Of the 16 major diatom species in Big Dry Creek, only 1 is sensitive to organic pollution 

 (class 3) and 3 are most tolerant of organic pollution (class 1). The remaining 12 major species 

 are somewhat tolerant of organic pollution and these (class 2) species were abundant at all sites. 



The Trees. Diatom metrics indicate that this site suffered from minor impairment due to 

 organic loading and sedimentation (Table 4). The dominant diatom species here — Nitzschia 

 inconspicua — is a eutraphentic species that is somewhat tolerant of organic enrichment. The 

 Trees site supported the smallest species richness and species diversity values of all the sites, 

 which may result from competition for available nutrients (see below). 



This site supported the largest percentage of Rhopalodiales, which is the order of diatoms 

 that harbor nitrogen-fixing endosymbionts (cyanobacteria). The abundance of diatoms in this 

 group, together with the presence of Lyngbya, indicates likely nitrogen-limiting conditions at this 

 site. Percent Rhopalodiales declined downstream to a value of at the sewage treatment plant, 

 then recovered to low levels farther downstream. 



Mormon Church. A large percentage of motile diatoms indicated moderate impairment 

 from sedimentation at this site and only partial support of aquatic life uses. Minor impairment 

 from organic loading was also noted here. This site supported the second largest percentage of 

 diatoms in the order Rhopalodiales, which indicates that nitrogen was probably still limiting 

 here. The Mormon Church site shared over 70% of its diatom assemblage with The Trees site 

 and the two sites were virtually indistinguishable, both floristically and ecologically. Both sites 

 were dominated by Nitzschia inconspicua and Nitzschia filiformis . The latter species is a 

 brackish water diatom that was found in abundance only at these two sites. Its decline at the 

 sewage plant and downstream may indicate dilution in Big Dry Creek by discharges of fresher 

 water or by water of different ionic content. 



Sewage Plant. This site had the lowest biological integrity and worst water quality of all 

 the sites. Diatom metrics indicate moderate impairment and partial support of aquatic life uses 

 due to organic loading and sedimentation (Table 4). Two diatom species that are very tolerant of 

 organic pollution — Biremis circiimtexta and Nitzschia palea — were most abundant here. In 



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