Although intended primarily as a measure of organic loading 

 (Lange-Bertalot 1979) , in practice the pollution index responds 

 to a variety of disturbances and pollution types, including 

 salts, organics, temperature extremes, toxics, inorganic 

 nutrients, and siltation (Bahls 1993) . Taxa that tolerate one of 

 these types of pollution also tend to tolerate one or more of the 

 other types. Knowing the autecology of the dominant species 

 helps to identify which type or types of pollution is the most 

 probable cause of impairment. 



The pollution index and the number of diatom species 

 increased at Kurtz (RWl-9) , where diatom metrics indicated full 

 support with only minor impairment of beneficial uses. The 

 brackish water taxa recorded at Gackle were much less abundant 

 here, indicating fresher water than upstream. The dominant 

 species at Kurtz was Synedra famelica (Table 5) . 



At Union Bridge (RW2-D), Diatorna tenue replaced Synedra 

 famelica as the dominant diatom species (Table 5) . Diatoma tenue 

 is halophilous, meaning that it is stimulated by small amounts of 

 salt (Lowe 1974) . In a survey of saline seeps in eastern Montana 

 (Bahls and Bahls 1976) , Diatoma tenue preferred waters with lower 

 conductivities than those preferred by Synedra famelica. This 

 indicates that the Union Bridge site probably had fresher water 

 than the Kurtz site, which in turn had fresher water than the 

 Gackle site upstream. Diatom metrics indicated full support of 

 uses at Union Bridge with minor impairment caused by salts and 

 organic loading. 



The site below Circle (RW2-F) and the site upstream at Union 

 Bridge had very similar floras (similarity index = 66%) , in spite 

 of the intervening presence of the Circle STP . Diatoma tenue 

 accounted for almost 70% of the diatom cells below Circle. In 

 addition to being stimulated by small amounts of salt, Diatoma 

 tenue is typically found in waters where the oxidation of 



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