• 



algae and one genus of cyanobacteria (Anabaena) accounted for a small portion of the sample's 

 biomass. This site supported the smallest number of non-diatom genera (6) of all the sites. 



West Fork Yaak River. The periphyton community at this site was dominated by the 



filamentous red alga Batrachospennwn. This is a diverse and widespread genus in North 

 America. In Montana, Batrachospermum is found mostly in small, cold mountain streams in the 

 northwest part of the state. After Batrachospermum and diatoms, which ranked second in 

 biovolume, the West Fork sample was composed mostly of filamentous green algae {Spirogyra, 

 Hormidium, Microspora, and Mougeotia). Fourteen genera of non-diatom algae, representing 

 three algal divisions, were found in the sample from the West Fork (Table 3). This is excellent 

 algal diversity for a mountain stream. 



Diatoms (Table 4, Figures 1 and 2) 



The 15 major diatom species from the Yaak River and the West Fork of the Yaak River 

 are included in pollution tolerance classes 3 and 2 and are either sensitive to organic pollution or 

 only somewhat tolerant of organic pollution (Table 4). None of the major diatom species in 

 these streams are most tolerant of organic pollution (pollution tolerance class = 1). 



Yaak River above East Fork. A few abnormal diatom cells indicate possible minor 

 stress on the periphyton community at this site. All other diatom metrics indicate no stress, 

 excellent biological integrity, and full support of aquatic life uses. The dominant diatom species 

 here — Cocconeis placentula and Achnanthidium minutissimum — are attached species that are 

 resistant to disturbance and sensitive to organic pollution. Although the siltation index was well 

 below the threshold for minor impairment, one of the major species at this site {Planothidium 

 lanceolatiim) is adapted to living on sand grains. This site exhibited the best diatom diversity, 

 the least organic loading, and the least disturbance of the 3 Yaak River sites (Figures 1 and 2). 



