Summary 



In early September 2002, 9 periphyton samples were collected from 4 tributaries of the 

 Swan River in northwest 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. 



Goat Creek, Elk Creek, Jim Creek and Piper Creek all supported a mix of cyanobacteria, 

 green algae, and diatoms. In addition, all streams but Jim Creek supported the red alga 

 Audouinella and only Jim Creek supported the xanthophyte Trihonema. The non-diatom algae 

 of these streams indicated cool water of good quality. 



Seven of the major diatom species in tributaries of the Swan River are sensitive to 

 organic pollution and four are somewhat tolerant of organic pollution. The tolerant species 

 tended to be more abundant at the downstream stations and in Jim Creek. Jim Creek also had the 

 lowest pollution index value and the highest siUation index value of any site, indicating that this 

 stream supported larger sediment and organic nutrient loads than the other sites. Nevertheless, 

 values for these metrics were still within the range of excellent biological integrity for a 

 mountain stream. 



The dominant diatom species at all of the sites was either Achnanthes biasolettiana or 

 Achnanthidium minutissimum. Both are small, attached (non-motile) species that are adapted to 

 living in fast currents. Their abundance here is a consequence of the steep gradient and fast 

 current velocities of these streams. Both species prefer cool water temperatures and low nutrient 

 concentrations. 



Although diatom community metrics at several sites indicated moderate to severe stress, 

 these stresses appear to be natural in origin and related to the high gradients, cold temperatures, 

 and low nutrient concentrations of these streams. High values for the pollution index and low 

 values for the siltation index and percent abnormal cells indicate that organic enrichment, 

 sedimentation, and toxic metals had little or no effect on the benthic algae of these streams. 



Several ecological attributes of the diatoms were examined to characterize water quality 

 tendencies at the sampling sites. The four tributaries and stations within each tributary are fairly 

 uniform in their general water quality characteristics, which may be described as circumneutral, 

 fresh-brackish (TDS <900 mg/L), and mesotrophic, with continuously high concentrations of 

 dissolved oxygen. Diatoms living in these streams are mostly attached (non-motile), autotrophic 

 with respect to nitrogen, tolerate a wide range of nutrient concentrations, and live mainly in 

 water bodies, but regularly in wet places. They indicate some organic loading, which may be 

 natural, but zones where the oxidation of biodegradable organic matter is mostly complete. 



