DISCUSSION AND CONCLUSIONS 



On the basis of diversity measures, the most severely impacted 

 station on the Yellowstone through Billings was below the Corette plant (V). 

 The great abundance (42.3%) of Cymbella affinis here helped depress 

 diatom diversity to the lowest levels recorded in the present study. 

 Although nutrient data at this site are incomplete and dated, phosphate 

 does appear to be significantly more concentrated here than upstream 

 (Table II). However, the abundance of the saprophobic diatoms £. affinis 

 and Achnanthes minutissima and the relatively minor importance of Nitzschiae 

 indicate chemical water quality below the Corette plant to be rather good. 

 Because C. affinis is a summer diatom, i.e., it prefers warmer waters (7), 

 the stress causing depressed diversity at this location appears to be 

 brought on by elevated temperature from the thermal discharge rather 

 than by some chemical constituent introduced from the ash pond. 



While burdened with a much heavier nutrient load (Table II), Yegen 

 Drain (VII) had significantly higher diversity values than other study 

 sites (Table III). Yet when this load was released into the Yellowstone 

 River, diversity values were slightly depressed (Station VIII). One 

 explanation might be that Yegen Drain offers a greater diversity of 

 habitats and a physical environment, in terms of substrate, depth, tem- 

 perature and flow regime, favorable to a larger variety of benthic diatoms. 

 This situation deserves more attention and illustrates the fact that 

 factors other than pollution load are responsible for biological diversity 

 levels in streams, making them difficult to compare on this basis alone. 



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