small number of deep-channel segment-periods within which the benthic communities were 
categorized as “healthy" and, therefore, appropriate for use as a biological reference. These 
benthic communities were categorized using the methodology described on pp. 39-41 of U.S. 
EPA 2007. 
The revised methodology published in this addendum was applied to derive a new deep-channel 
biologically-based reference curve. The revised method yielded no segment-periods meeting the 
revised criteria outlined in Table III-1 above. This suggests that the occurrence of healthy 
benthic communities in the deep-channel designated use are currently insufficient to identify a 
corresponding set of “acceptable" violations of the instantaneous minimum dissolved oxygen 
criteria to develop a biologically-based reference curve. 
Comparisons of Degraded Reference Benthic Communities with the Published Deep- 
Channel Reference Curve 
While no benthic communities could be categorized as “healthy” in the most recent review, 25 
“degraded" reference benthic community segment-periods were identified. The EPA 
Chesapeake Bay Program Office and its partners conducted and reviewed analyses that showed 
that all 25 segment-periods (in the 1996-2005 time period) for which deep-channel benthic 
communities were categorized as “degraded" failed a dissolved oxygen criteria assessment 
conducted using the 10% default reference curve (Figure III-5). 
i 
0.8 
0.6 
E 
0.4 
0.2 
space 
Figure III-5. CFD graph of deep-channel instantaneous minimum dissolved oxygen criterion 
violation rates corresponding to benthic communities categorized as “degraded" (red lines) in 
relation to the 10% default reference curve (blue line). 
In the absence of a suitable reference community, a biological reference curve for the deep- 
channel instantaneous minimum dissolved oxygen criterion can not be constructed at this time. 
Under these circumstances, “a default reference curve such as the normal distribution curve 
25 
