based reference curves were identified using a measure of benthic community health - the 
Chesapeake Bay B-IBI (Weisburg et al. 1997). Using the revised methodology outlined in this 
addendum chapter, the EPA Chesapeake Bay Program Office and its partners identified two 
distinct sets of “healthy” and “degraded” (average B-IBI < 3.0, SD < 1.0) benthic communities, 
with correspondingly distinct violation rates (Figure III-4). The EPA Chesapeake Bay Program 
Office, in coordination with its partners, further determined that a reference curve constructed 
from the 100 lh percentile of healthy violation rates (x) for each point in time (y) accurately 
distinguished between healthy and degraded benthic communities with zero error in 
classification. 
A step-by-step guide to the derivation of this curve (Figure III-4), including the x-y coordinate 
values for plotting the curve, is provided in Appendix C. 
—*- r.RRPH 1999 ?ooi 
-•— POTM HI 9987000 
space 
Figure III-4. Dissolved Oxygen deep-water criteria violation rates corresponding to healthy 
(blue) and degraded (red) benthic communities. Recommended new deep-water biologically- 
based reference curve represented by the 100 th percentile of healthy violations is shown in black. 
APPLICATION OF A REFERENCE CURVE FOR DEEP-CHANNEL 
INSTANTANEOUS MINIMUM DISSOLVED OXYGEN CRITERIA 
In the case of the deep-channel instantaneous minimum dissolved oxygen criterion, the 
application of a biological reference curve was recommended in U.S. EPA 2007 (p. 43). 
Appendix D provides a more thorough review of history of EPA guidance regarding application 
of the deep-channel reference curve. 
This 2007 recommendation for application of a biologically-based reference curve for 
assessment of the deep-channel dissolved oxygen criterion was based on the identification of a 
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