Appendix B. 
Shape of the Biologically-based Reference Curve 
The shape of the biologically-based reference curve is an important factor in identifying 
acceptable violations of the Chesapeake Bay water quality criteria for dissolved oxygen. The 
shape of biologically derived reference curve has thus far reinforced the suitability of the 
hyperbolic 10% default reference curve when a biologically-based reference curve is 
unavailable. An alternative hypothesis, however, is that comparing the total area under a CFD 
assessment curve to the total area under the biologically-based reference curve is a better 
measure of the degree to which healthy biological communities can tolerate violations of the DO 
criteria than the existing “point” method. Arguments put forth to support this proposal include: 
(1) a segment-period may exceed the biologically-based reference curve in one area of CFD 
space while the overall area of its exceedance is within than that represented by the biologically- 
based reference curve; (2) there is high variability in the shape of CFD curves and the data do not 
allow identification of combinations of time and volume that lead to poor B-IBI scores in a 
segment; and (3) the proposed “area” method has lower error rates than the published “point” 
method, even with the modifications proposed by EPA to the latter method. 
With regard to arguments 1 and 2, application of the method modifications outlined in this 
addendum, Chesapeake Bay benthic communities are now being accurately classified as 
“healthy” or “degraded” when there is sufficient data to do so. As a result, the Chesapeake Bay 
data support a rather specific combination of time and volume that forms the boundary between 
healthy and degraded benthic communities in the deep-water designated use (Figure B-l). 
space 
Figure B-l. Dissolved oxygen violation curves associated with healthy (blue) and degraded (red) 
benthic communities in deep-water designated use habitats. The deep-water biologically-based 
reference curve (yellow) is also shown. 
46 
