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. 



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. 



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 samples were assigned a unique number 

 compatible with the Montana Diatom Database, e.g., 1885-01. The 

 first part of this number (1885) designates the sampling site 

 (Box Elder Creek Reach BE022) ; the second part of the 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. 



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. 



On completion of the project, station information, sample 

 information, and diatom proportional count data will be entered 

 into the Montana Diatom Database. One set of diatom slides will 

 be deposited in the University of Montana Herbarium in Missoula. 

 The other set of slides will be retained by Haiinaea in Helena. 



