Ecological Attributes (Table 5) 



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 at the 

 six sites (Table 5). In least-impaired mountain streams in Montana, diatom assemblages are 

 typically dominated by mesotraphentic freshwater nitrogen autotrophs that tolerate only a small 

 amount of BOD loading, require continuously high concentrations of dissolved oxygen, and 

 prefer pH values greater than 7.00. 



For Chicago Gulch and Collar Gulch, the modal category for diatom motility was "not 

 motile" at four of the six sites. However, most diatoms were '"moderately motile" in the tributary 

 of Chicago Gulch. This was the only site for which the diatoms indicated impairment from 

 sedimentation. "Variable" motility was the modal category for the upper site on Collar Gulch. 



With regard to pH, the modal category was "alkaliphilous" only for the tributary and 

 lower sites on Chicago Gulch. For the upper site on Chicago Gulch and for all three sites on 

 Collar Gulch, the modal pH category was "circumneutral", which indicates that these sites had 

 significantly lower pH values than the other two sites. 



For salinity, the modal category was "fresh" for all sites except upper Collar Gulch. 

 Here, the modal category was "very fresh", which indicates a concentration of total dissolved 

 solids less than 200 mg/L and chloride content less than 100 mg/L (Van Dam et al. 1994). This 

 site had a unique diatom assemblage dominated by three species that require low concentrations 

 of dissolved solids and low pH values. This same site was dominated by autotrophic diatoms 

 that tolerate only low concentrations of organic compounds, whereas autotrophs that tolerate 

 high concentrations of organic compounds were in the majority at most of the other sites. 



Most sites were unclassified with regard to oxygen demand or the majority of diatoms 

 require continuously high concentrations of dissolved oxygen. Exceptions were the tributary and 

 lower sites on Chicago Gulch, where the D.O. requirement for most diatoms was "fairly high" 

 and "moderate", respectively. 



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