Introduction 



This report evaluates the biological integrity', support of aquatic life uses, and probable 

 causes of stress or impairment to aquatic communities in the Yaak River and the West Fork of 

 the Yaak River in northwestern Montana. The purpose of this report is to provide information 

 that will help the State of Montana determine whether these streams are 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 aquatic life use support in this report is based on the species composition 

 and structure of periphyton (benthic algae, phytobenthos) communities at four sites that were 

 sampled in August of 2003. Periphyton is a diverse assortment of simple photosynthetic 

 organisms called algae that live attached to or in close proximity of the stream bottom. Some 

 algae form long filaments or large gelatinous colonies that are conspicuous to the unaided eye. 

 But most algae, including the ubiquitous diatoms, can be seen and identified only with the aid of 

 a microscope. 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 Barbour et al. (1999) list several 

 advantages of using periphyton in biological assessments. 



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 ftmctional organization 

 comparable to that of natural habitats withm a region" (Karr and Dudley 1981). 



