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 Ward Creek were compared to numeric 

 biocriteria or threshold values developed for streams in the 

 Rocky Mountain and Great Plains Ecoregions of Montana (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 that are known to be impaired 

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

 diatom metrics indicated impairment at both sites (see Table 6), 

 a within-stream comparison of sites (Protocol II, Bahls 1993) 

 could not be used. 



The criteria in Tables 3 and 4 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. 



Quality Assurance. Several steps were taken to assure that 

 the study results are accurate and reproducible. 



Upon receipt of the samples, station and sample information 

 were recorded in a laboratory notebook and the samples were 

 assigned a unique number compatible with the Montana Diatom 

 Database, e.g., 2169-01. The first part of this number (2169) 

 designates the sampling site (Ward Creek at Browns Lake) ; the 

 second part of this number (01) designates the number of 

 periphyton samples that have been collected at this site to date 

 for which data have been entered into the Montana Diatom 

 Database . 



