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

 matter using sulfiiric 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 1998). 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 Mountains and Great Plains ecoregions of Montana (Tables 2 and 3). 

 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 Tables 2 and 3 are 

 valid only for samples collected during the summer field season (June 21 -September 21). 



The criteria in Tables 2 and 3 distinguish among four levels of stress or impairment 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. 



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