INTRODUCTION 



This report evaluates the support of aquatic life uses, and 

 probable causes of impairment to those uses, in the Teton River 

 of northcentral Montana, and in a tributary, McDonald Creek. 

 This evaluation is based on the species composition and community 

 structure of periphyton (benthic algae) communities at two sites 

 on McDonald Creek and nine sites on the Teton River that were 

 sampled in July and August 1998. 



The periphyton or phytobenthos is a diverse assortment of 

 simple photosynthetic organisms, called algae, that live attached 

 to or in close proximity of the stream bottom. Most algae, such 

 as the diatoms, are microscopic. Although not visible to the 

 naked eye, diatoms often carpet a stream bottom with a slippery 

 brown film. Some algae, such as the filamentous greens, are 

 conspicuous and their luxuriant growth in response to nutrient 

 enrichment may deplete dissolved oxygen, interfere with fish 

 spawing, clog irrigation intakes, and cause other problems. 

 Collectively, the phytobenthos accounts for practically all of 

 the primary production and much of the biological diversity in 

 the mountain streams of Montana (Bahls et al . 1992). 



Stevenson and Bahls (1999) list several advantages for using 

 periphyton in biological assessments of streams: 



• Algae are universally present in large numbers in all 

 streams and unimpaired periphyton assemblages typically 

 support a large number (>30) of species; 



• Algae have rapid reproduction rates and short life cycles, 

 making them useful indicators of short-term impacts,- 



• As primary producers, algae are most directly affected by 

 physical and chemical factors, such as temperature, 



