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. 



Some algae, such as the filamentous greens, are conspicuous 

 and their excessive growth may be aesthetically displeasing, 

 deplete dissolved oxygen.^ interfere with fishing and fish 

 spawning, clog irrigation intakes, create tastes and odors in 

 drinking water, and cause other problems. 



PROJECT AREA AND SAMPLING SITES 



The project area is located in Bighorn, Rosebud, and Custer 

 Counties in southeastern Montana. The Tongue River heads in the 

 Bighorn Mountains of northcentral Wyoming and flows northeasterly 

 for about 150 miles to where it enters the Yellowstone River at 

 Miles City, Montana. The project area encompasses the reach of 

 river from the Wyoming line to the mouth of the river at Miles 

 City . 



The project area is within the Northwestern Great Plains 

 Ecoregion (Woods et al . 1999) . The surface geology of the 

 watershed consists of Paleocene coal-bearing deposits of the Fort 

 Union Formation and Eocene deposits of the Wasatch Formation 

 (Renfro and Feray 1972) . Vegetation is mixed grassland (USDA 

 1976) . The main land use is livestock grazing, with some 

 irrigated crop production along the river. 



Periphyton samples were collected at 6 sites on the Tongue 

 River on August 17-18, 2001 (Table 1, Maps 1-6). The uppermost 



