Most diatoms at most of the sites were highly motile nitrogen autotrophs that tolerate 

 high levels of organics and prefer brackish or brackish-fresh eutrophic and alkaline waters. The 

 modal categon, 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. 



Modal categories at some sites represent a significant improvement in water quality 

 compared to other sites. For example, most diatoms at Taylor's were "not motile" and most 

 diatoms in Sand Creek were "moderately motile". At other sites, the modal category was 

 "highly motile". At Wright's and Taylor's, the modal category was "brackish", but it was 

 "brackish-fresh" at Garoutte's and in Sand Creek and "fresh" at both sites near the mouth of 

 Prairie Elk Creek. 



Modal categories suggest possible inferior conditions at one or both of the upstream sites 

 on Prairie Elk Creek. At Garoutte's, the modal category' for oxygen demand was "low", whereas 

 it was "not classified" at the remaining sites. At Wright's, the modal category for saprobity was 

 "alpha-mesosaprobous", while it was "beta-mesosaprobous" at Garoutte's and "not classified" at 

 the remaining stations. 



References 



.APHA. 199S. Standard Methods for the E.xarrunation of Water and Wastewater. 20"' Edition. American Pubhc 

 Heahh Association, Washington, D.C. 



Bahls, L.L. 1979. Benthic diatom diversitv' as a measure of water quality. Proceedmgs of the Montana 

 Academy of Sciences 38:1-6. 



Bahls, L.L. 1993. Periphyton Bioassessment Methods for Montana Streams (revised). Montana Department of 

 Health and Environmental Sciences, Helena. 



Bahls, L.L., Bob Bukantis, and Steve Tralles. 1992. Benchmark Biology of Montana Reference Streams. Montana 

 Department of Health and Environmental Sciences, Helena. 



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