University of California Water Resources Center

The objectives of this project were to: 1) assess the biotic integrity of benthic macroinvertebrate communities in 7 stream systems subjected to grazing and irrigation within Bridgeport Valley using the California Stream Bioassessment Protocol (CSBP); 2) determine if yearly variation exists for standard macroinvertebrate metrics; and 3) identify correlations between CSBP visual habitat quality assessment, macroinvertebrate community, and habitat features. Thirty, 100 m study reaches were established across the 7 stream systems, representing the range of habitat type and quality found in Bridgeport Valley. In August 1999, macroinvertebrate collections, visual habitat quality assessments, and habitat feature measurements were conducted across 3 riffles within each of the 30 (n=90) study reaches according to the CSBP protocol. Macroinvertebrate collections were repeated during August of 2000 at 14 study reaches (n=42) to evaluate inter-annual variability in key CSBP macroinvertebrate metrics. Macroinvertebrate collections were sub-sampled, and all sub-sampled individuals were identified to family. Overall, biotic integrity of streams in Bridgeport Valley is relatively high (CSBP mean habitat quality assessments of “optimal” and “sub-optimal”; 69% EPT taxa), but CSBP habitat quality assessments indicate there is room for improvement across the Valley. Annual variation in macroinvertebrate community requires that comparisons across streams must occur within the same year, and that multiple years of data collection is justified. CSBP habitat quality visual assessment scores and habitat features were weakly correlated to aquatic macroinvertebrate community. While weak, these relationships did generally respond as expected from published work. We found the CSBP habitat quality assessment worksheet to be a valuable tool allowing trained individuals to identify relatively high and low habitat quality within managed systems. However, we must question the value of macroinvertebrate metrics as indicators of variable levels of grazing and irrigation impact on habitat quality typical of the eastern Sierra Nevada.