SUMMARY 



On July 14, 2000, a composite periphyton sample was 

 collected from natural substrates in Mol Heron Creek in the upper 

 Yellowstone River drainage of southcentral Montana for the 

 purpose of assessing whether Mol Heron Creek is water-quality 

 limited and in need of TMDLs . The sample was 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 periphyton community of Mol Heron Creek contained the 

 filamentous green alga Cladophora, an indicator of nutrient 

 enrichment. Several other factors also indicate some nutrient 

 enrichment and organic loading in Mol Heron Creek: (1) an 

 abundance of diatoms and green algae relative to cyanobacteria; 

 (2) the pollution tolerant species Cymbella silesiaca as one of 

 the co-dominant diatoms; (3) a relatively low pollution index; 

 and (4) the absence of diatoms in the family Epithemiaceae . 



Nevertheless, all but one of the diatom association metrics 

 for Mol Heron Creek indicated excellent biological integrity, no 

 impairment, and full support of aquatic life uses. The exception 

 was a small percentge (0.97%) of teratological cells, mostly of 

 Diatoma vulgare. However, this species is prone to developing 

 abnormal cells for no apparent reason. 



In summary, Mol Heron Creek above the sampling site is 

 judged to be fully supporting of aquatic life uses and not in 

 need of TMDLs. 



