Summary 



In July of 2003, periphyton samples were collected from 8 sites on 5 streams in the 

 Ninemile Creek TMDL planning area in west central Montana for the purpose of assessing 

 whether these streams are water-quality limited and in need of TMDLs. The samples were 

 collected following MDEQ standard operating procedures, processed and analyzed following 

 standard methods for periph>lon, and evaluated following modified USEPA rapid bioassessment 

 protocols for wadeable streams. "' 



A large percentage of motile diatoms suggest moderate impairment from sedimentation 

 and partial support of aquatic life uses at the lower site on Josephine Creek. The siltation index 

 suggests minor impairment from sedimentation but full support of uses at all of the remaining 

 sites except Big Blue Creek. However, the siltation index for Big Blue Creek approached the 

 threshold for minor impairment and this stream supported a large percentage of Planothidium 

 species, which are adapted to living on sand grains. 



Low or borderline values for the pollution index suggest minor impairment from organic 

 loading at all sites except Big Blue Creek, where the pollution index was normal for a mountain 

 stream. Most diatoms in lower McCormick Creek and upper Josephine Creek require only 

 moderate levels of dissolved oxygen, compared to the continuously high levels required by 

 diatoms in most mountain streams. Most diatoms in Josephine Creek (both sites) prefer alpha- 

 mesosaprobous conditions, which is a higher level of saprobity than typical for most mountain 

 streams. These three sites — lower McCormick Creek and the two sites on Josephine Creek — 

 appear to have higher levels of organic loading than the other sites. 



Most diatoms at all sites in McCormick Creek and Josephine Creek were eutraphentic, a 

 category that indicates elevated concentrations of inorganic nutrients. The majority of diatoms in 

 upper Stony Creek were oligotraphentic and the majority of diatoms in lower Stony Creek were 

 mesotraphentic. Most diatoms in Big Blue Creek and Cedar Creek tolerate a wide range of 

 nutrient concentrations ranging from oligotrophic to eutrophic. 



Among tributaries of Ninemile Creek, nitrogen- fixing blue-green algae were most 

 abundant in McCormick Creek and Josephine Creek. These were also the only streams to 

 support nitrogen-fixing diatoms. Nitrogen is most likely the limiting nutrient in these streams. 



Five of the sites supported deformed diatom cells. In large numbers, abnormal cells may 

 indicate metals toxicity. However, the percentage of abnormal cells was within acceptable limits 

 at all sites. The largest percentage of abnormal cells (0.63%) was recorded in Big Blue Creek. 



