these species were more abundant at the downstream site than they 

 were at the upstream site (Table 6), indicating an increase in 

 nutrient concentrations at the downstream site. This increase in 

 pollution-tolerant species was accompanied by a decline in the 

 Pollution Index from 2.91 to 2.60. However, both pollution index 

 values indicate excellent water quality and no impairment. 



Diatom species richness and diversity at the upstream site 

 were good to excellent for a mountain stream but only fair to 

 good for a prairie stream (Table 6) . The percent dominant 

 species at the upstream site indicated moderate stress. This 

 site was dominated by tychoplanktonic species of araphid diatoms 

 that indicate stable flows and an absence of physical distrubance 

 (Dr. Rex Lowe, Bowling Green State University, pers . comm.). 



The very low Disturbance Index (percent Achnanthidium 

 rninutissimum) at station 01 confirms a low level of physical 

 disturbance here. The relatively small percentages of A. 

 rninutissimum, an opportunistic pioneer species, indicated minimal 

 biological, chemical, or physical disturbance at both sites. 



The relatively large number of cells in the motile genera 

 Navicula and Nitzschia indicated moderate impairment from 

 siltation at the downstream site when compared to thresholds for 

 mountain streams (Table 6) . However, when compared to diatom 

 criteria for prairie streams, siltation caused no impairment at 

 either station 01 or station 02. 



A few diatom cells exhibiting physical abnormalities were 

 observed at both sites. This number of abnormal cells indicates 

 only minor 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. 



