SUMMARY 



On August 1, 2000, periphyton samples were collected from 

 two stations on Spring Creek above Choteau, Montana for the 

 purpose of assessing whether the creek is water-quality limited 

 and in need of TMDLs . The samples were collected following DEQ 

 standard operating procedures, processed and analyzed using 

 standard methods for periphyton, and evaluated following modified 

 USEPA rapid bioassessment protocols for wadeable streams. 



The Spring Creek watershed straddles the boundary between 

 the Northwestern Glaciated Plains and the Montana Valley and 

 Foothill Prairies Ecoregions and is classified B-2. For these 

 reasons, Spring Creek metrics were compared to criteria for both 

 mountain streams and prairie streams. Also, Spring Creek has 

 every indication of being a spring brook of the Teton River, with 

 which it is closely associated hydrologically . 



Spring Creek supported a rich non-diatom flora composed of 

 17 genera. Dominance by diatoms and green algae and low numbers 

 of cyanobacteria indicate an ample supply of nutrients, including 

 nitrogen. A decline in the number of non-diatom genera at the 

 downstream station (S-3) may be due to competition with aquatic 

 mosses at this site. 



The large number of Achnanthes minutissima at S-3 resulted 

 in a rating of moderate impairment and partial support of aquatic 

 life uses. The reasons for this rating were borderline values 

 for percent dominant species and the disturbance index. The 

 disturbance at S-3 is probably natural in origin and caused by 

 grazing by macroinvertebrates (e.g., snails). 



All other diatom metrics indicated good to excellent 

 biological integrity, little or no impairment, and full support 

 of aquatic life uses at both sites. 



