outfall from the Lewistown wastewater treatment plant. 



The two replicate samples from site 03 generated a percent 

 community similarity index of 84% (Table 5) . In addition, diatom 

 metrics from both sites yielded the same bioassessment rating: 

 minor impairment but with full support of aquatic life uses. 

 These results are acceptable from a quality assurance standpoint. 



A large percent dominant species indicated moderate 

 impairment and partial support of aquatic life uses at 3SPRC05 

 (Table 5) . The dominant species here was the eutrophic diatom 

 Cocconeis pediculus . This diatom has a concave valve surface, by 

 which it is adapted to living as an epiphyte on Cladophora and 

 other filamentous algae. Its abundance at site 05 is directly 

 related to dominance at this site by Cladophora (Table 4) . 

 Cladophora and Cocconeis indicate that the probable cause of 

 impairment at this site is nutrient enrichment. 



Sites 03 and 05 shared only about a third of their diatom 

 floras, indicating that a moderate amount of environmental change 

 (degradation) occurred between them. Both sites are downstream 

 from the outfall of the Lewistown wastewater treatment plant. 



All of the sites sampled in August, except the one just 

 below Big Springs, had a few teratological diatom, valves. This 

 may indicate small concentrations of toxic chemicals, including 

 heavy metals. None of the sites sampled in May and August 

 supported a large number of diatoms in the family Epithemiaceae . 

 This indicates that phosphorus, not nitrogen, was likely the 

 limiting nutrient in Big Spring Creek. 



The overall impairment ratings for Big Spring Creek would 

 not change if metrics for sites 03 and 05 in the B-2 segment 

 below Lewistown were compared to criteria for plains streams 

 instead of mountain streams. Site 03 would still have good 



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