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 Mountain and Great Plains ecoregions of Montana (Tables 2 and 3). 

 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 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. In cold, high-gradient mountain streams, natural stressors will often mimic the 

 effects of man-caused impairment on some metric values. 



Quality Assurance 



Several steps were taken to assure that the study results are accurate and reproducible. 

 Upon receipt of the samples, station and sample attribute data were recorded in the Montana 

 Diatom Database and the samples were assigned a unique number, e.g., 2658-01 . The first part 

 of this number (2658) designates the sampling site (Dearborn River near Bean Lake) and the 

 second part (01) designates the number of periphyton samples that that have been collected at 

 this site for which data have been entered into the Montana Diatom Database. 



Sample observations and analyses of soft (non-diatom) algae were recorded in a lab 

 notebook along with information on the sample label. A portion of the raw sample was used to 

 make duplicate diatom slides. The slides used for the diatom proportional counts will be 

 deposited in the Montana Diatom Collection at the University of Montana Herbarium in 

 Missoula. The duplicate slides will be retained by Hannaea in Helena. Diatom proportional 

 counts have been entered into the Montana Diatom Database. 



