for the Tongue River indicated good to excellent water quality tf^ 

 and no impairment of biological integrity. 



Three of the major diatom species in the Tongue River- - 

 Cyclotella meneghiniana, Nitzschia frustulum, and Nitzschia 

 reversa- -are considered salt-tolerant species. These species 

 peaked in abundance at the two downstream stations (Table 5) . 

 This probably reflects a buildup in salts from tributaries and 

 from irrigation return flows. However, despite continued drought 

 and record low flows in the Tongue River during 2001, salinity 

 did not have an overt effect on the river's flora. 



Diatom species richness and species diversity values both 

 indicate good to excellent biological integrity and no impairment 

 of aquatic life uses (Table 5) . The lowest species richness and 

 diversity values were recorded at TR02 and TR03 . This decline in 

 species diversity probably results from a decrease in habitat 

 diveristy caused by the temperature- and flow-stabilizing effects ^^^ 

 of the Tongue River Dam and Reservoir. 



TR02 and TR03 also supported the largest populations of 

 Cocconies pediculus . Cocconeis pediculus is adapted to living 

 attached to filaments of Cladophora, and the abundance of this 

 diatom at TR02 and TR03 is directly related to the abundance of 

 Cladophora at these sites. This abundance of Cladophora results 

 from the flow- and temperature-stabilizing effects of the Tongue 

 River Dam and Reservoir upstream. Cocconeis pediculus accounted 

 for over half of the diatom cells counted at TR03 (Table 5) and 

 its dominance here indicated moderate impairment and partial 

 support of aquatic life uses due to habitat homogeneity. 



The disturbance index (percent abundance of Achnanthes 

 minutissima) was very low at all sites (Table 5) . This indicates 

 that physical scour, invertebrate grazing, and chemical toxicity 

 were not controlling factors for the periphyton community of the (B 



10 



