Summary 



In July 2003, periphyton samples were collected from 3 sites on Logan Creek in the 

 Flathead-Stillwater River TMDL planning area in northwestern Montana for the purpose of 

 assessing whether this stream is 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 indicated minor stress but full support of aquatic life uses at all 

 three sites on Logan Creek. An elevated percentage of motile diatoms indicated minor 

 sedimentation at the upper two stations: Logan Creek above Cyclone Creek and Logan Creek 

 above Star Meadows. .A large number oi Achnanthidiian minutissimuin at these sites also 

 indicated minor disturbance, which probably resulted from natural causes. These two sites 

 supported virtually identical diatom assemblages, which had excellent species richness and 

 diversity for a mountain stream. The non-diatom algae at these sites were a healthy mix of 

 cyanobacteria, green algae, and the chrysophyte Vaucheria (above Cyclone Creek only). 

 Nitrogen was probably the limiting nutrient above Star Meadows. 



The periphyton community at the Tally Lake Campground was significantly different 

 from the communities at the upstream sites. The Tally Lake sample was dominated by two 

 genera of nitrogen-fixing cyanobacteria {Rivularia and Nostoc), which indicate that nitrogen was 

 probably the limiting nutrient here. The rare and pollution-sensitive brown alga Herihaudiella 

 was abundant here and ranked third in biovolume. The diatom assemblage at Tally Lake 

 Campground was dominated by attached species that are sensitive to organic pollution. A large 

 percentage of Cocconeis placentiila indicated minor stress that was probably natural and related 

 to the prevailing substrate at this site. Although diatom metrics indicate that this site had good 

 biological integrity, modal categories for diatom ecological attributes indicate that the site had 

 significantly higher alkalinity, larger concentrations of inorganic nutrients (phosphorus), and 

 smaller concentrations of dissolved oxygen than the two upstream sites. 



