After the sorting and identification is complete, BMIs are assessed using three different metrics that
include two richness and one tolerance metric. These metrics are integrated into an overall stream score,
which is given an integrity rating. Numbers can be calculated from abundance ratings by assigning a simple
numbering system (A = 6; C = 3; R = 1). The calculation procedure for each metric is described below:
- Total Taxa: This metric is simply the total number of different kinds from the entire
- EPA Taxa: The total number of different kinds from the Ephemeroptera (mayflies),
Plecoptera (stoneflies), and Trichoptera (caddisflies) orders.
- Biotic Index: Multiply the abundance number by the tolerance rating to determine the
tolerance score. Add the entire tolerance score column and the abundance column. Divide the total
tolerance by the total abundance.
- Stream Score: The overall stream score is determined using a point scale system, which
combines the metrics into a single number and integrity rating.