sites on the same stream varied from minor (40-60% between the upper and lower McCormick 

 Creek sites) to moderate (20-40% between the two sites on Stony Creek and Josephine Creek). 



The diatom order Rhopalodiales includes genera {Epithemia and Rhopalodia) that are 

 known to harbor nitrogen-fixing endosvmbionts within their cells. These symbiotic nitrogen- 

 fixers are single-celled cyanobacteria (blue-green algae). Nitrogen is likely the limiting nutrient 

 in waters that support large numbers of diatoms in the order Rhopalodiales. Among tributaries to 

 Ninemile Creek, diatoms in the order Rhopalodiales were present only in samples collected from 

 McCormick Creek (both sites) and the lower site on Josephine Creek. 



The following paragraphs highlight the key findings for each stream and each site based 

 upon the major diatom species and core diatom metrics in Table 5. 



Stony Creek. Both sites on Stony Creek had good biological integrity and provided 

 fiill support of aquatic life uses. Borderline values for the pollution index and siltation index 

 suggest minor impairment from organic loading and sedimentation at both sites. There was 

 slightly more sedimentation and organic loading at the downstream site than at the upstream site 

 (Table 5). A few teratological diatom cells were counted at the downstream site but none at the 

 upstream site. 



The major diatom species at the upstream site suggest slow current velocities and cold 

 water {Diatoma mesodon and Meridian circulare), low pH {Eunotia muscicola), and sandy 

 substrates {Planothidium spp.). The large percentage of Navicula perpusilla — an aerophilic 

 species — suggests that the sample was collected from substrates that are intermittently wetted. 

 Major species at the downstream site suggest faster current velocities (larger % Achnanihidium 

 minulissimum), somewhat warmer water temperatures, sandy substrates, and higher levels of 

 inorganic nutrients {Fragilaria and Synedra spp.), The two sites shared 37% of their diatom 

 assemblages, which indicates that a minor to moderate change in environmental conditions 

 occurred between them. 



10 



