v.. 



biodegradable compounds is nearly complete, where concentrations 

 of inorganic nutrients are high, and where nitrogen is present in 

 the form of ammonia (Lowe 1974) . 



Dominance by Diatoma tenue and the resulting depressed 

 species diversity index indicated moderate impairment with 

 partial support of aquatic life uses below Circle. One may 

 conclude from the autecology of Diatoma tenue that the primary 

 cause of impairment here was nutrient, primarily nitrogen, 

 enrichment, with salts playing a minor role. The most likely 

 source of this enrichment is the Circle STP, perhaps exacerbated 

 by background loads of salts and nutrients from upstream. 



Deformed valves of Diatoma tenue also indicated moderate 

 impairment and partial support of uses below Circle (Table 5) . 

 Diatom deformities may result from a variety of stressors, 

 including heavy metals (McFarland et al . 1997). Given that 

 j:iiato;T7a tenue is typically found in waters where ammonia is 

 present, ammonia is a possible cause of the diatom deformities 

 observed below Circle. 



While Union Bridge and the site below Circle had the largest 

 similarity index, the site below Circle and the next downstream 

 site below Bluff Creek (RW2-B) had the smallest similarity index 

 of any pair of adjacent stations on the Redwater River (Table 5) . 

 In the 15 miles between these two sites, there was a significant 

 improvement in water quality. The dominant diatom below Bluff 

 Creek {Fragilaria construens) is a sensitive species and it 

 accounted for less than 10% of the cells at this site, and at 

 other sites, hence it is not listed in Table 5. Another 

 indicator of recovery here is the increase in cells of the family 

 Epithemiaceae {Epithemia and Rhopalodia spp . ) , which indicates a 

 decline in available nitrogen relative to phosphorus (Stevenson 

 and Pan 1999) . The site below Bluff Creek had the largest 

 diversity index and probably the best overall water quality and 



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