Diatoms (Table 4) 



Of thel8 major diatom species in Sand Creek and Prairie Elk Creek, only Achnanthidium 

 minutissimum is sensitive to organic pollution (class 3). A mimitissimum was most abundant in 

 the lower reaches of Prairie Elk Creek. Seven of the major species are most tolerant of organic 

 pollution (class 1). These species tended to be most abundant in the upper and middle reaches of 

 Prairie Elk Creek and in Sand Creek. The remaining 10 major species are somewhat tolerant of 

 organic pollution and these (class 2) species were abundant at all sites (Table 4). 



Garoutte's. This site was dominated by highly motile and pollution tolerant species of 

 Navicula and Nitzschia. As a result, diatom metrics indicate moderate impairment from organic 

 loading and sedimentation at this site (Table 4). 



Wright's. This site was also dominated by highly motile species, which indicated higher 

 than usual levels of sedimentation for a prairie stream. The siltation index at this site was near 

 the threshold between minor impairment and moderate impairment. The pollution index at this 

 site suggests minor impairment from organic loading. Although this is an improvement from the 

 moderate rating upstream at Garoutte's, the Wright site supported a larger percentage brackish 

 water species {Nitzschia agnita, Nitzschia filiformis, Tabidaria fasciculate), indicating that this 

 site had saltier water than upstream. The Wright and Garoutte sites shared only about a third of 

 their diatom assemblages, which suggests that a moderate change in environmental conditions 

 occurred between the two sites. 



Taylor's. Diatom metrics showed improved conditions at this site compared to the 

 upstream sites. Sedimentation levels were normal for a prairie stream and the pollution index 

 was higher, indicating only minor impairment from organic loading. However, like the Wright 

 site, this site supported a large percentage of brackish-water diatoms, which indicates higher 

 levels of dissolved solids. The higher salinities at this site may be responsible for the smaller 

 diatom species richness and diversity values observed here. The Taylor site supported the largest 

 percentage of nitrogen fixing diatoms (5.40%), which may indicate that this site is nitrogen 

 limited. The Taylor and Wright sites shared slightly less than half of their diatom assemblages, 

 which suggest that minor environmental change occurred bet\\een them. 



