Tongue River in the summer of 2001. 



The siltation index, which is the percent abundance of 

 motile diatoms in the genera Navicula, Nitzschia and Surirella, 

 indicated minor impairment at TR04 and moderate impairment at 

 TR05 (Table 5) . Reimeria sinuata, which is also a motile and 

 free-living diatom, peaked in abundance at TR04 . No impairment 

 from siltation was indicated at the three upstream sites or at 

 the lowest site at Miles City. 



A small number of teratological diatom cells was counted at 

 each of the four uppermost stations, indicating minor impairment 

 (Table 5) . The cause of this impairment is unknown. 



The two upstream sites on the Tongue River supported 

 relatively large numbers of diatoms in the family Epithemiaceae 

 (Table 5) . Diatoms in this family harbor small nitrogen- fixing 

 cyanobacteria in their cells and an abundance of these diatoms 

 typically indicates a deficiency of nitrogen in the water column 

 relative to phosphorus. Nitrogen deficiency was particularly 

 acute at TR02 below Tongue River Dam. 



Similarity index values indicate the amount of floristic 

 similarity between adjacent sampling sites. The two upstream 

 sites, above and below Tongue River Reservoir, had less than 20% 

 of their diatom populations in common (Table 5), indicating a 

 very large change in environmental conditions. Sites TR04 and 

 TR05 shared less than a third of their diatom floras, also 

 indicating a substantial change in conditions between these 

 sites. Sites TR02 and TR03 shared nearly half of their diatom 

 floras, indicating that a smaller change occurred between these 

 two sites. Similarity index values for the remaining station 

 pairs were somewhere in between these extremes. 



11 



