The samples were examined to estimate the relative abundance and rank by biovolume of 

 diatoms and genera of soft (non-diatom) algae according to the method described in Bahls 

 (1993). Soft algae were identified using Smith (1950), Prescott (1962, 1978), John et al. (2002), 

 and Wehr and Sheath (2003). These books also served as references on the ecology of the soft 

 algae, along with Palmer (1969, 1977). 



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

 matter using sulfuric acid, potassium 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). At 

 least 400 diatom cells (800 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 the Integrated Taxonomic Information System ( http://ww\v. itis.usda.gov ). For taxa not 

 yet 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 (Table 6). 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 1979, 1996, Van Dam et al. 1994). 



Values for selected diatom 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 2 1 -September 21). 



