Oscillatoria is considered to be quite tolerant of organic 

 pollution (Palmer 1969) . Diatoms ranked second at Station 1 and 

 various genera of green algae ranked third through fifth. Seven 

 genera of non-diatom algae were recorded at Station 1, which is 

 about average for mountain streams in Montana (Bahls 1993) . 



The sample from Station 2 contained 13 genera of non-diatom 

 algae. Diatoms dominated the biovolume at this site, followed by 

 the green alga Oedogonium and the cyanobacterium Oscillatoria 

 (Table 4) . Cladophora ranked fourth in biovolume. 



The non-diatom algae from these sites indicated significant 

 organic loading, especially at the upstream site (Station 1) . 

 Theffincrease in the number of non-diatom genera at the downs trealh 

 station may reflect warming water temperatures, slower "eB^f-fent 

 velocities, ''arid' a conversion of organic' iiutrients to irioirganiC 

 nutrients .« 



DIATOMS 



Major diatom species in the Shields River were either 

 sensitive to organic pollution or somewhat tolerant of organic 

 pollution (Table 5) . The three tolerant species- -Melosira 

 varians, Navicula capitatoradiata, and Navicula reichardtiana- - 

 were all more abundant at Station 1 than at Station 2. 



The pollution tolerance index at Station 1 (2.55) was low 

 for a mountain stream and borderline on minor impairment. This, 

 plus the abundance of Oscillatoria and the absence of diatoms in 

 the family Epithemiaceae, indicate a probable source of organic 

 nitrogen loading above Station 1. 



The siltation index at Station 1 was well within the range 

 for moderate impairment and partial support of aquatic life uses . 



8 



