indicate nitrogen-limiting conditions, that is, low nitrogen to 

 "phosphorus ratios (Stevenson and Pan 1999) . 



Metric values from study sites were compared to numeric 

 biocriteria developed for streams in the Great Plains Ecoregions 

 of Montana (Table 3) using Protocol I in Bahls (1993) . 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, causes, and degrees 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. 



Protocol II (Bahls 1993) is based on the percentage of 

 change in metric values at study sites when compared to values 

 measured at an upstream, control site. Protocol II may be used on 

 relatively short segments of stream where an upstream control 

 site fully supports its aquatic life uses, that is, where it has 

 a rating of "good" or "excellent" biological integrity using 

 Protocol I. Protocol II could not be applied to Careless Creek 

 because a sample was not collected from an upstream control site,- 

 it could not be applied in Lodgepole Creek because both sites in 

 this drainage suffered moderate impairment under Protocol I. 



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 

 streams were sampled for the purpose of biocriteria development. 

 (Protocol II can be used at any time of the year.) 



