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Summary 



On July 29, 2003, periphyton samples were collected from three sites on Little 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. 



AH three sites exhibited minor impairment but full support of aquatic life uses. The 



dominant alga at all three sites was Rhizoclonium, a free-living and mat forming filamentous 

 green alga that often causes problems in standing waters in the western United States. Diatoms 

 and other genera of filamentous green algae were also common at all three sites. 



Diatom metrics indicate minor impairment from organic loading and sedimentation at the 

 Cohagen and Uall Creek sites, but otherwise excellent biological integrity for a prairie stream. 

 The Cohagen site was the only site that supported free-living nitrogen-fixing cyanobacteria 

 (Tolypothrix) and this site also had the largest percentage of nitrogen-fixing diatoms. The 

 abundance of nitrogen-fixing algae indicates that biologically available inorganic nitrogen was 

 probably in short supply (limiting) relative to phosphorus. The diatom assemblage at Cohagen 

 consisted mostly of facultative nitrogen heterotrophs that can use organic matter as a source of 

 nitrogen. The Cohagen and Uall Creek sites shared about 60% of their diatom assemblages, 

 indicating that they were quite similar both floristically and ecologically. 



Diatom metrics and ecological attributes at the Van Norman site indicate significantly 

 improved water quality compared to the two upstream sites. The pollution index here was just 

 below the threshold for minor impairment and the sedimentation index was well below the 

 threshold for minor impairment. The predominance of pollution sensitive species and other 

 indicators show that this site had significantly lower concentrations of organic matter and higher 

 concentradons of dissolved oxygen than the Cohagen and Uall Creek sites. The Van Norman 

 site supported the smallest percentage of nitrogen-fixing diatoms, indicating that biologically 

 available inorganic nitrogen was probably not in short supply relative to phosphorus. This site 

 shared only about 30% of its diatom assemblage with the two upstream sites, indicating that it 

 was quite dissimilar from these two sites both floristically and ecologically. 



