CHAPTER 3 
Biologically-based Reference Curves: Revisions to the 
Methodology and Applications 
BACKGROUND 
The published dissolved oxygen criteria assessment methodology currently used for assessing 
Chesapeake Bay water quality criteria attainment involves the use of cumulative frequency 
distribution (CFD) curves in a two-dimensional space of percent time and percent space (U.S. 
EPA 2003). Minimum concentrations of dissolved oxygen must be present to support species and 
their various life stages requiring protection. Dissolved oxygen criteria provide threshold 
conditions established for the designated uses such that water quality conditions that exceed this 
threshold are considered impaired. 
However, it is recognized that all water quality parameters are inherently variable in space and 
time. There will be small regions that persistently exceed the threshold due to poor flushing or 
other natural conditions. The Chesapeake Bay dissolved oxygen criteria have several durations 
reflecting the various tolerances of different life stages and effects (U.S. EPA 2003, 2008). Small 
regions or time periods of degraded condition should not lead to a degraded assessment for the 
segment (U.S. EPA 2003). Recognition that ephemeral exceedances of the threshold in both time 
and space do not represent persistent impairment of the segment leads to an assessment 
methodology that allows these conditions to be classed as acceptable while conditions of 
persistent and wide spread impaired condition will be flagged as unacceptable. (E. Perry, Pers. 
Comm. 2005). 
During an independent scientific peer review of the EPA published CFD procedures, reviewers 
raised specific concerns about the method for deriving the biological reference curves (STAC 
2006). At the time, there were no apparent solutions to resolve the concerns that were raised. 
However, during recent application of criteria assessment procedures to model simulated 
outputs, evaluation of the resultant model outputs put the spotlight back on the criteria 
assessment process and the underlying biological reference curve methodology. 
Work by the EPA Chesapeake Bay Program Office and its partners suggested that application of 
the currently published application of the Benthic-Index of Biotic Integrity (B-IBI) (Weisberg et 
al. 1997) did not accurately distinguish between healthy and degraded communities with 
corresponding distinct sets of dissolved oxygen violations. EPA Chesapeake Bay Program Office 
analysts and partners worked with recognized Chesapeake Bay benthic community experts 2 to 
revise the published methods for identifying “healthy” and “degraded” benthic communities. 
During this process, it was determined that the B-IBI provides a robust delineation of healthy and 
degraded benthic communities with corresponding distinct dissolved oxygen violation rates. 
2 Dr. Dan Dauer, Old Dominion University and Dr. Roberto Llanso, Versar, Inc. 
