An elevated number of Achnanthidium minutissimum indicated minor stress at the two 



ft 

 downstream sites on the Nez Perce Fork. This stress is probably natural in origin and caused by w 



steep gradients, fast current velocities, cold temperatures, and low nutrient concentrations. 



Otherwise, diatom metrics indicated excellent biological integrity and full support of aquatic life 



uses at both of the downstream sites. 



The upstream and middle sites on Nez Perce Fork shared less than 20 percent of their 

 diatom assemblages, indicating that major floristic and environmental changes occurred between 

 the two sites. On the other hand, the two downstream sites were very similar, floristically, 

 indicating virtually no change occurred between them. 



Modal Categories of Ecological Attributes (Table 5) 



Several ecological attributes assigned by Stevenson and Van Dam et al. (1994) were 

 selected from the diatom reports in the appendix and modal categories of these attributes were 

 extracted to characterize water quality tendencies in streams of the Bitterroot River TMDL m* 



planning area (Table 5). With few exceptions, most of the diatoms that inhabit these streams 

 may be characterized as non-motile, alkaliphilous, and autotrophic, while tolerating high levels 

 of organic nitrogen. They prefer eutrophic, fresh-brackish waters, and exert a moderate demand 

 for dissolved oxygen. These categories are defined by Van Dam et al. (1994) 



A few sites exhibit significant departures from these trends. For example, most diatoms 

 in Ditch Creek and at the middle and lower sites on Nez Perce Fork prefer circumneutral (rather 

 than alkaline) waters and exert a continuously high (rather than moderate) demand for dissolved 

 oxygen. The modal category for saprobity at the upper site on Overwhich Creek is alpha- 

 mesosaprobous/polysaprobous, which indicates a higher level of organic loading than at most of 

 the other sites in the study area. At the lower site on Overwhich Creek, the modal category for 

 dissolved oxygen is "fairly high" and the modal category for trophic state is meso-eutraphentic, 

 which indicate better oxygenated waters and lower nutrient concentrations than at most of the 

 other sites. The modal category for dissolved oxygen in Buck Creek is also "fairly high", in 

 contrast to "moderate" at most of the other sites. ^f 



13 



