G. olivaceoides was also abundant in upwelling areas of the 

 Nyack Floodplain in the Middle Fork of the Flathead River (Bahls, 

 unpublished data) . Groundwater upwelling from the hyporheic zone 

 of a river is typically richer in nutrients than ambient stream 

 water, and may significantly influence both the composition and 

 the production of the benthic community (Stanford and Ward 1993) . 



When a species accounts for more than half of the diatom 

 cells in a community it raises a red flag. Dominance by any 

 species is usually in indicator of stress, either natural or 

 cultural, or of conditions that are especially suitable for the 

 reproduction of that species. In the case of G. olivaceoides , 

 there may be an upwelling of cold, nutrient-rich water that 

 favors large numbers of this diatom in August at the Two Mile 

 Bridge site. Algal nutrients derived from limestone outcrops 

 observed in this area (Christian Levine, MDEQ, pers . comm. ) may 

 also contribute to the large population of this taxon, as well as 

 to the excessive growths of filamentous algae that have been 

 noted in this reach. 



The August collection at the Natural Bridge site indicated 

 excellent biological integrity with no impairment of aquatic life 

 uses. In June, depressed diatom diversity and dominance by 

 Gomphonema olivaceoides indicated full support of beneficial uses 

 with minor impairment. Again, this "impairment" was probably 

 natural in origin and related to the peculiar hydrologic and 

 geologic conditions that pertain in this reach of the Boulder 

 River, namely the upwelling of cold, nutrient-rich water in an 

 area of limestone outcrops. 



Similarity index values were calculated for the two dates on 

 which periphyton samples were collected at the upstream reference 

 site (Table 4) . Diatom floras at adjacent sample sites were more 

 similar in June, when streamflow was presumably larger, than they 

 were in August, when flows were smaller and species composition 



