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) . 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. 



The criteria in Table 2 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 sample, station and sample information 

 were recorded in a laboratory notebook and the sample was 

 assigned a unique number compatible with the Montana Diatom 

 Database: 0787-05. The first part of this number (0787) 

 designates the sampling site (Bear Creek at mouth) ; the second 

 part of the number (05) 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. Following the diatom 

 proportional count, the slide used for the count will be 

 deposited in the University of Montana Herbarium in Missoula. 

 The other slide will be retained by Hannaea in Helena. 



On completion of the project, station information, sample 

 information, and diatom proportional count data will be entered 

 into the Montana Diatom Database. 



