Using the newly delineated “healthy” and “degraded” benthic communities, EPA Chesapeake 
Bay Program Office analysts worked to produce a set of revised biological reference curves that 
minimize the error in distinguishing between “healthy” and “degraded" segments. In this chapter 
and its associated appendices, updates to the methodology involving development of 
biologically-based reference curves with Chesapeake Bay benthic macroinvertebrate monitoring 
program data are provided. Further, directions on application of reference curves for open-water, 
deep-water and deep-channel designated uses are provided for completing the Chesapeake Bay 
dissolved oxygen criteria attainment computations. 
ISSUES WITH DISSOLVED OXYGEN CRITERIA ASSESSMENT WITH THE 
PREVIOUSLY PUBLISHED BIOLOGICALLY-BASED REFERENCE CURVES 
The current published method for assessing dissolved oxygen (DO) impairments in Chesapeake 
Bay incorporates the use of a cumulative frequency distribution as the final step of assessment 
(U.S. EPA 2003). In this step, a set of DO violation rates for a particular segment-designated use 
(e.g. “CB4MH Deep Water”) are plotted as a cumulative frequency distribution (CFD) and 
compared to a “biological reference curve” comprising a cumulative frequency distribution of 
“acceptable violation rates” of the DO criteria. If the assessment curve exceeds, at any point , the 
reference CFD, then the given segment is considered “impaired (Figure III-1). 
Figure III-l. Conceptual graph illustrating the CFD assessment procedure. The red line is an 
example of a hypothetical “healthy” assessment curve; the blue line is the hypothetical reference 
curve. 
It has been recognized, however, that by combining violation rates from all healthy areas into 
one biologically-based reference curve, we create a curve that theoretically represents 
approximately the median of all curves included. Thus, a large percentage of the presumably 
“acceptable” violation rate CFDs that were pooled in order to generate the biologically-based 
reference curve may fail an assessment conducted against that same biologically-based reference 
curve. A more detailed evaluation confirmed this concern. In Figure III-2 below, the CFD for 
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