1 . 
Restrict dataset to 1996-2006 time period. For the Fixed station samples included in the analyses, use the “grand 
score” only from the CIMS data base with these samples. 
2. Use 3-year rolling time periods rather than single years. This brings the reference community ID method into 
better alignment with the DO criteria assessment method for which reference communities are being identified. 
3. Require a B-1B1 score sample size > 10. This improves the spatial representation of the B-1BI score. 
4. “Healthy” reference communities are those with an average B-IBI score > 3.0, and a standard deviation (SD) < 
1.0, rather than a minimum. Using the average is consistent with published methods used by Chesapeake 
benthic experts to assess benthic communities (e.g. Llanso et al. 2009). 
APPLICATION OF A REFERENCE CURVE FOR OPEN-WATER 30-DAY MEAN 
DISSOLVED OXYGEN CRITERIA: SUMMER SEASON 
Reference curves for the 30-day mean open-water dissolved oxygen criterion (June 1-September 
30 only) were based on criteria levels that would not impair biological communities (U.S. EPA 
2003). Analyses conducted by the EPA Chesapeake Bay Program Office and its partners suggest 
that the B-IBI does not provide an appropriate reference community for assessment of open- 
water dissolved oxygen criteria violations. Even with the latest improvements in the assessment 
methodology to distinguish between healthy and degraded benthic communities, Figure III-3 
illustrates that the health of the benthic community is not an appropriate indicator of open-water 
low dissolved oxygen conditions as defined by the summer season open-water 30-day mean 
dissolved oxygen criterion. This result is demonstrated by the cloudplot (Figure III-3) 
representing Chesapeake Bay Program segments deemed “healthy” and “degraded” according 
the updated assessment methodology. 
! 
Figure III-3. Open water “healthy” and “degraded” benthic communities are not distinguished 
by violations of the summer open water DO 30-day mean criterion 
APPLICATION OF A REFERENCE CURVE FOR DEEP-WATER MEAN DISSOLVED 
OXYGEN CRITERIA 
Reference curves for the 30-day mean deep-water dissolved oxygen criteria (June 1-September 
30 only) were based on criteria levels that would not impair biological communities (U.S. EPA 
2003). Reference areas for derivation of the original 2003 published deep-water biologically- 
23 
