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) . 



Periphyton is a diverse assortment of simple photosynthetic 

 organisms, called algae, and other microorganisms that live 

 attached to or in close proximity of the stream bottom. Most 

 algae, such as the diatoms, are microscopic. Diatoms are 

 distinguished by having a cell wall composed of opaline glass: 

 hydrated amorphous silica. Diatoms often carpet a stream bottom 

 with a slippery brown film. 



^ 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) . 



