Modal Categories (Table 6) 



Several ecological attributes assigned by Stevenson and Van Dam et al. (1994) were 

 selected from the diatom reports in the appendix. Modal categories of these attributes were 

 extracted to characterize water quality tendencies in tributaries of the Madison River (Table 6). 



The majority of diatoms at most sites in the sample set are non-motile, alkaliphilous, 

 nitrogen autotrophs that prefer fresh waters, moderate BOD levels, high oxygen levels, and 

 elevated concentrations of inorganic nutrients. However, the modal categories at some sites 

 represent significant departures in water quality when compared to most other sites in the sample 

 set. These departures, which may reflect increases or decreases in water quality, are discussed 

 below. 



Diatom species that prefer circumneutral (as opposed to alkaline) pH values were most 

 abundant at the upper site on Blaine Spring Creek. This site is likely to have lower pH values 

 than the remaining sites. 



The modal category for oxygen demand was less than "continuously high" at six sites. 

 Most diatoms were in the "fairly high" category in Buford Creek and Blaine Spring Creek near 

 mouth. Moderate oxygen demand was the modal category at the upper site on Elk River, the 

 lower site on Gazelle Creek, and the two middle sites on Blaine Spring Creek. 



Beta-mesosaprobous was the level of saprobity at all but one site. This represents a 

 dissolved oxygen saturation of 70-85% and 2-4 mg/L of biochemical oxygen demand (BOD5). 

 The level of saprobity was higher (alpha-mesosaprobous) in Blaine Spring Creek below the fish 

 hatchery. The alpha-mesosaprobous level corresponds to 25-70% saturation of dissolved oxygen 

 and 4-13 mg-'LBODs. 



At two sites — Antelope Creek and Blaine Spring Creek at bypass — the majority of 

 diatoms represented species that tolerate a wide range of inorganic nutrient concentrations, from 

 oligotrphentic to eutraphentic. Diatoms indicate eutrophic conditions at the other sites. 



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