species in tributaries of the Madison River are either sensitive to organic pollution or only 

 somewhat tolerant of organic pollution. None of the major diatom species is most tolerant of 

 organic pollution (Table 5). 



In general, diatom species richness, diversity, and equitability were excellent. Most sites 

 supported more than 50 species and diversity values in excess of 4.00. Only one site (Blaine 

 Spring Creek at bypass) had diversity values that indicated unusual stress, and this stress was 

 probably natural in origin. At none of the sites did the dominant species contribute more than 

 half of the cells to the diatom assemblage (Table 5). 



High diatom diversity in these streams infers the absence of natural stressors, such as 

 steep gradients, fast currents, low light, low nutrients, and constant cold temperatures. The 

 predominance of non-motile, free-living taxa {Diatoma spp., Fragilaria vaucheriae, Melosira 

 varians, Staurosira constriiens, Staurosirella leptostauron, Synecira ulna) relative to attached 

 species {Achnanthidium spp., Cocconeis placentula, Rhoicosphenia ahbreviata) implies that 

 most sites have gentle gradients and slow current velocities compared to other mountain streams. 

 This is confirmed by the relatively low disturbance index at most sites (Table 5). 



Besides the absence of natural stressors, high diatom diversity in these streams also 

 suggests moderate nutrient enrichment (little competition for available nutrients) and complex 

 microhabitats that are similar to prairie streams. Pollution index values, which indicate the 

 amount of organic loading, are lower than in most mountain streams. Many are near or below 

 the threshold for minor impairment (2.50). Similarly, siltation index values tend to be higher in 

 Madison River tributaries than in most mountain streams, with the notable exception of the upper 

 site on Blaine Spring Creek (Table 5). 



All but one of the sites supported teratological (deformed or physically abnormal) diatom 

 cells. In large numbers, abnormal cells may indicate metals toxicity. However, the percentage 

 of abnormal cells was within acceptable limits at all sites. The lower site on Gazelle Creek 

 supported the largest percentage of abnormal cells (Table 5). 



