Introduction 



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

 causes of stress or impairment to aquatic communities in streams of the Cut Bank — Two 

 Medicine TMDL planning area in north central 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 flilly 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 19 sites on 10 streams 

 that were sampled in August and September of 2002. 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 colonies and 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 functional organization 

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



