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 and modal categories of these attributes were 

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



The majority of diatoms at most sites in the Big Hole TMDL planning area were non- 

 motile, alkaliphilous, nitrogen autotrophs that prefer fresh waters, moderate BOD levels, high 

 oxygen levels, and eutrophic conditions or variable levels 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. 



Although most of the sites were dominated by non-motile diatoms, SWMPCOl and 

 LOSTCOl were dominated by highly motile diatoms and SWLGCOl was dominated by 

 moderately motile diatoms. These are the sites that are most likely to have sedimentation 

 problems. 



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

 abundant at SWMPCOl, LMCHCOl, and ELKHCOl and 02. These sites are likely to have lower 

 pH values than the remaining sites. 



Nitrogen autotrophs were in the majority at all sites except LOSTCOl and CAMPCOl. 

 At site 01 on Lost Creek, facultative heterotrophs (mainly Navicula minima and Nitzschia 

 inconspicua) accounted for the majority of the diatom cells. This site likely receives heavier 

 loads of organic nitrogen than other sites in the sample set. At site 01 on Camp Creek, most cells 

 were represented by species that have not been classified with regard to nitrogen uptake (e.g., 

 Achnanthidium affine). 



The modal category for oxygen demand was less than "continuously high" at several 

 sites. Most diatoms were in the "fairly high" category at SWMPCOl and DfV^DCOl and 02. 



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