Summary 



In October 2002, periphyton samples were collected from 14 sites on 8 streams in the Bitterroot 

 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 using standard methods for periphyton, and evaluated 

 following modified USEPA rapid bioassessment protocols for wadeable streams. 



Diatom metrics indicated moderate impairment from sedimentation and partial support of 

 aquatic life uses at the lower sites on Riemel Creek and Overwhich Creek. Minor impairment from 

 sedimentation was indicated at several sites: Meadow Creek (both sites), Martin Creek (both sites), the 

 upper site on Overwhich Creek, and the upper site on the Nez Perce Fork. The sedimentation index 

 approached but did not exceed the threshold for minor impairment in upper Reimel Creek, Deer Creek, 

 and Buck Creek. 



Species of Planothidium were abundant in samples from some of these streams. Diatoms in the 

 genus Planothidium are adapted to living attached to grains of sand. Sandy substrates of decomposed 

 granite are probably common in streams that drain the Idaho Batholith, a region of largely granitic rocks. 



The pollution index indicated minor impairment from organic loading at four sites: upper 

 Meadow Creek, Reimel Creek (both sites), and upper Overwhich Creek. The pollution index approached 

 but did not exceed the threshold for minor impairment in lower Meadow Creek and upper Nez Perce 

 Fork. The upper site on Overwhich Creek had the highest saprobity rating: alpha-mesosaprobous/poly- 

 saprobous, which indicates an elevated level of organic loading. 



An elevated percentage of teratological diatoms indicated possible chronic toxicity from heavy 

 metals in Martin Creek (both sites) and Deer Creek. Deer Creek supported the largest percentage of 

 teratological cells (1.33%). 



Ditch Creek and the middle and lower sites on the Nez Perce Fork supported large numbers of the 

 diatom Achnanthidium minutissimum, an indicator of disturbance. Large numbers of/4, minutissimum 

 often indicate natural stresses caused by steep gradients, fast current velocities, cold temperatures, 

 and low nutrient concentrations. Diatoms in Ditch Creek and at the lower two sites on the Nez 

 Perce Fork also demand higher levels of dissolved oxygen than diatoms at other sites, indicating 

 that these sites are better aerated than the others. 



The majority of diatoms in streams of the study area are non-motile, alkahphilous, eutraphentic 

 autotrophs that exert a moderate demand for dissolved oxygen. Most diatoms are in the saprobity class 

 beta-mesosaprobous and indicate fresh-brackish waters. Exceptions include Ditch Creek and the lower 

 sites on the Nez Perce Fork, where most diatoms prefer circumneutral pll and continuously high levels of 

 dissolved oxygen; upper Overwhich Creek, where most diatoms are in saprobity class alpha-mesosap- 

 robous/polysaprobous; lower Overwhich Creek, where oxygen demand is fairly high and trophic status is 

 meso-eutraphentic; and Buck Creek, where oxygen demand is fairly high. 



