3) restriction of reference segment-periods to those for which at least 10 observations are 
available; 
4) refined definition of a “healthy” benthic community as one for which the mean B-IB1 
score is at least 3.0; 
5) the standard deviation of the mean is less than 1.0; and 
6) use Grand Score in computations involving fixed data. 
The rationale underlying each of these six modifications is described in further detail below. 
Restrict Dataset to Data Collected Beginning in 1996 
Criteria violation results of dissolved oxygen criteria attainment assessments are compared with 
a reference CFD curve (e.g., standard 10% reference or biologically-based reference CFD curve), 
representing allowable amounts of criteria exceedance in a healthy habitat. When an appropriate 
biological reference community is identified and sufficient data are deemed available, a 
biological reference curve of acceptable percent exceedance is generated using a CFD of 
violation rates for “healthy” biological communities in that designated use. A review of the 
plotting methodology is provided in U.S. EPA 2008 (see Appendix A). 
Historically, the benthic monitoring work of the Chesapeake Bay Benthic Monitoring Program 
consisted of fixed station monitoring with sampling usually taking place in August and 
September (Chesapeake Bay Program 1989). The sampling design was primarily intended to 
assess long-term trends in living resources over decadal, annual and seasonal time scales. 
Derivation of the original dissolved oxygen biologically-based reference curves relied on the 
1985-2005 Chesapeake Bay benthic monitoring program dataset in order to take advantage of the 
full two decades of monitoring results. However, data collection methods have undergone 
revision during the 21 years of monitoring. In 1996, a stratified random sampling component was 
added to the benthic monitoring program in order to provide confidence limits on estimates of 
impaired waters in Chesapeake Bay. In order to ensure adequate spatial resolution of benthic 
community health, STAC (2009), in accordance with recognized Chesapeake Bay benthic 
community experts, recommended truncating the reference data set to start in 1996 when the 
updated sampling procedures were established. The data period was extended one year to 2006 
to include the most recently available data. The use of the 1996-2006 Chesapeake Bay benthic 
monitoring program data set is an update to previously published methods (U.S. EPA 2007, 
Chapter 4). The recommended data set represents a consistent period of improved assessments of 
Chesapeake Bay health condition. 
Use Sequential 3-year Time Periods Rather Than Single Years 
The biologically-based reference curve derivation methodology, as outlined in U.S. EPA 2007 
(see Chapter 4), used single year assessments to determine the health of the benthic community 
for the purposes of identifying acceptable dissolved oxygen criteria exceedances. However, 
dissolved oxygen criteria assessments are conducted on sequential 3-year time frames for each 
segment (U.S. EPA 2003); two year time steps are used in reporting for 303d listing cycles (e.g. 
the 2008 303d listing cycle used 2004-2006 data, the 2010 303d listing cycle used 2006-2008 
18 
