Cyanobacteria, many species of which can fix atmospheric nitrogen, were most common 

 in Nelson Creek and the upper reaches of Horse Creek, Sand Creek, and the Redwater River. 

 Nitrogen is most hkely the limiting nutrient at these sites. Cyanobacteria were absent from 

 Prairie Elk Creek and lower Sand Creek. 



Algae in the division Euglenophyta {Euglena and Phacus) are known to be especially 

 tolerant of organic pollution (Palmer 1969). These algae were found only in samples collected 

 from the Redwater River below the airport and at Highway 13. The filamentous chrysophyte 

 Tribonema is known to favor cool water temperatures. Tribonema was common in samples 

 collected fi-om upper Horse Creek, Prairie Elk Creek, and lower Sand Creek. 



Diatoms (Table 4) 



Of the 12 major diatom species in McCone County streams, ovAy Achnanthidium 

 mimitissimum is sensitive to organic pollution (class 3). A minutissimum was most abundant in 

 Prairie Elk Creek and lower Sand Creek (Table 4). Eight of the major species are most tolerant 

 of organic pollution (class 1). These species were abundant at all sites except the lower 2 sites 

 on the Redwater River. The remaining 3 major species are somewhat tolerant of organic 

 pollution and these (class 2) species were most abundant in lower Horse Creek and lower 

 Redwater River (Table 4). 



Horse Creek. Diatom metrics indicate moderate organic loading at the upper and lower 

 sites and severe organic loading at the middle site. Minor to moderate sedimentation was 

 evident at the lower two sites (Table 4). Some sedimentation and internal organic loading may 

 be natural in prairie streams. Diatom species richness, diversity, and equitability values were 

 moderately low to extremely low at the two downstream sites. The cause of this low diversity is 

 likely a combination of organic loading and excessive salinity. Some of the most common 

 diatom species in Horse Creek tolerate elevated concentrations of dissolved solids. Nitrogen- 

 fixing diatoms were abundant at the upper site on Horse Creek, indicating that nitrogen is likely 

 the limiting nutrient here. The three Horse Creek stations had little in common floristically, 

 indicating that major changes in environmental conditions occurred between the sites. 



