The diatom flora at Dent Bridge was dominated by Cymbella 

 sinuata (Table 8) . Cymbella sinuata (= Reimeria sinuata) is a 

 free-living and motile species (Round et al . 1990). Although not 

 included in calculating the siltation index, it probably should 

 be included by virtue of its motility and free-living lifestyle. 

 If C. sinuata had been included in calculating the siltation 

 index, it would have resulted in a rating of moderate impairment 

 with partial support at this site. 



The other major diatom at Dent Bridge was Navicula 

 tripunctata, another motile species. N. tripunctata is more 

 sensitive to organic pollution than Navicula cryptotenella, but 

 is still considered an "eutrophic" species (Lowe 1974) . 



The very large percent abundance of Cymbella sinuata at Dent 

 Bridge, and the resulting low diversity index, indicated moderate 

 impairment with only partial support at this site (Table 8) . 

 Dent Bridge was the only site on either McDonald Creek or the 

 Teton River where periphyton metrics indicated less than full 

 support of aquatic life uses . The specific cause of the low 

 diversity at this site is unclear, but sedimentation and the 

 homogeneity of microhabitats (lack of habitat diversity) is 

 likely responsible. 



The similarity index comparing the Dent Bridge diatom flora 



with the Kerr Bridge diatom flora was only about 10% (Table 8) . 

 Adjacent riffles in the same reach of stream can be expected to 

 have about 60% of their diatom floras in common (Bahls 1993). A 

 great distance between sites (about 20 miles in this case) , 

 intervening tributaries and pollution sources, and large changes 

 in stream habitat will all result in lower similarity values. 



The site near the mouth of the Teton River near Loma had a 

 much more equitable distribution of diatom species than did the 

 Dent Bridge site (Table 8) . Low diatom diversity, an elevated 



17 



