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 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 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 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 21 -September 21). 



