SUMMARY 



In July and August 1999, composite periphyton samples were 

 collected from natural substrates in Careless Creek, Lodgepole 

 Creek, and the South Fork of Lodgepole Creek in the middle and 

 lower Musselshell River drainage of central Montana. Samples 

 were collected following MDEQ standard operating procedures, 

 processed and analyzed using standard methods for periphyton, and 

 evaluated following modified USEPA rapid bioassessment protocols 

 for wadeable streams. 



Diatom metrics indicated that Careless Creek fully supports 

 its aquatic life uses when compared to biocriteria for prairie 

 streams. A case may be made, however, based on algal floristics, 

 that lower Careless Creek is transitional between cold and warm 

 water aquatic life, and would b'e more appropriately classified as 

 B-2 rather than C-3. If such were the case, diatom metrics for 

 Careless Creek would be compared to biocriteria for mountain 

 streams and this comparison would indicate moderate impairment 

 from siltation and only partial support of designated uses. 



Diatom metrics at the site near the mouth of Lodgepole Creek 

 indicated moderate impairment and only partial support of aquatic 

 life uses due to siltation, organic loading, and nutrient 

 enrichment. Nitrogen is the likely nutrient of concern in 

 Lodgepole Creek. 



The South Fork of Lodgepole Creek had low diatom diversity 

 and species richness for a prairie stream, resulting in a rating 

 of moderate impairment and partial support of aquatic life uses. 

 The low diversity was due mainly to the unusually large numbers 

 of Achnanthes rninutissima at this site. The abundance of this 

 species here and the presence of other cool -water and pollution 

 sensitive algal taxa, indicate that the stress operating in the 

 South Fork may be natural rather than cultural in origin. 



