impairment for either a mountain stream or a prairie stream. 



However, diatom species diversity at both sites indicated 

 minor impairment for a mountain stream and moderate impairment 

 for a prairie stream (Table 6) . Species richness also indicated 

 minor to moderate impairment at J 3 . Diatom diversity and 

 species richness were significantly higher at J 1 than at J 3; 

 values at J 1 were very near the threshold for no impairment of a 

 mountain stream and for minor impairment of a prairie stream. 



The low diversity at both sites was due, in part, to the 

 large percent abundance of Achnanthes minutissima . This is an 

 opportunistic "weed" species that colonizes recently disturbed 

 habitats. Its large abundance at both sites indicated that 

 moderate biological, chemical, or physical disturbance had 

 occurred before the samples were collected. It is not known 

 whether this disturbance was natural or cultural in origin. 



The relatively small number of cells in the motile genera 

 Navicula and Nitzschia indicated no impairment from siltation at 

 either site (Table 6) . 



Fifteen diatom valves (7.5 cells) exhibiting physical 

 abnormalities were observed at each site, indicating moderate 

 impairment. The percentage of teratological cells has been 

 correlated with ambient concentrations of heavy metals in certain 

 Colorado streams (McFarland et al . 1997). Teratological cells 

 may also result from other forms of pollution and environmental 

 stress . 



Only a few cells of diatom species in the family 

 Epithemiaceae were counted, indicating that nitrogen was probably 

 not limiting to algal growth at either site. Diatoms in this 

 family often harbor nitrogen- fixing cyanobacteria within their 

 cells and are most abundant in waters where nitrogen is the 



