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 Dearborn River 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 

 needof 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 fiilly 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 6 sites on 3 

 streams that were sampled in August and September of 2002. Periphyton is a diverse assortment 

 of simple photosynthetic organisms called algae that hve attached to or in close proximity of the 

 stream bottom. Some algae form long filaments or large gelatinous 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 genetic 

 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). 



