The periphyton samples from both sites contained a healthy- 

 mix of green algae, diatoms, red algae, and cyanobacteria (Table 

 5) . Non-diatom genus richness was good, with 10 genera observed 

 at J 1 and 11 genera at J 3 . Cyanobacteria (Gloeothece) were 

 most abundant at J 1, while green algae iCladophora) ranked first 

 in biovolume at J 3 . Diatoms were intermediate in abundance and 

 biovolume at both sites. 



The decline in nitrogen- fixing cyanobacteria and the 

 increase in green algae from J 1 to J 3 may indicate an increase 

 in nutrient concentrations between these sites. Certain 

 cyanobacteria (blue-green algae) are typical of mountain streams 

 with lower nutrient levels in Montana (Bahls et al . 1992). They 

 cannot compete with diatoms and green algae under moderate to 

 heavy nutrient loading. 



Cladophora, which was occasional at J 1 but frequent at J 3 , 

 is a widespread alga that often becomes a nuisance in waters that 

 are enriched with nutrients. Ankistrodesmus and Scenedesmus are 

 planktonic algae that are common in larger and nutrient-enriched 

 hardwater streams and rivers . 



Red algae are generally more sensitive to pollution than 

 other types. Audouinella, a red alga found in the Judith River 

 and in many other Montana streams, was common at J 1 and rare at 

 J 3 (Table 5) . 



DIATOMS 



The major diatom species in the Judith River were either 

 sensitive to or only somewhat tolerant of organic pollution 

 (Table 6) . The percent abundance of tolerant and sensitive 

 species was about the same at the two sites, resulting in almost 

 identical pollution indexes. The pollution indexes indicated no 



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