Nitzschia dosterium, Nitzschia palea, Nitzschia obtusa) tolerate both elevated concentrations of 

 dissolved solids and heavy organic loading. Nitrogen-fixing diatoms were present but not 

 abundant at the upper site on Horse Creek, indicating that nitrogen may be the limiting nutrient 

 here. The two Horse Creek stations had little in common floristically, indicating that a major 

 change in environmental conditions occurred between them. 



Pasture Creek. Pasture Creek had the best water quality and biological integrity of the 

 three streams. Diatom metrics indicate little or no excess sedimentation and only minor organic 

 loading for a prairie stream. Major diatom species at the upstream site, especially Fragilaria 

 famelica and Tabularia fasciculata, indicate significantly higher levels of salinity than at the 

 downstream site, where Rhoicosphenia abbreviata was the dominant species. The latter species 

 indicates higher current velocities and cooler, fresher waters than upstream. The downstream 

 site also supported the largest percentage of Rhopalodiales among samples in this sample set, 

 indicating that nitrogen may be the limiting nutrient here. The two sites on Pasture Creek shared 

 about 43% of their diatom assemblages, which indicates that only a minor to moderate change in 

 environmental conditions occurred between them. 



Modal Categories (Table 5) 



Several ecological attributes v/ere selected from the diatom reports in the appendix and 

 modal categories of these attributes were extracted to characterize water quality tendencies in 

 tributaries of the Redwater River (Table 5). 



Most diatoms at several sites were highly motile nitrogen heterotrophs that tolerate 

 elevated levels of organics and prefer brackish, eutrophic, and alkaline waters. These sites have 

 more salinity and more organic loading than the typical prairie stream in eastern Montana. The 

 modal category for some of the attributes and sites was "not classified", which means that the 

 ecological preferences of the diatoms that comprise the largest group have yet to be determined. 



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