Summary 



In August 2002, periphyton samples were collected from 2 sites on Beaver Creek in the 

 upper Madison River TMDL planning area in southwestern Montana for the purpose of assessing 

 whether this stream is 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. 



The sample collected at the upper site on Beaver Creek was dominated by Hydrurus 

 foetidus, a macroscopic and gelatinous chrysophyte that is common in unshaded mountain 

 streams in North America. In addition to ample sunlight, Hydntrus foetidus requires cold 

 temperatures (<10"C) and low (circumneutral) pH. Hydnirus foetidus was absent from the 

 sample collected at the lower site on Beaver Creek. 



Diatoms were frequent in both samples collected from Beaver Creek and the diatom 

 associations at both sites were dominated by species that are either sensitive to organic pollution 

 or only somewhat tolerant of organic pollution. Values for the pollution index and siltation 

 index indicate no impairment from organic loading or sedimentation, respectively. Slightly 

 depressed diversity values and slightly elevated values for percent dominant species, disturbance 

 index, and percent abnormal cells indicate that the periphyton community was subjected to minor 

 stresses. However, these stresses were probably natural in origin and caused by low water 

 temperatures and low nutrient concentrations coupled with rapidly growing diatom populations. 



The two sites shared 68% of their diatom associations, which indicates that there was 

 little or no difference between them in terms of floristics or ecological conditions. The major 

 distinction between the two sites appears to be the greater exposure to sunlight at the upstream 

 site, which allowed for optimum growth oi Hydrurus foetidus. 



