The dominant diatom at Rock Creek Station 2 was Achnanthes 

 minutissima (Table 5). This pioneer species is among the first 

 to colonize bare substrates after they have been disturbed; it 

 has a broad ecological amplitude, but is sensitive to low 

 concentrations of dissolved oxygen. The large percentage of this 

 taxon (42.10%) resulted in minor impairment due to the elevated 

 disturbance index and percent dominant species. The disturbance 

 at this site may have been natural in origin, e.g., grazing by 

 macroinvertebrates . 



All three sites had at least one teratological cell, but not 

 enough to suggest chemical toxicity or more than minor impairment 

 (Table 5) . Diatoms in the family Epithemiaceae were absent or 

 scarce at all three sites, suggesting that phosphorus, and not 

 nitrogen, was limiting to algal growth. 



The two Rock Creek stations had about half of their diatom 

 assemblages in common (Table 5) , which is to be expected given 

 the intervening tributaries between these sites. The diatom 

 assemblage at Rock Creek Station 2 was actually more similar to 

 the assemblage at Cottonwood Creek Station 2 than it was to the 

 assemblage at Rock Creek Station 1. 



Using Rock Creek Station 1 as a local reference site under 

 Protocol II (Table 6) , Rock Creek Station 2 proved to have good 

 to excellent biological integrity and no impairment of aquatic 

 life uses. 



10 



