shared about a third of its diatom assemblage with the Cemetery Road site. Both the Cemetery 

 Road and airport sites had normal diatom species richness and diversity for a prairie stream. 



At Highway 13, diatom metrics indicate severe impairment from sedimentation but only 

 minor impairment from organic loading. Diatom species richness, diversity, and equitability 

 were very low as a result of dominance by Nitzschia reversa. This is a facultative planktonic 

 species and its abundance here indicates probable ponding and very low current velocities. The 

 sedimentation inde.x is very high here because Nitzschia reversa belongs to the highly motile 

 genus Nitzschia. This site shared only about one-fifth of its diatom assemblage with the next 

 upstream site, indicating that a major environmental change occurred between the two sites. 



Sand Creek. Diatom metrics indicate severe impairment from organic loading at the 

 upper site on Sand Creek. Several pollution tolerant species dominated the diatom assemblage at 

 this site. Not all of these species are motile, however, and the sedimentation index indicates only 

 minor impairment, both here and at the downstream site near the mouth. Although depressed, 

 diatom species richness and diversity were within the normal range for a prairie stream. 



Along with Prairie Elk Creek, the lower site on Sand Creek had the best biological 

 integrity of all sites in the sample set. Only minor impairment from sedimentation and organic 

 loading was evident. This site supported the second largest percentage of the pollution sensitive 

 species Achnanthidium minutissimum. Diatom species richness, diversity, and equitability were 

 excellent and within the normal range for a prairie stream. This site shared less than one-third of 

 its diatom assemblage with the upper site on Sand Creek, which indicates that a moderate 

 amount of environmental change occurred between them. 



Modal Categories (Table 5) 



Several ecological attributes were selected from the diatom reports in the appendix and 

 modal categories of these attributes were extracted to characterize water quality tendencies in 

 streams of McCone County (Table 5). 



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