% 



Diatoms 



Seven of the major diatom species in tributaries of the Swan River are sensitive to 

 organic poHution and four are somewhat tolerant of organic poHution (Table 4). The tolerant 

 ones tended to be more abundant at the downstream stations and in Jim Creek. The dominant 

 diatom species at all of the sites was QiXher Achnanthes biasolettiana ov Achnanthidium 

 minutissimum. Both are small, attached (non-motile) species that are adapted to living in fast 

 currents. Their abundance here is a consequence of the steep gradient and fast current velocities 

 of these streams. Both species prefer cool water temperatures and low nutrient concentrations. 



Although diatom community metrics at several sites indicated moderate to severe stress, 

 these stresses appear to be natural in origin and related to the high gradients, cold temperatures, 

 and low nutrient concentrations of these streams. High values for the pollution index and low 

 values for the sedimentation index and percent abnormal cells indicate that organic enrichment, 

 sedimentation, and toxic metals did not have a significant effect on the benthic algae of these 

 streams. 



Goat Creek. Low species diversity, high percent dominant species, and a high 

 disturbance index indicate minor to moderate stress in Goat Creek (Table 4). The dominant 

 species here — Achnanlhes biasolettiana and Achnanthidium minutissimum — indicate that this 

 stress was likely natural in origin. High similarity index values between adjacent stations in 

 Goat Creek indicate little or no change in the diatom associations from station to station, hence 

 little or no intervening input of pollutants or habitat alterations. 



Elk Creek. Both sites on Elk Creek had very low diatom diversity and species richness 

 and very high values for the disturbance index and percent dominant species (Table 4). Again, 

 Achnanthidium minutissimum was the dominant species, indicating a natural origin of the 

 stressors operating in Elk Creek. The pollution index, siltation index, and percent abnormal cells 

 all indicate good water quality here. The two sites on Elk Creek had very similar diatom floras, 

 indicating that little or no perturbation occurred between them. 



