Diatoms (Table 5) 



Diatoms were present in all of the samples. Most of the 34 major diatom species in 

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

 tolerant of organic pollution. Only one of the major diatom species (Nitzschia palea) is most 

 tolerant of organic pollution and this species was most abundant in Black Bear Creek (Table 5). 



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

 supported more than 40 species and diversity values in excess of 4.00. One site (lower Murray 

 Creek) had 73 species and a diversity index of 4.99. These are exceptionally high values for a 

 mountain stream. Only one site — upper Douglas Creek — had a diversity value that indicates 

 unusual stress. This was also the only site where the dominant species contributed more than 

 half of the cells to the diatom assemblage (Table 5). The cause of stress in upper Douglas Creek 

 was probably excess inorganic nutrients. 



High diatom diversity in these samples suggests the absence of extreme natural stressors, 

 such as steep gradients, fast currents, low light, low nutrients, and constant cold temperatures, 

 which may prevail in the extreme upper reaches of these streams. The abundance of non-motile, 

 free-living taxa {Diatoma spp., Fragilaria spp., Melosira varians, Pseudostarosira brievistriata, 

 Staurosira construens, and Synedra spp.), attached species {Achnanthidium spp., Cocconeis 

 placentula, Rhoicosphenia abbreviata), and motile, free-living taxa {Navicula spp., Nitzschia 

 spp., Surirella mimita) suggests a wide variety of substrates, gradients, and current velocities. 

 The disturbance index at most sites was relati\ely low, which suggests moderate gradients and 

 slower current velocities than most mountain streams (Table 5). 



Besides the absence of natural stressors and the presence of complex microhabitats, high 

 diatom diversity in these streams also suggests moderate nutrient enrichment (little competition 

 for available nutrients). 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. 



