Montana streams (Bahls et al . 1992). 



Plafkin et al . (1989) and Stevenson and Bahls (1999) list 

 several advantages of 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, dissolved salts, and toxins; 



• Sampling is quick, easy and inexpensive, and causes minimal 

 damage to resident biota and their habitat; 



• Standard methods and criteria exist for evaluating the 

 composition, structure, and biomass of algal associations; 



• Identification to species is straightforward for the 

 diatoms, for which there is a large body of taxonomic and 

 ecological literature; 



• Excessive algae growth in streams is often correctly 

 perceived as a problem by the public. 



• Periphyton and other biological communities reflect the 

 biological integrity^ of waterbodies; restoring and 

 maintaining the biological integrity of waterbodies is a 

 goal of the federal Clean Water Act; 



• Periphyton and other biological communities integrate the 

 effects of different stressors and provide a measure of 

 their aggregate impact; and 



• Periphyton and other biological communities may be the only 

 practical means of evaluating impacts from non-point sources 

 of pollution where specific ambient criteria do not exist 

 (e.g., impacts that degrade habitat or increase nutrients) . 



^ Biological integrity is defined as "the ability of an 

 aquatic ecosystem to support and maintain a balanced, integrated, 

 adaptive community of organisms having a species composition, 

 diversity, and functional organization comparable to that of 

 natural habitats within a region" (Karr and Dudley 1981) . 



