and identified to species. The following were used as the main 

 taxonomic and autecological references for the diatoms: Krammer 

 and Lange-Bertalot 1986, 1988, 1991a, 1991b; Patrick and Reimer 

 1966, 1975. Lowe (1974) was also used as an ecological reference 

 for the diatoms. 



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 Antelope Creek and Potter Creek were 

 compared to numeric biocriteria or threshold values developed for 

 streams in the Rocky Mountain and Great Plains Ecoregions of 

 North America (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) . 



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., 1999-01. The first part of this number (1999) 

 designates the sampling site (Antelope Creek near mouth) ; 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 . 



