35 
Assessing dissolved oxygen criteria attainment is challenging because of the 
complexity of both the criteria and the Bay itself. To fully assess all the criteria 
components, data need to be collected at a spatial intensity that adequately represents 
the four designated-use habitats of Chesapeake Bay tidal waters at different times of 
the year (U.S. EPA 2003c, 2004b). Similarly, data must be collected during all the 
applicable seasons and at frequencies sufficient to address the various criteria dura¬ 
tion components. The different dissolved oxygen criteria apply to different 
designated-use areas and multiple criteria apply to the same designated-use area. The 
dissolved oxygen criteria components also apply over different time periods to 
protect species during critical life stages or during particularly stressful times of the 
year. To fully assess each dissolved oxygen component in each designated-use 
habitat over the appropriate time periods will require an extensive monitoring 
program and a detailed assessment methodology. The Chesapeake Bay Program 
currently conducts extensive water quality monitoring throughout the Bay tidal 
waters and the EPA published a detailed dissolved oxygen criteria assessment 
methodology with the new water quality criteria (Chesapeake Bay Program 1989; 
U.S. EPA 2003a, 2004a). The existing Bay water quality monitoring was not suffi¬ 
cient to cover all the criteria components, however, and some details in the 
assessment methodology remain unresolved. 
For the 2006 303(d) listing cycle, the states’ 
listing decisions were based primarily on 
previous listings. Tidal waters that had been 
listed as impaired in 2004 were not removed 
from part 5 of their listing unless all the appli¬ 
cable criteria components were shown in 
attainment (see Chapter 8 for further details). 
The Chesapeake Bay Program partners had the 
capacity (data, assessment methodology) to 
assess only the 30-day mean dissolved oxygen 
criteria and, in some cases, the instantaneous 
minimum dissolved oxygen criteria. The 
remaining dissolved oxygen criteria were not 
assessed because the existing water quality 
monitoring programs and the published assess¬ 
ment methodologies were inadequate for full 
assessment. In most spatial assessment units, 
the 30-day mean criterion was not attained and 
those assessment units would have been listed 
whether or not the other applicable dissolved 
oxygen criteria were also assessed (Figure IV- 
1). In many smaller tidal tributaries, however, 
the 30-day mean criterion was attained and 
those spatial assessment units were listed either Figure IV-1. Listing status of the Chesapeake Bay open-water 
l4 . . ,,, . „ c . , ... designated use based on dissolved oxygen standards, 
as impaired (part 5) due to previous listing or 
chapter iv 
Refinements to the Chesapeake Bay Dissolved Oxygen Criteria Assessment Procedures 
