Bioavailability of metals in stream food webs and hazards to brook trout (Salvelinus fontinalis) in the upper Animas River watershed, Colorado

The water quality, habitats, and biota of streams in the upper Animas River watershed of Colorado, USA, are affected by metal contamination associated with acid drainage. We determined metal concentrations in components of th e food web of the Animas River and its tributaries-periphyton (aufwuchs), b enthic invertebrates, and livers of brook trout (Salvelinus fontinalis)-and evaluated pathways of metal exposure and hazards of metal toxicity to stre am biota. Concentrations of the toxic metals cadmium (Cd), copper (Cu), lea d (Pb), and zinc (Zn) in periphyton, benthic invertebrates, and trout liver s from one or more sites in the upper Animas River were significantly great er than those from reference sites. Periphyton from sites downstream from m ixing zones of acid and neutral waters had elevated concentrations of alumi num (Al) and iron (Fe) reflecting deposition of colloidal Fe and Al oxides, and reduced algal biomass. Metal concentrations in benthic invertebrates r eflected differences in feeding habits and body size among taxa, with great est concentrations of Zn, Cu, and Cd in the small mayfly Rhithrogena, which feeds on periphyton, and greatest concentrations of Pb in the small stonef ly Zapada, a detritivore. Concentrations of Zn and Pb decreased across each trophic linkage, whereas concentrations of Cu and Cd were similar across s everal trophic levels, suggesting that Cu and Cd were more efficiently tran sferred via dietary exposure. Concentrations of Cu in invertebrates and tro ut livers were more closely associated with impacts on trout populations an d invertebrate communities than were concentrations of Zn, Cd, or Pb. Coppe r concentrations in livers of brook trout from the upper Animas River were substantially greater than background concentrations and approached levels associated with reduced brook trout populations in field studies and with t oxic effects on other salmonids in laboratory studies. These results indica te that bioaccumulation and transfer of metals in stream food webs are sign ificant components of metal exposure for stream biota of the upper Animas R iver watershed and suggest that chronic toxicity of Cu is an important fact or limiting the distribution and abundance of brook trout populations in th e watershed.