After the identification of soft algae, the raw periphyton 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 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 2001; Krammer 2002. Where applicable, conventions 

 for diatom nomenclature followed those adopted by the hitegrated Taxonomic hiformation 

 System (http://www.itis.usda.gov). For taxa not included in ITIS, 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 Great Plains 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 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. 



