limiting nutrient-. 



The relatively large similarity index (72.94) indicated that 

 the two sites had very similar diatom floras and that little or 

 no change in environmental conditions had occured between the two 

 sites. Adjacent reaches on the same stream, without intervening 

 tributaries or pollution sources, can expect to have at least 60% 

 of their diatom floras in common (Bahls 1993) . 



In summary, dominance by Achnanthes minutissima and a 

 relatively large number of abnormal frustules indicated moderate 

 impairment at both sites when compared to biocriteria for both 

 mountain streams and prairie streams. Moreover, low diatom 

 diversity values at both sites indicated moderate impairment when 

 compared to diversity biocriteria for prairie streams. It is not 

 known whether the stress that resulted in the large populations 

 of A. winutissirna, the low diversity, and the larger than normal 

 number of teratological cells was natural or cultural in origin. 



LITERATURE CITED 



APHA. 1998. Standard Methods for the Examination of Water and 

 Wastewater. 20th Edition. American Public Health 

 Association, Washington, D.C. 



Bahls, L.L. 1979. Benthic diatom diversity as a measure of 

 water quality. Proc . Mont. Acad. Sci. 38:1-5. 



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. 



Barbour, M.T., J. Gerritsen, B.D. Snyder, and J.B. Stribling. 



1999. Rapid Bioassessment Protocols for Use in Streams and 

 Wadeable Rivers: Periphyton, Benthic Macroinvertebrates and 

 Fish. Second Edition. EPA/841-B-99-002 . U.S. EPA, Office 

 of Water, Washington, D.C. 



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