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 for Mizpah 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, 

 metrics for the lower site on Mizpah Creek (LMC-7) were also 

 compared to metrics generated from an upstream, least-impaired 

 reference site (UMC-1) using Protocol II in Bahls (1993) . 

 Protocol II is based on the percentage of change in metric values 

 at the study site(s) from values measured at the local reference 

 stream. Criteria for evaluating biological integrity using 

 Protocol II are given in Table 12 in Bahls (1993) . 



Protocol II may be used if an upstream reference site is 

 available that fully supports its aquatic life uses, that is, if 

 it has a rating of "good" or "excellent" biological integrity 

 using Protocol I. Mizpah Creek station UMC-l exhibited only 

 minor impairment using Protocol I and therefore qualifies as a 

 local reference site. 



