SUMMARY 



Composite periphyton samples were collected from natural 

 substrates at 2 sites on Mizpah Creek in northern Powder River 

 County in southeastern Montana. Samples were collected following 

 standard operating procedures of the Montana Department of 

 Environmental Quality, processed and analyzed using standard 

 methods for periphyton, and evaluated following modified USEPA 

 rapid bioassessment protocols for wadeable streams. 



Diatom metrics assigned both sites to the same use support 

 categories to which they were assigned in 1999. UMC-1 fully 

 supported aquatic life uses, with only minor impairment noted 

 from organic loading and a few abnormal diatom frustules. UMC-1 

 proved to be a suitable local reference site for use in Protocol 

 II. 



LMC-7 provided only partial support of aquatic life uses 

 when compared to UMC-1 and to other least -impaired prairie 

 streams. The major causes for less than full support at LMC-7 

 were organic loading, sedimentation, and a much different diatom 

 assemblage than the upstream control site (UMC-1) . Some of the 

 organic loading at LMC-7 may be internal and therefore natural in 

 origin. 



The algal assemblages at the two sites also indicated that 

 LMC-7 had larger concentrations of nitrogen and dissolved solids, 

 and higher water temperatures, than did UMC-1. 



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