and rapid division rates, are better suited for detecting the 

 short-term effects of the oil spill. 



DIATOMS 



All of the major diatom species in Casino Creek and Big 

 Spring Creek were Class 3 or pollution-sensitive taxa (Table 5) . 

 Class 3 diatoms typically decrease in abundance with increasing 

 concentrations of organic waste (Lange-Bertalot 1979) . However, 

 all of these major species except Achnanthes rninutissima exhibit 

 eutrophic tendencies, meaning that they grow best in higher 

 concentrations of inorganic nutrients such as nitrogen and 

 phosphorus (Lowe 1974) . 



Three of the major diatom species- -Diato/na vulgare, 

 Gomphonema olivaceum, and Nitzschia dissipata- -are cool season 

 diatoms. These diatoms often dominate the diatom assemblages of 

 mountain streams in the fall, winter, and spring (L. Bahls, 

 unpublished data) . 



Species diversity values at all sites were normal for 

 healthy mountain streams (Table 5) . Diatom diversity in Big 

 Spring Creek was slightly higher just below Casino Creek than it 

 was above Casino Creek. The lowest diversity value was recorded 

 in Big Spring Creek below Lewistown. 



Casino Creek had the lowest pollution index value of 2.56. 

 Nevertheless, this was still within the acceptable range for 

 .mountain streams in Montana. Pollution index values for Big 

 Spring Creek above and below Casino Creek were virtually the 

 same . 



Siltation index values indicated problematic levels of fine 

 sediments at all sites (Table 5) . If this assessment had been 

 conducted during the summer index period, Big Spring Creek above 



