INTRODUCTION 



Near the first of March 1999, several hundred gallons of used 

 motor oil were released into Casino Creek, a tributary of Big 

 Spring Creek south of Lewistown, Montana. Several days later, 

 Anne Tews, an employee of the Montana Department of Fish, 

 Wildlife and Parks, conducted a field assessment of the spill. 



This report is based on 4 composite periphyton samples that 

 were collected by Tews during her field assessment. This report' 

 evaluates the effects of the oil spill on the species composition 

 and community structure of periphyton (benthic algae) communities 

 in Casino Creek and Big Spring Creek. Using biocriteria for 

 wadeable mountain streams in Montana, the report also assesses 

 the impacts of the spill on aquatic life uses in the two streams. 



The periphyton or phytobenthos community is a basic 

 biological component of all aquatic ecosystems. Periphyton 

 accounts for much of the primary production and biological 

 diversity of streams in western Montana (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, 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 



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