Little Dry Creek near mouth of Uall Creek. This site supported somewhat larger 

 percentages of tolerant species and motile species, resulting in a smaller pollution index and a 

 larger siltation index than the Cohagen site (Table 4). Nevertheless, diatom metrics indicated 

 only minor impairment and full support of aquatic life uses. This site shared about 60% of 

 its diatom assemblage with the Cohagen site, which indicates that the two sites were virtually 

 identical, both floristically and ecologically. Both the Cohagen site and the Uall Creek site had 

 exceptional diatom species richness, diversity, and evenness for a prairie stream. 



Little Dry Creek near Van Norman School. This site had the highest pollution index, 

 the lowest sedimentation index, and the best biological integrity of all three sites. The large 

 pollution index, due mostly to dominance by the pollution-sensitive species Achnanthidium 

 minutissimum, was just below the threshold (2.25) for no impairment and excellent biological 

 integrity. The percentage of motile diatoms at Van Norman in the genera Navicula, Nitzschia. 

 and Surirella was well below the threshold (50.0) for minor impairment from sedimentation in 

 prairie streams. A strict comparison of diatom metric values in Table 4 to biocriteria in Table 2 

 would indicate that the Van Norman site was subject to minor impairment from organic 

 loading and biological or physical disturbance, which may have been natural in origin. 



The Van Norman site shared only 34% of its diatom assemblage with the Uall Creek site 

 (Table 4) and only 28% with the Cohagen site. Hence, of the three sites this one was the most 

 unique in terms of both floristics and ecology. Flow regime sets this site apart from the upper 

 sites. Little Dry Creek flows all the time at Van Norman, whereas the upper sites consist of a 

 series of pools (Rebecca Ridenour, MDEQ, personal communication). 



Of the three sites, Van Norman supported the lowest percentage of diatoms in the order 

 Rhopalodiales, indicating that inorganic nitrogen was probably more available here than it was 

 upstream. This site also had excellent diatom species richness and diversity for a prairie stream. 



Modal Categories (Table 5) 



