Diatom species diversity and species richness also declined 

 from AraC-01 to AraC-02 (Table 7) . This decline in diversity was 

 due mainly to an abundance of Hannaea arcus at AraC-02. This 

 diatom and Hydrurus foetidus, which dominated the periphyton at 

 this site, both indicate cold waters of high quality. Hence the 

 low diatom diversity and species richness values at this site are 

 likely due to natural stressors, such as cold water temperatures, 

 low concentrations of dissolved solids, and moderate current 

 velocities . 



Sandbar Creek. Both sites on Sandbar Creek had elevated 



percentages of abnormal diatoms (Table 7) . The percentage at 

 SbrC-01 (3.71%) indicated moderate impairment and partial support 

 of aquatic life uses; the percentage at SbrC-02 (10.26) indicated 

 severe impairment and nonsupport of aquatic life uses. 



In addition, both sites on Sandbar Creek had depressed 

 diatom diversity and species richness values, indicating moderate 

 to severe stress (Table 7). Diatoma mesodon, the dominant diatom 

 species at the upstream site, is a cold water stenotherm (Lowe 

 1974) . I have recorded large numbers of this species in mine 

 adit discharges and in streams that receive mine adit discharges 

 (unpublished data) . It appears to be a good indicator of stream 

 sites that are fed primarily by groundwater. 



The dominant diatom species at the downstream site (SbrC-02) 

 was Achnanthidium minutissimum (synonym: Achnanthes minutissima) 

 (Table 7) . This is a pioneer species which, when present in very 

 large numbers, indicates some form of chemical, physical or 

 biological disturbance (Barbour et al . 1999). Although diatom 

 associations in unpolluted streams in Montana may be composed of 

 up to 70% Achnanthidium minutissimum, much larger percentages of 

 this diatom aln\ost always occur in streams that receive mining 

 wastes (unpublished data) . The large percentage of this diatom 



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