which was probably related to increasing organic loading and salinity. Sites 1 and 2 and sites 3 

 and 5 shared similar diatom floras, but sites 2 and 3 were quite dissimilar (Table 9). 



In Dupuyer Creek, there was no indication from the diatoms of either excessive organic 

 loading or excessive sedimentation for a prairie stream (Table 9). Organic loading appeared to 

 be highest at the 2 upstream sites and decreased at the downstream site. Sedimentation was 

 highest at the middle site, but still within acceptable limits for a prairie stream. There was no 

 indication of excessive salinity in Dupuyer Creek. Adjacent sites in Dupuyer Creek shared 

 about half their diatom floras, indicating only minor changes in ecological conditions. 



Modal Categories. Several ecological attributes were selected from the diatom reports 

 in the appendix and modal categories of these attributes were extracted to characterize water 

 quality tendencies in streams of the Cut Bank — Two Medicine planning area (Tables 10-12). 



Modal categories indicate that diatoms in Old Maids Coulee are primarily eutraphentic, 

 alpha-mesosaprobous, and either facultative nitrogen heterotrophs or nitrogen autotrophs that can 

 tolerate high levels of organic nitrogen (Table 10). In Cut Bank Creek, modal diatom categories 

 indicate alpha-mesosaprobous conditions at the upstream site, improving to beta-mesosaprobous 

 at the two downstream sites. These categories represent oxygen saturation ranges of 25-70% and 

 70-85%, and BOD concentrations of 4-13 mg/L and 2-4 mg/L, respectively. Most diatoms at all 

 3 sites in Cut Bank Creek tolerate a wide range of trophic conditions, and are therefore classified 

 as "variable" with respect to trophic state (Van Dam et al. 1994). Most diatoms indicate fresh- 

 brackish waters at all sites in Old Maids Coulee and Cut Bank Creek (Table 10). 



Non-motile diatoms indicating fresh-brackish waters were prevalent at all mountain sites 

 in the Two Medicine drainage (Table 11). Diatoms in the fresh-brackish category grow best in 

 waters that have less than 500 mg/L chloride and less than 900 mg/L total dissolved solids (Van 

 Dam et al. 1994). Most diatoms at mountain sites in the Two Medicine drainage indicate either 

 alkaline or circumneutral waters with moderate to continuously high concentrations of dissolved 

 oxygen. Most diatoms at these sites are autotrophs that tolerate high levels of organic nitrogen 



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