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 mounting medium 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 (Table 3) . 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 

 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 available. Although 

 Big Spring Creek is shown on a map of Montana ecoregions (Omernik 

 and Gallant 1987) as flowing mostly through the Northwestern 

 Great Plains, the Lewistown area is relatively cool and moist and 

 compares more favorably to the Montana Valley and Foothill 

 Prairie ecoregion. For this reason, and because Big Spring Creek 

 supports cold- and cool-water fisheries and associated aquatic 

 life, metric values will be compared to criteria developed from 

 mountain streams . 



In some cases, natural stressors (e.g., high gradient, low 



6 



