SUMMARY 



In August 2000, composite periphyton samples were collected 

 from East Rosebud Creek near Roscoe, 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. 



The very small biovolume of algae at Station 1 indicates 

 very cold and nutrient -poor water entering the study reach. An 

 increase in biovolume and an occasional pollution tolerant alga 

 at Station 2 indicate that a slight increase in nutrients and 

 water temperature occurred between Station 1 and Station 2. 



Dominance of the diatom assemblage by Achnanthes minutissima 

 indicates moderate stress at Station 1 and minor stress at 

 Station 2. This stress is likely natural in origin and caused by 

 the swift, cold, and nutrient-poor waters entering the study 

 reach from the mountains upstream. 



The decline in abundance of Achnanthes minutissima from 

 Station 1 to Station 2 was accompanied by an increase in diatom 

 diversity and diatom species richness, and by a small decrease in 

 the pollution index. Such changes also reflect a slight increase 

 in nutrients and water temperature between the two sites. 



The siltation index also increased between the two sites, 

 but values at both stations were within acceptable limits for a 

 mountain stream. A few abnormal diatom valves were observed at 

 Station 1, but numbers of deformed valves were within acceptable 

 limits. The two sites had almost two-thirds of their diatom 

 associations in common, indicating that only a small change in 

 water quality occurred between Station 1 and Station 2. 



