Spirogyra ("pond scum") was the dominant alga at the lower site on Nez Perce Fork, 

 perhaps indicating slower current velocities, warmer water temperatures and higher nutrient 

 concentrations than the upper sites. Zygnema, another filamentous green, ranked second at the 

 lower site and diatoms ranked third. Zygnema is widespread and prefers neutral to slightly acidic 

 waters. Audouinella was common and ranked fourth at the lower site. 



Diatoms (Table 4) 



All but one of the major diatom species from the Bitterroot River TMDL planning area 

 are included in pollution tolerance classes 3 and 2, and are either sensitive to organic pollution or 

 only somewhat tolerant of organic pollution (Table 4). The one exception was Gomphonema 

 parvulum, which is most tolerant of organic pollution (pollution tolerance class = 1). 

 Gomphonema parvulum was a major species (>5% relative abundance) only in Ditch Creek, 

 which demonstrated good biological integrity except for minor natural stresses (see below). 



Stresses indicated at some of the sites appear to be natural in origin. For example, high 

 values for the disturbance index and percent dominant species in Ditch Creek and the middle and 

 lower sites on the Nez Perce Fork (Table 4) indicate minor stress related to steep gradients, fast 

 currents, cold temperatures, and low nutrient concentrations. High values for the pollution index 

 and low values for the siltation index and percent abnormal cells indicate that organic 

 enrichment, sedimentation, and toxic metals did not have a significant effect on the benthic algae 

 at these sites. These three sites also had the highest ratings for dissolved oxygen (continuously 

 high), indicating that they had the steepest gradients and highest saturation of dissolved oxygen 

 (Table 5). 



Meadow Creek. Both sites on Meadow Creek supported an elevated percentage of 

 motile diatoms (Navicula and Nitzschia), which indicated minor impairment from sedimentation 

 (Table 4). The siltation index was larger at the upstream site than at the downstream site. The 

 dominant diatom species at the upstream site was Planothidium lanceolatum, a specialized 

 species adapted to living attached to sand grains. This species was also common at the 

 downstream site. The pollution index indicated minor impairment from organic loading at the 



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