characteristic of the biota that changes in some predictable way 

 with increased human influence (Barbour et al'. 1999) . 



One additional metric was calculated for this study: 

 percent of cells in the diatom family Epithemiaceae . This family 

 is represented in rivers by two genera, Epithemia and Rhopalodia, 

 which commonly harbor endosymbiotic nitrogen- fixing bluegreen 

 algae (cyanobacteria) within their cells. A diatom association 

 that contains a large percentage of cells in these genera may 

 indicate nitrogen-limiting conditions, that is, low nitrogen to 

 phosphorus ratios (Stevenson and Pan 1999) . 



Metric values from study sites were compared to numeric 

 biocriteria developed for streams in the Rocky Mountain 

 Ecoregions of Montana (Table 3) . These criteria are based on 

 metric values measured in least- impaired reference streams (Bahls 

 et al . 1992) and on metric values measured in streams that are 

 known to be impaired by various sources and causes of pollution 

 (Bahls 1993) . 



The criteria in Table 3 distinguish among four levels of 

 impairment and three levels of aquatic life use support: no 

 impairment or only minor impairment (full support) ; moderate 

 impairment (partial support) ; and severe impairment (nonsupport) . 

 These impairment levels correspond to excellent, good, fair, and 

 poor biological integrity, respectively. 



Only metrics generated from periphyton samples collected in 

 summer (June 21 -September 21) may be compared with confidence to 

 the biocriteria in Table 3 . Metric values change seasonally and 

 summer is the season in which reference streams and impaired 

 streams were sampled for the purpose of biocriteria development. 

 Because the samples analyzed for this report were collected in 

 winter, comparisons of metrics with the biocriteria in Table 3 

 must be made with caution. 



