Tolypothrix was absent from the middle and lower sites on Little Dry Creek, perhaps 

 indicating that these sites had warmer waters and/or more biologically available nitrogen than the 

 upstream site. Little Dry Creek near the mouth of Uall Creek and Little Dry Creek near the Van 

 Norman School supported the same three genera of filamentous green algae as the upstream site. 

 In addition, Cladophora, another mat-forming and potential problem alga, was common at the 

 Van Norman site (Table 3). 



Diatoms (Table 4) 



Of the 15 major diatom species in Little Dry Creek, 2 are sensitive to organic pollution 

 (class 3) and 2 are most tolerant of organic pollution (class 1). Sensitive and most tolerant 

 species were abundant at all three sites (Table 4). The remaining 1 1 major species are somewhat 

 tolerant of organic pollution and these (class 2) species were also abundant at all three sites. All 

 three sites had excellent diatom species richness and diversity for a prairie stream. 



Little Dry Creek at Cohagen. Diatom metrics indicate minor impairment from 

 organic loading and sedimentation at this site but otherwise excellent biological integrity and full 

 support of aquatic life uses (Table 4). The dominant diatom species at this site were Nitzschia 

 amphibia and Nitzschia inconspicua. Both of these species are facultative nitrogen heterotrophs 

 that prefer eutrophic, alkaline, and brackish-fresh waters and need periodically elevated 

 concentrations of organically bound nitrogen (Van Dam et al. 1994). 



Of the three sites, the Cohagen site also supported the largest percentage of diatoms in the 

 order Rhopalodiales (Table 4). Diatoms in the order Rhopalodiales harbor endophytic nitrogen- 

 fixing symbionts (cyanobacteria). The large percentage of Rhopalodiales at Cohagen, coupled 

 with an abundance of the nitrogen-fixing cyanobacterium Tolypothrix, indicates that biologically 

 available inorganic nitrogen was probably in short supply (limiting) relative to phosphorus. With 

 an abundance of both autotrophic and heterotrophic algae at this site, any increase in organic or 

 inorganic nitrogen would likely result in an increase in algal growth. 



