SUMMARY 



In early August 2000, periphyton samples were collected from 

 one station each on Antelope Creek and Potter Creek north of 

 Livingston, Montana for the purpose of assessing whether these 

 streams are 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. 



Both streams flow through the semi-arid sagebrush steppe of 

 the Shields River Valley, and Potter Creek does so for its entire 

 length. In addition. Potter Creek heads at a relatively low 

 elevation compared to other streams in the area. For these 

 reasons, diatom metrics were compared to biocriteria for both 

 mountain streams and prairie streams. 



An unusually large percentage of motile diatoms indicated 

 that Potter Creek was moderately impaired by sediment when judged 

 against criteria for mountain streams. However, when compared to 

 sedimentation criteria for prairie streams. Potter Creek was 

 assessed as borderline but unimpaired. Potter Creek also had a 

 larger than normal percentage of teratological cells which may 

 have been caused by toxic ammonia generated by internal organic 

 loading. 



Diatom metrics indicated no impairment or only minor 

 impairment in Antelope Creek. Both streams showed signs of 

 nutrient, especially nitrogen, enrichment. These included large 

 numbers of Nitzschia palea and small percentages of diatoms in 

 the family Epithemiaceae, both indicators that nitrogen was not 

 limiting to algal growth in either stream. The two streams had 

 about half of their diatom assemblages in common. 



