all common and widespread algae that may become a nuisance in 

 waters that are enriched with nutrients. Spirogyra, Oedogonium, 

 and Mougeotia are particularly abundant in warmer standing or 

 slow-moving waters. 



The abundance of green algae and diatoms relative to 

 cyanobacteria probably indicates moderate nutrient enrichment at 

 both sites on Billman Creek. Generally, cyanobacteria (blue- 

 green algae) indicate lower nutrient levels in mountain streams 

 in Montana (Bahls et al . 1992K They cannot compete with diatoms 

 and green algae under moderate to heavy nutrient loading. The 

 presence of an euglenoid alga {Phacus) at Station 2 suggests some 

 organic loading at this site. 



DIATOMS 



Together with Surirella minuta, four species of Navicula 

 dominated the diatom assemblage at Station 1 (Table 6) . Most of 

 these species are somewhat tolerant of organic loading and 

 nutrient enrichment, and they are all motile species adapted to 

 living in aggrading or depositional habitats. 



Most diatom metrics at Station 1 indicate good or excellent 

 biological integrity (Table 6) . However, the unusually high 

 percentage of motile taxa resulted in a siltation index that 

 indicates severe impairment for a mountain stream. When compared 

 to siltation criteria for plains streams, the siltation index 

 still indicates moderate impairment and partial support of 

 aquatic life uses (Table 6) . This comparison (with prairie 

 stream criteria) is probably valid given the unique nature of 

 Billman Creek (Patrick Newby, MDEQ, personal communication) . 



Although the percentage of motile taxa at Station 2 was 



smaller (Table 6) , it nevertheless indicates moderate impairment 

 for a mountain stream and minor impairment for a plains stream. 



8 



