Yaak River at \\'hitetail Campground. The percent Achnanthidium minutissimum 

 (disturbance index), percent dominant species, and percent abnormal cells at this site indicate 

 minor stress from unknown sources. Other diatom metrics indicate no stress, excellent 

 biological integrity, and full support of aquatic life uses. However, the siltation index at this site 

 approached (but did not exceed) the threshold for minor impairment and the pollution index was 

 smaller than it was above the East Fork, which indicates an increase in organic loading. The 

 decline in the pollution index was due to increases in abundance of pollution tolerant species, 

 primarily Synedra rumpens and Synedra ulna. This site shared 41% of its diatom assemblage 

 with the site above the East Fork, which indicates that a minor to moderate change in overall 

 environmental conditions occurred between these sites. 



Yaak River at Sylvanite. Achnanthidium minutissimum accounted for over 41 percent 

 of the diatom cells at this site, which indicates minor disturbance. The cause of this disturbance 

 is unknown and may be physical, chemical, or biological in origin. About 1% of the diatom cells 

 here were abnormal, but this value is within the range of minor impairment. The cause of these 

 abnormal cells is unknown. Other metrics indicate no impairment and full support of aquatic 

 life uses. However, diatom species richness and diversity were significantly lower here than they 

 were upstream (Table 4, Figure 1 ). The pollution index was about the same as it was at Whitetail 

 Camp Ground and the siltation index was intermediate between the low value above the East 

 Fork and the high value at Whitetail Camp Ground. Although numerically insignificant, the very 

 large cells of Didymosphenia geminata and their polysaccharide stalks accounted for most of the 

 biomass at this site. D. geminata is a boreal species that often produces large standing crops in 

 streams. This site shared 52% of its diatom assemblage with the Whitetail site, which indicates a 

 minor change in environmental conditions and diatom species composition. 



^^'est Fork Yaak River. The large percentage oi Synedra rumpens (63.12%)) and the 

 much larger than usual percentage of abnormal diatom cells (4.77%) each indicates moderate 

 impairment and only partial support of aquatic life uses at this site. Synedra rumpens 

 tolerates some organic enrichment as well as elevated concentrations of heavy metals. Diatom 

 species richness and diversity were ver>' low here (Table 4 and Figure 1). All of these metrics 

 suggest toxicity from heavy metals. Although this site supported a relatively large number of 



