The siltation index for Potter Creek indicated only fair 

 biological integrity, moderate impairment, and partial support of 

 aquatic life uses for a mountain stream (Table 6) . The siltation 

 index for Potter Creek indicated full support but borderline 

 impairment when compared to biocriteria for a prairie stream. 



An unusually large percentage of teratological cells (1.64%) 

 was observed in the sample from Potter Creek, resulting in a 

 rating of fair biological integrity, moderate impairment, and 

 partial support of aquatic life uses (TabJe 6) . Since the 

 criteria for abnormal cells are the same for mountain streams and 

 prairie streams, the fair rating applies in both cases. 



A number of factors are known to cause abnormalities in 

 diatom cells, including heavy metals (McFarland et al . 1997). 

 Salinity and ammonia are other possible causes. Salinity is not 

 the likely cause of teratological cells in Potter Creek because 

 most of the major diatom species that are present (Table 6) 

 indicate fresh to only slightly brackish water. Heavy metals are 

 also an unlikely cause. Deformities caused by heavy metals have 

 been reported only from poorly buffered, circumneutral waters 

 (McFarland et al . 1997)-. 



In summer, with decreased flows, higher temperatures, and 

 decompositon of algae and aquatic macrophytes, a certain amount 

 of internal organic loading may occur in low gradient streams. 

 Some of the abnormalities observed in Potter Creek may have been 

 caused by ammonia generated from this internal organic loading. 

 Thus, the percentage of deformed cells observed in Potter Creek 

 in August may be within the normal range for such low gradient 

 streams . 



