SUMMARY 



On August 25, 2001, periphyton samples were collected at two 

 stations on the upper Stillwater River near Cooke City for the 

 purpose of assessing whether the river is water-quality limited 

 and in need of TMDLs . The samples were collected following DEQ 

 standard operating procedures, processed and analyzed using 

 standard methods for periphyton, and evaluated following modified 

 USEPA rapid bioassessment protocols for wadeable streams. 



Low diatom species diversity and a large percentage of the 

 tolerant diatom Encyonema rninutum indicate moderate impairment 

 and partial support of aquatic life uses below Daisy Creek. 

 Above the Wilderness boundary, impairment was minor and aquatic 

 life uses were fully supported. 



A relatively large number of teratological diatom cells also 

 indicates moderate impairment below Daisy Creek. The cause of 

 these deformities is probably heavy metals. The number of 

 teratological cells was much smaller above the Wilderness 

 boundary, indicating some recovery between the two sites. 



All of the major diatom species in the upper Stillwater 

 River are either tolerant of a wide range of trophic conditions 

 or exhibit their most vigorous growth in stronger mesotrophic to 

 eutrophic waters. Among the tolerant species is Encyonema 

 minutum, which dominated the diatom assemblage below Daisy Creek. 



Periphyton samples were collected from these same two sites 

 in August 1994. A bioassessment based on these samples, using 

 the same methods used here, resulted in the same ratings of 

 biological integrity: "Fair" (moderate impaimnent) below Daisy 

 Creek and "Good" (minor impairment) above the Wilderness 

 boundajry. As in 2001, low diatom diversity was responsible for 

 the less than excellent ratings in 1994. 



