Relatively small numbers of Achnanthes minucissima indicated 

 little or no physical, chemical or biological disturbance and 

 relatively stable periphyton communities throughout Big Spring 

 Creek (Table 5) . Cocconeis placentula, an epiphytic diatom, 

 probably peaked below the hatchery because of an abundance of 

 macrophyte hosts at this site. The minor impairment indicated by 

 the large relative abundance of this taxon is the result of 

 natural factors- -stable flows and heavy plant growth- -operating 

 at this site. 



Diatom species diversity was healthy and relatively constant 

 over the length of Big Spring Creek (Table 5) . The number of 

 diatom species was largest at 50 below the hatchery, then 

 declined downstream to between 34 and 38 at the remaining sites. 

 Diatom associations with more than 30 species are considered 

 normal and healthy. 



The siltation index was smallest below the hatchery. Here 

 and at Burleigh's Easement, the small siltation index values 

 indicated no impairment. At Carroll Trail, Spring Creek Colony, 

 and near the mouth, elevated siltation index values indicated 

 minor impairment but still full support of aquatic life uses. 



No abnormal or teratological cells were observed during the 

 diatom proportional counts . 



The diatom association at Burleigh's Easement had less than 

 40% of its flora in common with the control site below the 

 hatchery (Table 5) . Adjacent sites on the same stream, without 

 intervening pollution sources or tributaries, can be expected to 

 have at least 60% of their floras in common (Bahls 1993) . 

 Dissimilarity between these two sites may be related to natural 

 marl (calcium carbonate) deposits on the stream bottom at 

 Burleigh's Easement. Marl deposits are created when diurnal pH 

 peaks, created by plant photosynthesis, cause calcium carbonate 



