SUMMARY 



On July 13, 2000, a composite periphyton sample was 

 collected from natural substrates at the mouth of Bear Creek in 

 the upper Yellowstone River drainage of southcentral Montana for 

 the purpose of assessing whether Bear 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 of Bear Creek was dominated by diatoms, the 

 chrysophyte Hydrurus foetidus, and by the filamentous green alga 

 Ulothrix. Cyanobacteria (blue-green algae) were of secondary- 

 importance in Bear Creek, indicating that the stream might be 

 receiving some cultural inputs of nutrients. Typically, green 

 and blue-green algae share dominance in streams of the Middle 

 Rockies Ecoregion and dominance by diatoms and green algae may 

 indicate higher than normal levels of nutrient enrichment. 



The diatom flora of Bear Creek was dominated by Hannaea 

 arcus, the unofficial State Diatom of Montana. This species 

 prefers cold, flowing waters and is reported to be indifferent to 

 small amounts of organic pollution. Major diatom taxa in Bear 

 Creek also included one other "sensitive" species and two species 

 that are somewhat tolerant of pollution. 



Diatom association metrics indicated good to excellent water 

 quality, little or no impairment, and full support of aquatic 

 life uses, except for a borderline percent dominant species value 

 (50.85%), which indicated moderate impairment. This dominance by 

 Hannaea arcus also resulted in a depressed species diversity 

 index that indicated minor impairment. A small percentage 

 (0.24%) of abnormal diatom cells (again, Hannaea arcus) may 

 indicate chronic toxicity from heavy metals in Bear Creek. 



