INTRODUCTION 



This report evaluates the support of aquatic life uses, and 

 probable causes of impairment to those uses, in Big Spring Creek 

 near Lewistown, in central Montana. This evaluation is based on 

 the species composition and cummunity structure of periphyton 

 (benthic algae) communities at five sites on Big Spring Creek 

 that were sampled in 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 individual diatoms are 

 not visible to the naked eye, they 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 spawning, 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, 

 nutrients, and toxins; 



