Modal Categories 



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 in 

 streams of the Dearborn River TMDL planning area (Table 6). In all six of the streams that were 

 sampled for periphyton in 2002, the majority of diatoms were non-motile autotrophs that tolerate 

 high levels of organic nitrogen and indicate fresh-brackish waters and moderate organic loading 

 (beta-mesosaprobous conditions). 



For other ecological attributes, the modal category departed from the typical condition at 

 some sites. For example, most diatoms at the upper site on the main stem Dearborn River and at 

 both sites on the Middle Fork indicated circumneutral water, e.g., pH ~7.0. However, most 

 diatoms at the lower site on the Dearborn River and at both sites on the South Fork prefer pH 

 values >7.0 (alkaliphilous). This may be due in part to different geological features (limestone 

 outcrops?) at these sites. 



The majority of diatoms at both sites on the Dearborn River and both sites on the Middle 

 Fork require "continuously high" levels of dissolved oxygen (Table 6). In the South Fork, the 

 modal category for dissolved oxygen was only "moderate" at the upstream site and "fairly high" 

 at the downstream site. Note that the modal category for dissolved oxygen actually indicated 

 increased saturation of dissolved oxygen from the upstream site to the downstream site on the 

 South Fork of the Dearborn River. This parallels the improvement in the pollution index 

 between these two sites that was noted earlier (Table 5). 



The modal category for trophic status was "variable" at sites on the main stem Dearborn 

 River and on the Middle Fork Dearborn River. This means that most diatoms at these sites are 

 able to tolerate a wide range of trophic conditions ranging from oligotrophic to eutrophic. In the 

 South Fork, however, the modal category was "eutraphentic", which indicates that most diatoms 

 at these two sites prefer elevated concentrations of algal nutrients (C, N, and P). The source of 

 these elevated nutrients may natural (e.g., limestone) or anthropogenic, or both. 



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