Of the three most tolerant (Class 1) diatom species in 

 0' Fallon Creek, two (Nitzschia agnita and N. agnita) are brackish 

 water species and one (Nitzschia palea) prefers polysaprobic 

 conditions. All of the somewhat tolerant (Class 2) diatom 

 species in Table 6 prefer waters with high concentrations of 

 electrolytes. Hence, the two most likely causes of pollution in 

 0' Fallon Creek are salinity and organic loading. Organic loading 

 in 0' Fallon Creek may be internal and result from the breakdown 

 of aquatic plants. 



Diatom species richness in 0' Fallon Creek ranged from 33 at 

 OF-7 to 70 at the next downstream site (AF-3) . Diatom species 

 diversity (Shannon) also increased between these two sites. 

 Based on changes in major species, this increase in diversity 

 appears to have resulted mainly from a decrease in salinity. Low 

 diversity values indicate minor impairment at all sites except 

 AF-5, where moderate impairment is indicated by the large 

 percentage of Nitzschia frustulum (Table 6) . N. frustulum, one 

 of the most common diatoms in the Fort Union Region, prefers 

 eutrophic conditions and alkaline fresh waters (Lowe 1974) . 



A very large percentage of motile diatoms in the genera 

 Navicula and Nitzschia indicate severe impairment by siltation at 

 the upstream site (OF-7), moderate impairment at AF-5, and minor 

 impairment at the remaining three sites. In general, impairment 

 from both siltation and pollution decreases in a downstream 

 direction (Table 6) . 



A few abnormal diatom cells were counted at three of the 

 five sites on 0' Fallon Creek, indicating minor impairment. The 

 cause of these abnormal cells is unknown. Diatoms in the family 

 Epithemiaceae accounted for only a small percentage of diatom 

 cells in 0' Fallon Creek, indicating that nitrogen was probably 

 not the limiting nutrient. 



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