Introduction 



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

 causes of stress or impairment to aquatic communities in the Shields River near Livingston, 

 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. 

 Previous periphyton reports have examined biological integrity in the upper reaches of the 

 Shields River (Bahls 2001b) and in tributaries to the Shields River (Bahls 2000, 2001a). 



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 stnicture of periphyton (benthic algae, phytobenthos) communities at sites that were sampled 

 in 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 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, di\ersity, and functional organization 

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



