Summary 



In the summer of 2003, periphyton samples were collected from 42 sites on 22 streams in 

 the Big Hole River TMDL planning area in southwestern Montana for the purpose of assessing 

 whether these streams are water-quality limited and in need of TMDLs. The samples were 

 collected following MDEQ standard operating procedures, processed and analyzed following 

 standard methods for periphyton, and evaluated following modified USEPA rapid bioassessment 

 protocols for wadeable streams. 



Diatom metrics indicate minor impairment from both organic loading and sedimentation 

 at most of the sites that were sampled for periphyton. However, the pollution index at one site — 

 Swamp Creek site 01 — indicated moderate impairment from excessive organic nutrients and 

 only partial support of aquatic life uses. Siltation index values at Swamp Creek site 01 and 

 Lost Creek site 01 suggest severe impairment from sedimentation and non-support of aquatic 

 life uses. The siltation index value at Sawlog Creek site 01 indicates moderate impairment. 



Large percentages of teratological diatom cells (>10%) indicate elevated concentrations 

 of heavy metals and severe impairment of aquatic life uses at both sites on Elkhorn Creek. A 

 smaller percentage of abnormal cells (3-10%) suggest moderate impairment from heavy metals 

 at Lost Creek site 02. Several other sites supported still smaller percentages of abnormal diatom 

 cells (<3%). 



Diatoms were present in all of the samples. Most of the 27 major diatom species are 

 either sensitive to organic pollution or only somewhat tolerant of organic pollution. Only two of 

 the major diatom species are most tolerant of organic pollution. 



In general, diatom species richness, diversity, and equitability were excellent. Most sites 

 supported more than 50 species and diversity values greater than 4.00. Ninety species were 

 counted in one sample (LMCHC02) and another site (MCVYCOl) had a diversity index of 5.22. 

 These are exceptionally high values for mountain streams. Only one stream (Elkhorn Creek) had 

 diversity values that indicated unusual stress. At only two sites, both on Elkhorn Creek, did the 

 dominant species contribute more than half of the cells to the diatom assemblage. 



High diatom diversity in these streams infers the absence of natural stressors, such as 

 steep gradients, fast currents, low light, low nutrients, and/or constant cold temperatures. The 

 predominance of non-motile, free-living taxa relative to attached species implies that most sites 

 have gentle gradients and slow current velocities compared to other mountain streams. This is 

 confinned by the comparatively low disturbance index at most sites. 



Besides the absence of natural stressors, high diatom diversity in these streams also 

 suggests moderate nutrient enrichment (little competition for available nutrients) and complex 

 microhabitats that are more similar to prairie streams. Pollution index values, which indicate the 

 amount of organic loading, are generally low for mountain streams. Many are at or below the 

 threshold for minor impairment. Similarly, siltation index values tend to be higher in Big Hole 

 tributaries than in most mountain streams. 



