Introduction 



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

 causes of stress or impairment to aquatic communities in Logan Creek in the Flathead-Stillwater 

 River TMDL Planning Area of northwestern Montana. The purpose of this report is to provide 

 information that will help the State of Montana determine whether Logan Creek 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 aquatic life use support in this report is based on the species composition 

 and structure of periphyton (aka benthic algae, phytobenthos) communities at three sites on 

 Logan Creek that were sampled in early July of 2003. Periphyton is a di\erse 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 periph>l;on 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 communiry of organisms having a species composition, diversit)', and functional organization 

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



