After the identification of soft algae, the raw periph>ton samples were cleaned of organic 

 matter using sulfuric acid, postassium dichromate, and 3% 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 1 998). Approximately 

 350 diatom cells (700 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 2001; Krammer 2002. Diatom naming conventions followed 

 those adopted by the Academy of Natural Sciences for USGS NAWQA samples (Morales and 

 Potapova 2000). 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 1996, Van Dam et al. 1994). 



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 

 the distribution of 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 biocriteria in Table 2 are valid only for samples collected 

 during the summer field season (June 21 -September 21). 



The criteria in Table 2 distinguish among four levels of stress or impairment and three 

 levels of aquatic Ufe 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. 



