• 



Many of the sites supported teratological (deformed or physically abnormal) diatom cells. 

 In large numbers, abnormal cells may indicate metals toxicity. The percentage of abnormal cells 

 was within acceptable limits in all streams except Elkhom Creek and Lost Creek (Table 5). 



Similarity inde.x values indicate that ecological changes between adjacent sites on the 

 same stream varied from un-measurable (>60%) to minor (40-60%) to moderate (20-40%). The 

 similarity index ("percent community similarity") measures the cumulative percentage of cells of 

 each taxon that are shared by two stream sites. If diatom metrics indicate that the upstream site 

 has no impairment or only minor impairment, then the similarity index can be used to estimate 

 the degree of impairment that may or may not occur at downstream sites on the same stream. 



The diatom order Rhopalodiales includes genera {Epithemia and Rhopalodia) that are 

 known to harbor nitrogen-fixing endosymbionts within their cells. These symbiotic nitrogen- 

 fixers are single-celled cyanobacteria (blue-green algae). Nitrogen is likely the limiting nutrient 

 in waters that support large numbers of diatoms in the order Rhopalodiales. Among tributaries to 

 the Big Hole River, diatoms in the order Rhopalodiales accounted for more than 2% of the cells 

 at the following sites: FRANC02 (25.39%), DIVDOl (3.46%), DIVD02 (3.24%), MCVYCOl 

 (2.73%), and CAMPC03 (2.05%). 



The following paragraphs highlight the key findings for each stream and each site based 

 upon the major diatom species and core diatom metrics in Table 5. 



Doolittle Creek. Aside from a few teratological diatom cells, both sites on Doolittle 

 Creek had excellent biological integrity. However, values for both the pollution index and the 

 sedimentation index approached (but did not exceed) the threshold for minor impaimicnt at the 

 downstream site (Table 5). The two sites shared 56% of their diatom assemblages. This 

 indicates that only minor tloristic and ecological change occurred between them. The dominant 

 diatom species at the upstream site {Diatoma mesodon) indicates consistently cold water. 



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