DIATOMS 



All of the major diatom species in Cottonwood Creek are 

 either tolerant of a wide range of trophic conditions- -from 

 oligotrophic to eutrophic- -or produce their largest populations 

 only in stronger mesotrophic to eutrophic waters (Tables 5 and 

 6) . Two of these major species- -Cocconeis pediculus and 

 Epithemia sorex--are epiphytes with one concave valve surface 

 that is so adapted to living attached to filaments of green 

 algae . 



All three sites on Cottonwood Creek had excellent diatom 

 diversity, species richness, and equitability (Table 5 and 6). 

 The disturbance index (% Achnanthidium winutissimum) was low at 

 all sites, indicating very little chemical, physical, or 

 biological disturbance and high community stability. 



The pollution index indicated minor impairment but full 

 support of aquatic life uses at all three sites when compared to 

 biocriteria for mountain streams in Table 2 (Table 5) . When 

 compared to criteria for plains streams in Table 3, the pollution 

 index indicated minor impairment only at site 03, and no 

 impairment at sites 01 and 02 (Table 6) . 



The siltation index indicated minor impairment at sites 01 

 and 02, and moderate impairment with partial support of aquatic 

 life uses at site 03 when judged against biocriteria for mountain 

 streams in Table 2 (see Table 5) . The siltation index increased 

 in a downstream direction. When compared to the criteria for 

 plains streams, the siltation index indicated no impairment and 

 full support of aquatic life uses at all three sites (Table 6) . 



The siltation index is the percentage of diatoms in the 

 genera Navicula, Nitzschia, and Surirella. Diatoms in these 



