After Che identification of soft algae, the raw periphyton 

 samples were cleaned of organic matter using sulfuric acid, and 

 permanent diatom slides were prepared using Naphrax, a high 

 refractive index mounting medium, following Standard Methods for 

 the Examination of Water and Wastewater (APHA 1998) . Between 407 

 and 408 diatom cells (814 to 816 valves) were counted at random 

 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 the Judith River 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 

 metrics from both sites indicated impairment (see Table 6) , 

 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 



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