nutrient concentrations. High values for the pollution index and low values for the siltation 

 index and percentage of abnormal cells indicate that organic enrichment, sedimentation, and 

 toxic metals did not have a significant effect on the benthic algae at these sites. 



Dearborn River near Bean Lake. Aside from an elevated disturbance index and 

 percent dominant species {Achnanthidiiim minutissimum), which indicate minor natural stress, 

 this site had excellent biological integrity (Table 5). The natural stress here is probably the result 

 of cold temperatures, fast current velocities, and low nutrient concentrations. Five out of the six 

 major diatom species registered here are sensitive to organic pollution. 



Dearborn River below Highway 200. An increase in the percentage of pollution 

 tolerant (class 2) diatoms here resulted in a significant decrease in the pollution index when 

 compared to the upstream station near Bean Lake (Table 5). However, the pollution index here 

 did not drop below the threshold for minor impairment for either a mountain stream or a prairie 

 stream. Four of the 10 major diatom species recorded at this site are somewhat tolerant of 

 organic pollution, including three species — Fragilaria capucina, Fragilaria vaucheriae, and 

 Synedra ulna — that were not recorded upstream. Nevertheless, this site shared almost two-thirds 

 of its diatom association with the upstream site, indicating that there was very little change in 

 diatom floristics and environmental conditions between them. One teratological cell ofDiatoma 

 moniliformis was registered here, which may have been due to natural causes. 



Middle Fork near Rogers Pass. The very large percentage of Achnanthidium 

 minutissimum at this site resulted in depressed species richness and diversity, and elevated values 

 for the disturbance index and percent dominant species (Table 5). The moderate stress indicated 

 here is probably natural in origin and due to the steep gradient, low water temperatures, and low 

 nutrient concentrations at this site. Values for other metrics indicated no impairment from 

 organics, sediment, or toxics and excellent water quality. Next to A. minutissimum, the most 

 abundant diatom species at this site was Gomphonema pseudobohemicum. This species is rare in 

 the United States but is widely distributed in Europe in "slightly acid and electrolytically poor" 

 (low conductance) waters (Reichardt 1999). Like Achnanthidium minutissimum, Gomphonema 

 pseudobohemicum is sensitive to organic pollution. 



10 



