BOX 2 Benthic Community Metrics

Taxa Richness and Community Composition

  • Taxa richness
  • Occurrence of intolerant snail and mussel speciesa
  • Number of mayfly (Ephemeroptera) taxa
  • Number of stonefly (Plecoptera) taxa
  • Number of caddisfly (Trichoptera) taxa
  • Total number of EPT taxaa
  • Percent oligochaetes
  • Percent in the two most dominant taxa

Trophic and Functional-Feeding Group

  • Percent omnivores and scavengers
  • Percent collector-filterers
  • Percent predators

Abundance

  • Total abundance of individuals (combined quantitative samples, lower score given for extremely low or extremely high values)

a  

Metric applied to qualitative and quantitative samples combined. All other metrics applied to individual quantitative samples and resultant scores averaged.

Benthic Community

A modified version of the Benthic Index of Biotic Integrity (Kerans et al., 1992) is used to rate the condition of stream benthic communities. Twelve benthic community attributes in three areas (Box 2) are scored based on expected conditions at reference sites with water quality supportive of healthy benthic communities. Sampling effort consists of three Surber (riffle), three Hess (pool), and one qualitative sample. Metrics for EPT and intolerant snail and mussel species are computed by pooling all qualitative and quantitative samples. Total abundance is computed by pooling all quantitative samples. Metrics are computed separately for each quantitative sample at a station, then averaged by station.

The value obtained for each metric is scored poor, fair, or good (1, 3, or 5) against the best expected values for the reference sites. Scores are then summed to produce an index for each station that ranges from 12 to 60. That overall benthic community index for each stream location is classified as poor (score <30), fair (score 34-44), or good (score >45). Professional judgment is used to rate index scores between 30 and 33 as poor or fair.



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