the good equitability of diatom cells among species indicated 

 only minor impairment at these sites . 



Slightly more than 1% of the diatom cells in the sample from 

 Reach Oil were abnormal in shape (Table 5) . Smaller percentages 

 of abnormal cells were counted in Reaches 008, 006, and 003. 

 Among the causes of teratological deformities in diatom cells are 

 toxins and salinity. Healthy diatom assemblages typically do not 

 have abnormal cells, except for natural monstrosities represented 

 by post-auxospore cells (Erstlingzelle) . In mountain streams, 

 heavy metals may produce deformities, particularly among the 

 Fragilariaceae (McFarland et al . 1997). The cause of the 

 abnormal cells in Box Elder Creek is unknown. 



Representatives of the diatom family Epithemiaceae were 

 present at all of the Box Elder* Creek sites (Table 5) . Diatoms 

 in this family harbor endosymbiotic nitrogen- fixing cyanobacteria 

 within their cells and are usually most abundant where nitrogen 

 is the limiting nutrient. The percentage of diatoms in this 

 family peaked in Reaches 003 and 001 at the downstream end of the 

 study section. 



The similarity index indicates floristic affinities between 

 adjacent reaches. It can be used to judge the degree of change 

 in water quality and other variables between sites. The most 

 dissimilar adjacent reaches were Oil and 012, followed by 012 and 

 022 (Table 5) . In fact, reaches 022 and 001 had about as much in 

 common, f loristically , as did reaches 022 and 012 . On the other 

 hand, reaches 008 and Oil were very similar, as were reaches 001 

 and 003, and 003 and 004 (Table 5). The duplicate samples from 

 Reach 003 approached 80% similarity, which is to be expected for 

 duplicate samples from the same site (Bahls 1993) . 



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