INTRODUCTION 



This report evaluates the biological integrity, support of 

 aquatic life uses, and probable causes of impairment to those 

 uses in Big Dry Creek and Little Dry Creek in Garfield County, 

 Montana. This evaluation is part of an assessment sponsored by 

 the Garfield County Conservation District with assistance from 

 the Natural Resources Conservation Service, U.S.D.A. 



The evaluations in this report are based on the structure 

 and species composition of the periphyton or phytobenthos 

 community. The periphyton community is a basic biological 

 component of all aquatic ecosystems. Periphyton accounts for 

 much of the primary production and biological diversity of 

 Montana streams (Bahls et al . 1992). 



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. Many 

 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. 



Some algae, such as the filamentous greens, are conspicuous 

 and their excessive growth may be aesthetically displeasing. 

 Algae may also deplete dissolved oxygen, interfere with fishing 

 and fish spawning, clog water filters and irrigation intakes, 

 create tastes and odors in drinking water, and generate toxins 

 that may be lethal to livestock and other animals. 



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

 several advantages for using periphyton in biological assessments 

 of streams : 



