Diatoms (Table 4) 



Six of the major diatom species in Beaver Creek are sensitive to organic pollution and 

 these were abundant at all three sites (Table 4). Six of the major species are somewhat tolerant 

 of organic pollution and these were present at all three sites. Nitzschia linearis, a somewhat 

 tolerant species that is also highly motile and tolerant of sedimentation, was abundant only at the 

 upper site. Only one of the major diatom species— Nitzschia palea — is most tolerant of organic 

 pollution (pollution tolerance class 1). Nitzschia palea was most abundant at the upstream site 

 and declined in relative abundance at the middle and lower sites (Table 4). 



Diatom metrics suggest the upper site was moderately impaired by sedimentation and 

 provided less than full support for aquatic life uses. This was due to a much larger than normal 

 percentage of motile diatoms, which approached the threshold for severe impairment in a 

 mountain stream. The pollution index was also depressed at the upper site, suggesting minor 

 impairment from organic loading (Table 4). Nitzschia palea, a nitrogen heterotroph, was the 

 second most abundant diatom species here. Otherwise, diatom species richness and diversity at 

 this site were acceptable, and no abnormal diatom cells were recorded. 



The middle site had the best biological integrity of the three sites on Beaver Creek. 

 Diatom metrics indicate only minor impairment from sedimentation and minor disturbance due 

 to the slightly elevated percentage of Achnanthidium minutissimum. The second most abundant 

 species here was Gomphonema minutum, an eutraphentic species that suggests elevated levels of 

 inorganic nutrients (Van Dam et al. 1994). Two abnormal diatom cells were recorded at this site, 

 which is normal background. This site shared about half of its diatom assemblage with the upper 

 site, indicating only minor changes in envirormiental conditions. 



A large percentage of motile diatoms suggests the lower site was moderately impaired by 

 sedimentation. G. minutum was the dominant diatom species here, indicating eutrophication. 

 The pollution index approached but did not drop below the threshold for minor impairment. No 

 abnormal diatom cells were observed and species richness and diversity were acceptable but on 

 the low side. This site shared about half of its diatom assemblage with the middle site. 



