After the identification of soft algae, the raw periphyton samples were cleaned of organic 

 matter using sulfuric acid, potassium dichromate, and hydrogen peroxide. Then 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). At 

 least 300 diatom cells (600 valves) were counted at random and identified to species. The 

 following were the main taxonomic references for the diatoms: Krammer and Lange-Bertalot 

 1986, 1988, 1991a, 1991b; Lange-Bertalot 1993, 2001; Krammer 1997a, 1997b, 2002; Reichardt 

 1997, 1999. Diatom naming conventions followed those adopted by the Integrated Taxonomic 

 Infomiation System (http://www.itis.usda.gov). Van Dam et al. (1994) was the main ecological 

 reference for the diatoms. 



The diatom proportional counts were used to generate an array of diatom association 

 metrics. A metric is a characteristic of the biota that changes in some predictable way with 

 increased human influence (Barbour et al. 1999). Diatoms are particularly useful in generating 

 metrics because there is a wealth of information available in the literature regarding the pollution 

 tolerances and water quality preferences of common diatom species (e.g., Lowe 1974, Beaver 

 1981, Lange-Bertalot 1 996, Van Dam et al. 1 994). 



Values for selected metrics were compared to biocriteria (numeric thresholds) 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 metric values 

 measured in streams that are known to be impaired by various sources and causes of pollution 

 (Bahls 1993). The criteria in Table 2 are valid only for samples collected during the summer 

 field season (June 21 -September 21) and distinguish among four levels of stress or impaimient 

 and three levels of aquatic life use support: (1) no impairment or only minor impairment (full 

 support); (2) moderate impairment (partial support); and (3) severe impairment (nonsupport). 

 These impairment levels correspond to excellent, good, fair, and poor biological integrity, 

 respectively. In cold, high-gradient mountain streams, natural stressors will often mimic the 

 effects of man-caused impairment on some metric values. 



