and Nitzschia resulted in pollution and siltation index values 

 that indicated moderate impairment and only partial support of 

 aquatic life uses (Table 5) . 



Nitzschia frusculum was the dominant diatom in Lodgepole 

 Creek. This species is widely distributed in eastern Montana and 

 was the most common diatom in the Southern Fort Union Coal 

 Region, where it tolerates large amounts of suspended sediment 

 and turbidity, although it does best at smaller concentrations 

 (Bahls et al . 1984) . 



The other major diatom species near the mouth of Lodgepole 

 Creek was Nitzschia palea (Table 5) . N. palea is a facultative 

 nitrogen heterotroph and, in large numbers, a good indicator of 

 elevated concentrations of organic nitrogen. In the Southern 

 Fort Union Region, this diatom "prefered fresh to brackish and 

 very hard to extremely hard waters,- it was also indifferent to 

 moderate amounts of suspended sediment, turbidity, and chloride 

 (Bahls et al . 1984) . 



South Fork Lodcrepole Creek 



The South Fork of Lodgepole Creek had very low diatom 

 diversity and species richness for a prairie stream (Table 5) . 

 These low values resulted in a rating of moderate impairment and 

 partial support of aquatic life uses. 



The low diversity in the South Fork was due in part to the 

 unusual abundance of the pollution sensitive species Achnanthes 

 winutissima (Table 5) . Although often abundant in mountain 

 streams, A. minutissiwa rarely accounts for more than a few 

 percent of the diatom cells in prairie streams. Its abundance in 

 the South Fork is another indicator of the cool water 

 temperatures that evidently prevailed in this stream prior to 

 sampling. This indicates that at least a portion of the stress 



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