Diatoms (Table 5) 



Diatoms were present in all of the samples. Most of the 27 major diatom species in 

 tributaries of the Big Hole River are either sensitive to organic pollution or only somewhat 

 tolerant of organic pollution. Only two of the major diatom species (Navicula minima and 

 Nitzschia palea) are 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. One site (LMCHC02) 

 had 90 species and another (MCVYCOl) had a diversity index of 5.22. These are exceptionally 

 high values for mountain streams. Only one stream (Elkhom Creek) had diversity values that 

 indicated unusual stress. .A.t onh- X\\o sites, both on Elkhom Creek, 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 mesodon, Fragilaria spp., Melosira 

 varians. Meridian circulare, Pseudostarosira brievistriata, Staiirosira construens. Staurosirella 

 spp., Synedra ulna) relative to attached species {Achnanthidium spp., Cocconeis placentula) 

 implies that most sites have gentle gradients and slow current velocities compared to other 

 mountain streams. This is confirmed by the comparatively 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 more similar to prairie streams. Pollution index values, which indicate the 

 amount of organic loading, are generally low for mountain streams. Many are at or below the 

 threshold for minor impairment. Similarly, siltation index values tend to be higher in Big Hole 

 tributaries than in most mountain streams. 



• 



