addition to tolerance of organic loading, B. circumtexta also tolerates elevated salinity levels. As 

 a nitrogen heterotroph, Nitzschia palea can use preformed organic matter as a source of nitrogen. 

 The sewage plant site did not support any nitrogen-fixing diatoms and shared only about one- 

 third of its diatom association with the Mormon Church site just upstream. 



Highway 200. This site had much improved biological integrity and water quality 

 compared to the sewage plant site and suffered only minor impairment from organic loading, 

 although it did support a large percentage of the pollution-tolerant species Gomphonema 

 parvulum. The Highway 200 site shared less than 30 percent of its diatom assemblage with the 

 upstream site at the sewage plant (Table 4). The Highway 200 site shared 37% and 33% of its 

 diatom assemblage with The Trees and the Mormon Church sites, respectively. 



Van Norman. This site was very similar to the Highway 200 site both floristically and 

 ecologically (Table 4). Diatom metrics indicate only minor impairment from organic loading 

 and sedimentation. The sedimentation index here was just above the threshold (50.00) for minor 

 impairment. The Van Norman site supported the largest percentage of nitrogen-fixing diatoms 

 below the sewage plant. 



Near Mouth. This site was similar to the next two sites upstream and registered only 

 minor impairment from organic loading. Diploneis pnella was the dominant diatom species at 

 all three sites. Diploneis puella is a motile, alkaliphilous, mesotraphentic, nitrogen autotroph 

 that requires high levels of dissolved oxygen and tolerates only small amounts of organic matter 

 (Van Dam et al. 1994). Nitzschia reversa ranked second in abundance near the mouth. This is a 

 tychoplanktonic diatom that produces large populations in quiet waters (pools). This site had the 

 largest diatom species richness and diversity values of all the sites sampled on Big Dry Creek 

 (Table 4). 



Modal Categories (Table 5) 



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