Seven abnormal diatom valves were counted at Station 1, placing 

 the site in the minor impairment category. All other diatom 

 metrics indicated full support of aquatic life uses at Station 1. 



The diatom assemblage at Station 2 was dominated by Cymbella 

 affinis, a common diatom in moderately enriched rivers in western 

 Montana. It is very abundant in the Clark Fork River below 

 Missoula in late summer (Weber 2 000) . Dominance by Cymbella 

 affinis resulted in a rating of minor impairment (Table 5) . 



Ten abnormal diatom valves were counted at Station 2, which 

 indicated moderate impairment and partial support of aquatic life 

 uses. This metric, however, was just above the threshold for 

 minor impairment. The cause and source of this impairment are 

 unknown . 



All other diatom metrics at Station 2 indicated excellent 

 biological integrity, no impairment, and full support of aquatic 

 life uses. The pollution index at Station 2 was significantly 

 larger than- it was at Station 1, and the siltation index was much 

 smaller, indicating that Station 2 had less organic loading and 

 siltation. The relatively large percentage of diatoms in thdl 

 family Epithemiaceae indicates that nitrogen was likely th^ 

 limiting nutrient at Station 2 .| 



Diatom species diversity and species richness declined 

 somewhat from Station 1 to Station 2, but these metrics still 

 indicated excellent biological integrity at the downstream site. 

 The two sites shared less than a third of their diatom floras, 

 indicating considerable floristic change and improvement in water 

 quality between them. 



