration was found to control metal concentrations and alkalinity in stream water. Primary rock-type and geologic age interact with hydrothermal alteration and pyrite mineralization to determine metal bioavailability to aquatic invertebrates. CCAR predicted the amount of accumulated metals in aquatic invertebrate whole body tissue, population and community responses to metals exposure; supporting the idea that the BLM is capable of predicting metal toxicity in aquatic invertebrates. The presence/absence of taxa was found to be a relatively insensitive indicator of metal contamination as compared to population density responses. This disparity in population response variables indicates that some native populations are comprised of subpopulations which are differentially sensitive to metals. We found that toxicity is not constant for all individuals of a population, a violation of an important assumption of the BLM. However, CCAR out performed the prediction of aquatic community responses to metals exposure as compared to CCU. More importantly, we observed deleterious effects in benthic macroinvertebrate communities an order of magnitude below water quality criteria suggesting that these standards are not protective of aquatic invertebrates from Colorado streams.