Comparative application of indices of biotic integrity based on periphyton, macroinvertebrates, and fish to southern Rocky Mountain streams

To assess the relative sensitivity of assessments using community metrics for macroinvertebrates, periphyton, and fish assemblages, we compared the results of three parallel assessments using these assemblages at 86 stream reaches sampled in 1994 and 1995 by the Regional Environmental Monitoring and Assessment Program (R-EMAP) in the mineralized zone or historical mining region of the Southern Rockies Ecoregion in Colorado. We contrasted assessments using community metrics for each taxa group selected to be diagnostic of the two large-scale stressor gradients identified in this ecoregion: discharges from historical hardrock metal mines and agriculture, particularly pasturing of livestock. While principal components analysis (PCA) extracted axes from the metrics for all three assemblages correlated with increased metal concentrations, the axes differed in their sensitivity to different environmental gradients. Two axes extracted from the fish metrics were correlated with dissolved metals, suspended solids, and sediment embeddedness or with sediment metals. Two axes extracted from the macroinvertebrate metrics partially separated these two stressor gradients, while the single correlated axis extracted from the periphyton metrics did not. The second macroinvertebrate PCA axis was correlated with an environmental gradient correlated both with agricultural effects and with stream size, as were the second and third periphyton PCA axes. The third fish PCA axis was correlated with stream size and slope, but was not sensitive to agricultural effects. Fish, macroinvertebrates, and periphyton differ in their sensitivity to different stressors, and combining metrics for these assemblages into a mixed assemblage index of biotic integrity may increase the utility of the multimetric approach to diagnose environmental stressors at impaired reaches.