water diatom that indicates meso-eutrophic conditions (Van Dam et 

 al . 1994). N. fonticola is also motile and is adapted to living 

 on unstable substrates. 



The two periphyton samples that were collected in May from 

 above and below Lewistown shared 64% of their diatom floras 

 (Table 5) . This indicates that the two sites were very similar, 

 floristically, and that little or no environmental change 

 occurred between them. However, the two sites both supported a 

 handful of teratological diatom valves, which may indicate the 

 presence of toxic chemicals in the water. 



August Samples. The sample collected just below Big Springs 

 in August was dominated by Diatoms hyemalis (Table 5) . This 

 oligotrophic diatom indicates cold waters that do not vary in 

 temperature by more than a few degrees seasonally. Although the 

 Shannon species diversity and percent dominant species metrics 

 for this site both indicated moderate impairment and partial 

 support of aquatic life uses, the thermal stresses that cause 

 this "impairment" may be considered natural and result from the 

 discharge of the Big Springs. 



Diatom metrics at sites 02 and 03 indicated minor impairment 

 but full use support in August (Table 5) . At site 02, a slightly 

 elevated disturbance index and percent dominant species were due 

 to an abundance -of Achnanthidiuw minutissimum. Site 02 only 

 shared about 12% of its flora with site 01, indicating that the 

 flora of Big Spring Creek had changed from a spring flora to more 

 of a stream flora at this point . 



A somewhat elevated number of motile diatoms at site 03 

 indicated minor impairment from siltation here (Table 5) . Sites 

 02 and 03 shared only about a quarter of their floras, indicating 

 that a moderate amount of environmental change had occurred 

 between them. These sites bracket the City of Lewistown and the 



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