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 on Big Dry Creek (Table 5). 



Modal categories were identical for the The Trees and the Mormon Church. Most 

 diatoms at these sites were highly motile, alkaliphilous, eutraphentic, and facultative nitrogen 

 heterotrophs that indicate brackish- fresh waters with 25-70% dissolved oxygen saturation and 4- 

 13 mg/L BOD5 (Van Dam et al. 1994). The modal category for oxygen demand was "moderate" 

 (>50% saturation) at these sites. Modal categories at the sewage plant remained the same except 

 for the nitrogen uptake and oxygen demand categories, which were "not classified". 



At Highway 200 and downstream, modal categories indicated water quality improvement 

 for sedimentation, salinity, and nutrient loading (Table 5). At these sites, most diatoms were 

 only moderately motile instead of highly motile (exception: Van Norman site), nitrogen 

 autotrophs instead of facultative nitrogen heterotrophs, and indicated fresh waters instead of 

 brackish-fresh waters. The modal category for trophic state was eutraphentic at all stations. 



References 



APHA. 1998. Standard Methods for the Examination of Water and Wastewater. 20* Edition. American Public 

 Health Association, Washington, D.C. 



Bahls, L.L. 1979. Benthic diatom diversity as a measure of water quality. Proceedings of the Montana 

 Academy of Sciences 38:1-6. 



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

 Heahh 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. 

 EPAy841-B-99-002. U.S. Environmental Protection Agency, Office of Water, Washington, D.C. 



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