After the identification of soft algae, the raw periphyton 

 samples were cleaned of organic matter using sulfuric acid and A 

 postassium dichromate, and permanent diatom slides were prepared 

 using Naphrax, a high refractive index mounting medium, following 

 Standard Methods for the Examination of Water and Wastewater 

 (APHA 1998) . 



Between 405 and 415 diatom cells (810 to 830 valves) were 

 counted at random and identified to species . The four volume 

 series by Krammer and Lange-Bertalot (1986, 1988, 1991a, 1991b) 

 was used as the main taxonomic and autecological reference for 

 the diatoms. Lowe (1974), Bahls et al . (1984), van Dam et al . 

 (1994) , and Lange-Bertalot (1996) were also used as ecological 

 references for the diatoms . 



The diatom proportional counts were used to generate an 

 array of diatom association metrics (Table 1) . A metric is a 

 characteristic of the biota that changes in some predictable way 

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



Metric values from the Stillwater River were compared to 

 numeric biocriteria or threshold values developed for streams in 

 the Rocky Mountain Ecoregions of Montana (Table 2) . 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 Tables 2 and 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. 



