algae, red algae, and cyanobacteria (Table 4) . The number of 

 non-diatom genera ranged from a low of 6 above Tongue River 

 Reservoir Co a high of 13 below the reservoir. This compares to 

 an average of 13 nondiatom genera (range 9-19) recorded from 

 prairie reference streams in Montana (Bahls 1993) . 



Diatoms dominated the periphyton sample collected above the 

 Tongue River Reservoir and ranked first in biomass at the three 

 lowest sites on the river (Table 4) . The filamentous green alga 

 Cladopbora ranked first in biomass at the two sites immediately 

 downstream from Tongue River Reservoir, where diatoms were 

 abundant and ranked second. Dominance by Cladophora at these two 

 sites is probably due to the flow- and temperature-stabilizing 

 effects of Tongue River Dam and Reservoir. 



Audouinella, a red alga and an indicator of clean water, was 

 present at all sites except at the mouth of the river (TR06) . 

 Euglena, an indicator of organic loading, was present at TR04 and 

 TR06, but was not recorded at other sites. 



Several indicator taxa were present among the cyanobacteria. 

 Anabaena, a nitrogen-fixing and potentially toxic genus, was 

 present at all sites. Merismopedia, a brackish water genus, was 

 recorded only at TR04, where it was rare. Nostoc, an indicator 

 of cool, nitrogen-poor waters, was found only at TR02 below 

 Tongue River Dam. The blue-green genus Phormidium was common at 

 all sites, where it grew mostly as an epiphyte. 



DIATOMS 



Major diatom species in Tongue River periphyton samples 

 (Table 5) are all sensitive to pollution (Class 3) or only 

 somewhat tolerant of pollution (Class 2) . The pollution index is 

 mainly an indicator of organic loading. Pollution index values 



