t 



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 Squirrel Creek were compared to numeric 

 biocriteria developed for streams in the Great Plains Ecoregions 

 of Montana (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 that are known to be 

 impaired by various sources and causes of pollution (Bahls 1993) . 



The criteria in Table 3 distinguish among four levels of 

 impairment and three levels of aquatic life use support: no 

 impairment or only minor impairment (full support); moderate 

 impairment (partial support) ; and severe impairment (nonsupport) . 

 These impairment levels correspond to excellent, good, fair, and 

 poor biological integrity, respectively. 



Besides the ecoregional biocriteria listed in Table 3, 

 Squirrel Creek metrics were also compared to metrics generated 

 from an upstream control site (#4) using Protocol II in Bahls 

 (1993) . Protocol II may be used on relatively short segments of 

 stream where an upstream control site fully supports its aquatic 

 life uses, that is, if it has a rating of "good" or "excellent" 

 biological integrity using Protocol I. Protocol II is based on 

 the percentage of change in metric values at study sites from 

 values measured at the upstream control site. Criteria for 

 evaluating biological integrity using Protocol II are given in 

 Table 12 in Bahls (1993) . 



For Protocol I, only periphyton samples collected in summer 

 (June 21-September 21) can be compared with confidence to 

 reference stream samples because metric values change seasonally 

 and summer is the season in which reference streams and impaired 



