INTRODUCTION 



This report evaluates the biological integrity, support of 

 aquatic life uses, and probable causes of impairment to those 

 uses, in the Shields River near Wilsall, Montana. The purpose of 

 this report is to provide information that will help the State of 

 Montana determine whether the Shields River is water-quality 

 limited and in need of TMDLs . 



The federal Clean Water Act directs states to develop water 

 pollution control plans (Total Maximum Daily Loads or TMDLs) that 

 set limits on pollution loading to water-quality limited waters. 

 Water-quality limited waters are lakes and stream segments that 

 do not meet water-quality standards, that is, that do not fully 

 support their beneficial uses. The Clean Water Act and USEPA 

 regulations require each state to (1) identify waters that are 

 water-quality limited, (2) prioritize and target waters for 

 TMDLs, and (3) develop TMDL plans to attain and maintain water- 

 quality standards for all water-quality limited waters. 



Evaluation of use support in this report is based on the 

 species composition and structure of the periphyton (benthic 

 algae, phytobenthos) community at two sites that were sampled on 

 September 19, 2000. The periphyton community is a basic 

 biological component of all aquatic ecosystems. Periphyton 

 accounts for much of the primary production and biological 

 diversity in Montana streams (Bahls et al . 1992) . 



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

 several advantages of using periphyton in biological assessments: 



• 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; 



