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

 matter using sulfuric acid, potassium dichromate, and 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 1993, 2001; Krammer 1997a, 1997b, 2002; Reichardt 

 1997, 1999. Diatom naming conventions followed those adopted by the Integrated Taxonomic 

 Information System ( http://www.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. 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 useftil 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 1 996, Van Dam et al. 1 994). 



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 

 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 criteria in Table 2 are valid only for samples collected during the summer 

 field season (June 21 -September 21). [Note: About half of the Blackfoot TMDL periphyton 

 samples were collected after September 21 .] These criteria distinguish among four levels of 

 stress or impairment and tliree levels of aquatic life use support: (1) no impairment or only 

 minor impairment (full support); (2) moderate impairment (partial support); and (3) severe 

 impairment (non-support). These impairment levels correspond to excellent, good, fair, and poor 



