Assessing the influence of water and substratum quality on benthic macroinvertebrate communities in a metal-polluted stream: an experimental approach

1. Field and laboratory experiments were conducted to assess the relative influence of water quality and substratum quality on benthic macroinvertebrate communities in the Animas River, a metal-polluted stream in south-western Colorado (U.S.A.). 2. A community-level in situ toxicity test measured direct effects of Animas River water on benthic invertebrates collected from a reference stream (Elk Creek). The effects of metal-contaminated biofilm were examined by comparing macroinvertebrate colonisation of clean and contaminated substrata placed in Elk Creek. A feeding experiment with the mayfly Baetis tricaudatus Dodds (Ephemeroptera: Baetidae) examined metal bioaccumulation and effects of metal-contaminated biofilm on growth and survival. 3. Animas River water was acutely toxic to most taxa, with greatest effects observed on mayflies (Heptageniidae, Ephemerellidae) and stoneflies (Taeniopterygidae and Capniidae). 4. Although Animas River biofilm was characterised by high concentrations of metals and low algal biomass, most taxa colonised substratum from the reference stream and the Animas River equally. The exceptions were Ephemerellidae, Taeniopterygidae and Simuliidae, which were less abundant on Animas River substratum. Mayflies grazing Animas River biofilm accumulated significantly more metals and showed reduced growth compared with organisms feeding on Elk Creek biofilm. 5. Results of our experiments demonstrated that effects of heavy metals on benthic community structure in the Animas River were complex, and that responses to metals in water and contaminated substratum were species-specific. Predicting recovery of benthic communities following remediation requires an understanding of these species-specific responses.