c 



Diatom diversity and species richness were normal for a 

 prairie stream, with only minor impairment indicated in Reaches 

 004 and 003 (Table 5) . The pollution index indicated minor 

 impairment at most sites, but moderate impairment in Reaches 006 

 and 001. The pollution index responds mostly to organic loading 

 (Lange-Bertalot 1979) . In Box Elder Creek, this loading is 

 probably internal and natural in origin, resulting from the 

 fertile, intermittent nature of the stream, its pooled channel 

 morphology, and its low frequency of flushing. 



The sedimentation index, which is the percentage of motile 

 diatoms, indicated moderate impairment at all sites except Reach 

 001, where only minor impairment was indicated (Table 5) . The 

 sedimentation index indicated moderate impairment even in the 

 reference reach (Reach 022), which was judged to be "proper 

 functioning" based on the relatively good condition of the 

 riparian habitat here (Amy Chadwick, RWRP, pers . comm. ) . This 

 may indicate that the sedimentation index is more responsive to 

 channel morphology and stream type than it is to the condition of 

 the riparian area. 



The disturbance index was uniformly low at all sites on Box 

 Elder Creek (Table 5) . The disturbance index is based on the 

 percent abundance of Achnanthes minutissima, which is a pioneer 

 species that colonizes disturbed habitats. The relatively low 

 numbers of this taxon indicate that Box Elder Creek supported a 

 mature algal assemblage and that there had been little physical, 

 chemical or biological disturbance to the periphyton in the days 

 and weeks prior to sampling. 



The dominant diatom species at each site generally accounted 

 for less than 25% of the cells (Table 5) . The only exceptions 

 were in Reaches 006 and 003, where Nitzschia palea and Nitzschia 

 frustulum, respectively, accounted for slightly more than a 

 quarter of the cells. This relatively low percent dominance and 



11 



