Summary 



In June and July of 2003, periphyton samples were collected from five sites on Prairie 

 Elk Creek and one site on Sand Creek in northeastern 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 using 

 standard methods for periphyton, and evaluated following modified USEPA rapid bioassessment 

 protocols for wadeable streams. 



Diatom metrics suggest moderate impairment from sedimentation at Garoutte's and 

 Wright's on Prairie Elk Creek, and in Sand Creek. Diatom metrics also suggest moderate 

 impairment from organic loading at Garoutte's and in Sand Creek. All other sites had minor 

 organic loading, and the two sites near the mouth of Prairie Elk Creek had minor sedimentation. 



Diatom salinity preferences indicate a downstream freshening of the water in Prairie Elk 

 Creek toward the mouth. Diatom floras at the two sites near the mouth of Prairie Elk Creek were 

 virtually identical, even though these two sites were sampled over a month apart. A longitudinal 

 downstream decrease in organic loading was also noted in Prairie Elk Creek. The sample 

 collected at Taylor's contained the largest percentage of nitrogen-fixing diatoms, which suggests 

 that nitrogen was the limiting nutrient at this site. 



Sand Creek shared many of the same diatom species with Prairie Elk Creek and diatom 

 metrics indicate that environmental conditions in Sand Creek site were veiy similar to those at 

 Garoutte's. Both sites suffered moderate impairment from organic loading and sedimentation, 

 and the majority of diatoms at both sites indicate alkaline, eutrophic, and brackish-fresh waters. 

 The sample from Sand Creek also contained nitrogen-fixing diatoms, but a smaller percentage 

 than measured at Taylor's on Prairie Elk Creek. 



