High altitude creates unique challenging conditions to biota that limit the diversity of benthic communities. Because environmental pollution may add further stress to life at high altitude, the present study explored the effect of metal pollution on the macroinvertebrate community composition in Andean streams between 3,500 to 4,500 meters above sea level (masl) during wet and dry seasons. At polluted sites, showing a high conductivity and a low pH, metal concentrations (e.g., Al, 13.07 mg/L; As, 3.49 mg/L; Mn, 19.65 mg/L; Pb, 0.876 mg/L; Zn, 16.08 mg/L) ranged from 8-fold up to 3,500-fold higher than at reference sites. The cumulative criterion unit allowed quantifying the potential toxicity of metal mixtures at the contaminated sites. Principal component analysis of physical chemical variables showed that reference sites were more likely to be structured by transparency, water discharge, and current velocity, while polluted sites appeared to be determined by metals and conductivity. Canonical correspondence analysis indicated a strong influence of highly correlated metals in structuring invertebrate communities, which were dominated by dipterans, coleopterans, collembolans, and mites at polluted sites. At reference sites crustaceans, ephemeropterans, plecopterans, and trichopterans were the most representative taxa. We concluded that severe metal pollution induced changes in macroinvertebrate community composition in high-altitude Andean streams, with a replacement of sensitive taxa by more tolerant taxa. Yet relatively species-rich communities persisted under harsh conditions.
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