Only periphyton samples collected in summer (June 21- 

 September 21) can be compared with confidence to reference stream 

 samples because metric values change seasonally and summer is the 

 season in which reference streams and impaired streams were 

 sampled for the purpose of biocriteria development. 



The previous analysis of these samples (Levine 1996) 

 employed Protocol II of Bahls (1993) . This protocol involves 

 comparing metrics from study sites to metrics generated from an 

 upstream control site. Protocol II is useful only where similar 

 physical conditions prevail at both the control site and the 

 study sites. The Natural Bridge site is more exposed to 

 sunlight, has a wider channel, slower current velocities and 

 smaller substrates than the upstream control site (Christian 

 Levine, MDEQ, pers . comm.) . Moreover, because diatom species 

 composition changes naturally in a longitudinal direction. 

 Protocol II should be used only where the control site and the 

 study site are close together, such as above and below an outfall 

 or a polluted tributary. Hence, Protocol II is not appropriate 

 for this study and was not used in this report. 



RESULTS AND DISCUSSION 



Results are presented in Table 4, located near the end of 

 this report following the Literature Cited section. Completed 

 diatom proportional . counts, pollution tolerance classes assigned 

 by Lange-Bertalot (1979) , and calculated percent abundances for 

 each species, are attached as Appendix A. 



Diatom metrics indicate that the control site below the East 

 Fork fully supported its beneficial uses. A slightly depressed 

 diversity index and a somewhat elevated disturbance index and 

 percent dominant species indicated minor stresses that were ■ 

 likely natural in origin (cold water and fast current velocity) . 



