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 the Shields River were compared to 

 numeric biocriteria or threshold values developed for streams in 

 the Rocky Mountain 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. 



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., 2013-01. The first part of this number (2013) 

 designates the sampling site (Shields River at McLeod) ; the 

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

 periphyton samples that have been collected at this site for 

 which data have been entered into the Montana Diatom Database. 



Sample observations and analyses of soft (non-diatom) algae 

 were recorded in a lab notebook along with station and sample 

 information provided by MDEQ. A portion of the raw sample was 

 used to make duplicate diatom slides. After completing the 



