(1993) . All samples were collected by MDEQ personnel as one 

 component of a suite of biological, habitat, and water quality 

 assessments . 



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) . 



After the identification of soft algae, raw periphyton 

 samples were "cleaned" of organic matter and permanent diatom 

 slides were prepared in Hyrax following Standard Methods for the 

 Examination of Water and Wastewater (APHA 1998) . For each slide, 

 400 diatom cells (800 valves) were counted at random and 

 identified to species using standard taxonomic references. 



The diatom proportional counts were used to generate an 

 array of diatom association metrics (Table 2) . A metric is a 

 characteristic of the biota that changes in some predictable way 

 with increased human influence (Barbour et al . 1999) . Metric 

 values from study sites are compared to numeric criteria for 

 Montana streams (Tables 3 and 4) . These criteria are based on 

 metric values measured in least- impaired reference streams (Bahls 

 et al . 1992) and on metric values measured in streams exhibiting 

 various levels of use support, which are known to be impaired by 

 various sources and causes of pollution (Bahls 1993) . 



Because of inherent differences in periphyton composition 

 and community structure between mountain streams and prairie 

 streams, two different sets of criteria are provided (Tables 3 

 and 4) . For the purpose of periphyton assessment, mountain 

 streams are those located in the Rocky Mountain and Montana 

 Valley and Foothill Prairie Ecoregions (Omernik and Gallant 

 1987) . These streams are generally classified B-l and B-2 in the 

 Montana Surface Water Quality Standards. 



