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SUMMARY 



On September 19, 2000, periphyton samples were collected at 

 two stations on the Shields River near Wilsall, Montana for the 

 purpose of assessing whether this stream 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. 



Station 1 had a high siltation index indicating fair 

 biological integrity, moderate impairment, and partial support of 

 aquatic life uses. An abundance of the cyanobacterium 

 Oscillatoria, a borderline pollution index, and the absence of 

 diatoms in the family Epithemiaceae also indicate that Station 1 

 likely suffers from organic nitrogen loading. 



The siltation index declined and the pollution index rose 

 significantly between Station 1 and Station 2, indicating an 

 improvement in water quality. Although diatom species diversity 

 and species richness declined somewhat between these two sites, 

 the number of non-diatom genera almost doubled (from 7 to 13) . 

 An increase in the percentage of diatom cells in the family 

 Epithemiaceae (from to 6%) indicates that nitrogen was probably 

 limiting at Station 2. 



A relatively large number of abnormal diatom valves were 

 counted at both sites. The value at Station 1 approached the 

 threshold for moderate impairment and the value at Station 2 

 barely exceeded this threshold. The cause of these abnormalities 

 is unknown. 



The two sites had less than one third of their diatom 

 associations in common, indicating that a significant floristic 

 change (and water quality improvement?) occurred between them. 



