enrichment 



DIATOMS 



Diatom association metrics at the Mill Creek site indicated 

 minor impairment but full support of aquatic life uses (Table 5) . 

 Slightly elevated percent dominant species and siltation indexes 

 and a few abnormal diatom cells resulted in an overall rating of 

 "good" biological integrity. 



The diatom association in Mill Creek was dominated by 

 Hannaea arcus (Table 5) . This species is common in mountain 

 streams and large cold lakes in northern latitudes. It tolerates 

 some nutrient enrichment but is sensitive to pollution from 

 sewage. Because of the elevated numbers of Hannaea arcus here, 

 the percent dominant species index sightly exceeded the threshold 

 for minor impairment. Since this species may be found in large 

 numbers in relatively pristine waters, its dominance in Mill 

 Creek may not be due to cultural enrichment. 



The sample from Pine Creek was dominated by Gomphonema 

 olivaceoides (Table 5) . This is a cosmopolitan cold water 

 species that often dominates the diatom assemblages of mountain 

 streams. In Montana, it is particularly abundant in streams 

 draining the Absaroka and Beartooth Mountains (unpublished data) . 

 Its dominance in Pine Creek, along with a small number of diatom 

 taxa and a low diversity index, probably indicates natural stress 

 due to cold water, fast currents, and low nutrients. 



The siltation index at both sites on Tom Miner Creek 

 indicated moderate impairment. and only partial support of aquatic 

 life uses (Table 5) . Both sites supported a large number of 

 diatoms in the genera Navicula and Nitzschia. These are motile 

 diatoms that are adapted to living on aggrading substrates. 



