Detecting changes in stream health: Advantages of multiple-community bioassessments

Biological assessments based on a single community type may underestimate the extent of biological impairment because communities differ in their biological and ecological traits and, therefore, their sensitivities to different types of natural and human-caused stressors.  In this study, we compared biological condition for macroinvertebrate, fish, and algal communities sampled contemporaneously in 567 streams nationwide to evaluate community differences in identifying changes in stream health. Each community was classified as “altered” if its composition, measured by a single indicator, was different than that measured at 90% of regional reference sites.  Otherwise, each community was classified as “unaltered.”  A stream site was classified as “altered” if one or more communities were so classified.  Macroinvertebrate assessments alone, classified 69% of urban streams and 37% of agricultural streams as “altered.” The inclusion of fish and algal community assessments increased the number of sites classified as “altered” by 18 and 20%, respectively, in urban streams, and by 49 and 87%, respectively, in agricultural streams. Our results suggest that the advantages of multiple-community assessments in detecting changes in stream health appear to be less in urban than in agricultural streams, perhaps due to the number and intensity of human-caused stressors adversely affecting stream health.