36 
as having “insufficient data to assess” (part 3). As nutrient loads are reduced and Bay 
water quality improves, assessing the complete array of applicable dissolved oxygen 
criteria to remove spatial assessment units from the “impaired” list will become 
more critical. 
Since Chesapeake Bay dissolved oxygen criteria were published in 2003, the capa¬ 
bility of fully assessing all the dissolved oxygen criteria for all four designated uses 
over all applicable time periods has progressed, but some limitations remain. The 
refined and expanded dissolved oxygen criteria assessment methodologies docu¬ 
mented in this chapter replace the methodologies previously published by U.S. EPA 
(2003a, 2004a). Work should continue in refining these methodologies to reduce 
uncertainty further and to increase confidence in the resulting assessments. Devel¬ 
oping, validating, and publishing EPA-recommended methodologies for assessing 
the full array of Chesapeake Bay dissolved oxygen criteria duration components will 
also prove critical. 
TEMPORAL PERIODS FOR ASSESSMENT 
OF DISSOLVED OXYGEN CRITERIA 
To assess dissolved oxygen criteria attainment, the time span over which the criteria 
apply must be clearly defined. In some cases, the temporal period is defined implic¬ 
itly as part of the criteria. For example, the dissolved oxygen criteria protective of 
the migratory fish spawning and nursery habitat designated use apply only to that 
time of year when spawning fish (and the resultant eggs and early juveniles) require 
higher dissolved oxygen levels compared to the rest of the year. In this example, 
dissolved oxygen criteria attainment should be assessed over the entire spawning 
season (February 1 through May 31) (U.S. EPA 2003a). Similarly, dissolved oxygen 
criteria in the deep-water and deep-channel designated uses apply only during the 
summer months — June 1 through September 30 — when the Bay stratifies and 
naturally reinforces the potential for lower dissolved oxygen concentrations in 
deeper waters. Therefore, assessment of dissolved oxygen criteria attainment in the 
deep-water and deep-channel designated uses should also be performed over the 
entire 4-month summer season (U.S. EPA 2003a). In all these cases, data are 
collected over the entire criteria season in each of the three years of the assessment 
period and these data are used to develop the cumulative frequency diagram (CFD) 
for assessing dissolved oxygen criteria attainment (see Chapter 2 and Appendix B for 
additional details). 
Open water is the only tidal water designated use in which the dissolved oxygen 
criteria apply year-round (U.S. EPA 2003a). In general, the Bay is most vulnerable to 
low dissolved oxygen during the summer when temperatures are high, oxygen solu¬ 
bility is low, and biological consumption of oxygen rises to its greatest level. Periods 
of low dissolved oxygen can also occur during the rest of the year, sometimes caused 
by high loading with subsequent slow consumption of organic material. The open- 
chapter iv 
Refinements to the Chesapeake Bay Dissolved Oxygen Criteria Assessment Procedures 
