^ Summary 



On July 29, 2003, periphyton samples were collected from six sites on Big Dry Creek 

 near Jordan, Montana for the purpose of assessing whether this stream is water-quality limited 

 and in need of TMDLs. The samples were collected following MDEQ standard operating 

 procedures, processed and analyzed using standard methods for periphyton, and evaluated 

 following modified USEPA rapid bioassessment protocols for wadeable streams. 



Diatom metrics indicate moderate impairment from sedimentation at the Mormon Church 

 site and the sewage plant site, and moderate impairment from organic loading at the sewage plant 

 site. The other four sites suffered minor impairment from organic loading and two of these sites 

 (The Trees and Van Norman) suffered minor impairment from sedimentation. 



All of the sites except The Trees were dominated by the filamentous green alga 

 Rhizoclonium. This free-floating and mat-forming alga has been reported to cause problems in 

 standing and slowly flowing waters in the western United States. 



The Trees and the Mormon Church sites were similar and different from the other sites in 

 that they supported a larger number of nitrogen-fixing algae, a larger percentage of nitrogen 

 heterotrophs, a larger number of cool-water algae, and a larger number of algae that tolerate 

 elevated levels of salinity. Nitrogen was probably the limiting nutrient at The Trees and the 

 Mormon Church. 



Algal communities at the Highway 200, Van Norman, and Near Mouth sites were similar 

 in composition. At these sites, diatom metrics and algal composition indicated fresher waters, 

 less organic loading, and less sedimentation than upstream. Most diatoms at all six sites indicate 

 eutrophic conditions. 



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