Combination of Biological and Habitat Indices for Assessment of Idaho Streams

States and tribes are encouraged to use multiple biological assemblages in assessment of water bodies. An assessment index for each assemblage provides information on aspects of the aquatic resource that may be unique in terms of stressor sensitivity, stressor type, or ecological scale. However, assessment results relative to impairment thresholds can disagree among indices for an individual water body, leading to uncertain overall water-body assessments. We explored options for combining stream indices for macroinvertebrates, fish, and habitat in ways that would yield the most consistent and sensitive results relative to established disturbance categories. Methods varied in the scoring or rating scales used to standardize each index value, the thresholds used to define impairment of aquatic life uses, and the ways of synthesizing multiple indices. The index compositing method that scores each index on a continuous scale and averages the scores after standardizing had superior accuracy, sensitivity, and precision. In addition, using the 25th quantile of reference sites instead of the 10th quantile resulted in a more balanced error rate among reference and degraded site categories.