Figure VI-17. Dissolved oxygen criteria reference curve for defining criteria attainment in 
deep-water designated use habitats. 
resulting cumulative frequency distribution curve, which serves as the recommended 
reference curve for the deep-water seasonal fish and shellfish designated use for 
assessing dissolved oxygen criteria attainment (see Appendix H for documentation 
of the validation curves used to confirm the reference curve). 
Deep-Channel Dissolved Oxygen Criteria Reference Curve 
The deep-channel seasonal refuge designated use contains dissolved oxygen concen¬ 
trations that are inadequate to support most Chesapeake Bay species, and the 
criterion is set to protect the survival of benthic organisms. Unfortunately, a biolog¬ 
ically-based reference curve could not be developed for the deep-channel use at this 
time. This area is assumed to be severely degraded and is not now sampled as part 
of the Chesapeake Bay Program long-term benthic monitoring program. No other 
appropriate biological data were available with which to identify reference sites. 
While a biologically-based reference curve is recommended for the future, a default 
reference curve such as the normal distribution curve representing approximately 10 
percent exceedance is appropriate in this case to account for anticipated natural 
criteria exceedances (Figure VI-18). States and other users must recognize that the 
deep-channel dissolved oxygen criterion is stated as an instantaneous minimum, thus 
any exceedance is assumed to have direct consequences to the survival of the 
bottom-dwelling community. 
REFERENCE CURVES FOR WATER CLARITY CRITERIA 
Reference areas for development of the water clarity criteria reference curve were 
identified as Chesapeake Bay Program segments or parts of segments where under¬ 
water bay grasses were abundant historically and thriving or increasing in coverage 
chapter vi • Recommended Implementation Procedures 
