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, Epithewia and Rhopalodia, 

 that 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 Painted Robe Creek were compared to 

 numeric biocriteria developed for streams in the Great Plains 

 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 periphyton samples collected in summer (June 21- 

 September 21) can be compared with confidence to reference stream 

 samples because metric values change seasonally and summer is the 

 season in which reference streams and impaired streams were 

 sampled for the purpose of biocriteria development. However, 

 late spring is an acceptable time to sample prairie streams 

 because flows often become depleted in summer. 



Quality Assurance. Several steps were taken to assure that 

 the study results are accurate and reproducible. Upon receipt of 

 the samples, station and sample information were recorded in a 

 laboratory notebook and samples were assigned a unique number 



