Table III-l. Chesapeake Bay dissolved oxygen criteria biologically-based reference curve 
derivation recommendations. 
U.S. EPA 2007, 2008 Addenda 
2010 Addendum 
Obtain dataset of all benthic index of biotic integrity (B-IBI) scores for 
time period 1985-2005 
Restrict dataset to data starting in 
1996; for random station data use 
‘total score’ and for fixed station 
samples use “grand score” only. 1 
For the relevant subset of Chesapeake Bay segments with open-water 
(OW) and deep-water (DW) and/or deep-channel (DC) designated uses: 
Match benthic stations and scores in a dataset with monthly open-water, 
deep-water, and deep-channel designated use boundaries. 
• Boundaries are derived using the standardized, automated 
method for identifying pycnocline boundaries documented in 
U.S. EPA 2008. 
Pycnocline boundaries are 
interpolated using the episodic 
pycnoclines approach defined in 
Chapter 2, this addendum. 
Pycnocline boundaries are then interpolated using the interpolator 
(Visual Basic program, Version 4.61, August 2006, Chesapeake Bay 
Program Office, as referenced in U.S. EPA 2008, Appendix A, p36). 
• Interpolator cells are matched with benthic station locations, and 
interpolated pycnocline boundaries are applied to each benthic 
station location. 
Benthic stations (and their associated B-IBI scores) are assigned to a 
designated use: OW, DW, or DC. 
No modification recommended 
To define the biological reference community for each designated use, all 
individual segment-years for which the minimum B-IBI was > 3.0 are 
identified. (Minimum sample size within a segment-year is recognized as 
n=l.) These are denoted as ‘healthy’ segment-years. 
a. Use 3-year rolling time periods 
rather than single years. 2 
b. Require a B-IBI score sample 
size n _> 10. 3 
C. “Healthy” reference 
communities are those with an 
average B-IBI score > 3.0 rather 
than a minimum, and standard 
deviation (SD) < 1,0. 4 
For the ‘healthy’ segment-years, the monthly (in the case of OW and 
DW) or instantaneous (DC) dissolved oxygen criteria violation rates are 
obtained based on the water quality profiles of sampling data collected by 
the Chesapeake Bay long term water quality monitoring program. 
No modification recommended. 
These season- and designated use-specific Chesapeake Bay dissolved 
oxygen criteria violation rates (e.g. percentage of a segment-designated 
use volumes failing the DO criteria in a given month; thus 4 measures 
per summer for OW and DW - June thru September) are used to define 
“acceptable” exceedances of the dissolved oxygen criteria. This 
definition of acceptable exceedances in space and time is based on the 
logic that if a healthy benthic community existed in the segment- 
designated use in that summer, then the degree of DO criteria violation 
that occurred did not lead to an impaired benthic community. 
No modification recommended. 
Source: U.S. EPA 2003, 2007, 2008. 
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