The periphyton samples from both sites contained a mix of 

 green algae, diatoms, red algae, and cyanobacteria (Table 5) . 

 Red algae are generally more sensitive to pollution than other 

 types, although Audouinella, the red alga found in Casino Creek, 

 is somewhat tolerant of pollution. Occasional cells of Euglena 

 were observed at C 2 . The presence of this pollution- tolerant 

 euglenoid alga suggests some organic loading at this site. 



Diatoms ranked first in biovolume at both sites, followed by 

 cyanobacteria (mostly Lyngbya) at C 1 and by green algae (mostly 

 Cladophora) at C 2 . The decline in nitrogen-fixing cyanobacteria 

 and the increase in green algae from C 1 to C 2 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 absent at C 1 but common at C 2 , is a 

 widespread alga that often becomes a nuisance in waters that are 

 enriched with nutrients. Protoderma, the only green alga found 

 at C 1, grows primarily on submerged macrophytes (Smith 1950) . 



DIATOMS 



The major diatom species in Casino Creek were both tolerant 

 of and sensitive to 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 index declined slightly between C 1 and C 2, 

 indicating a small increase in organic loading between these 

 sites. The pollution index indicated minor impairment for a 

 mountain stream and no impairment when compared to biocriteria 

 for a prairie stream (Table 4) . 



8 



