38 
open-water fish and shellfish designated use and the accompanying dis¬ 
solved oxygen criteria will fully protect the biological communities 
inhabiting shallow-water habitats. The open-water designated use extends 
into the intertidal zone and protects shallow-water organisms beyond under¬ 
water bay grasses. 
Unless a state has specifically delineated a sub-segment within a segment, attainment 
of the open-water designated use will be based on dissolved oxygen criteria attain¬ 
ment for the entire volume of the open-water designated use within the segment. 
Neither the need nor the requirement exists for a separate assessment of dissolved 
oxygen criteria attainment strictly within shallow waters (0-2 meters in depth). The 
importance of acquiring better temporal and spatial coverage of dissolved oxygen 
conditions in these shallow-depth habitats is not diminished however, since condi¬ 
tions in these areas vary greatly from the open water of the mid channels where the 
fixed stations are located. Shallow-water monitoring will provide the data needed to 
characterize dissolved oxygen conditions in shallow-water habitats more fully (see 
Chapter 7 for further details). 
ASSESSMENT OF SHORT-DURATION 
DISSOLVED OXYGEN CRITERIA 
Historically, the Chesapeake Bay Water Quality Monitoring Program consisted 
primarily of fixed-station monitoring conducted on a monthly or twice-monthly 
basis (Chesapeake Bay Program 1989). This sampling design was primarily intended 
to assess long-term trends in water quality and the status of living resources, 
capturing variability over decadal, annual, and seasonal time scales. The fixed- 
station monitoring was adapted to assess the 30-day mean dissolved oxygen criteria 
to measure dissolved oxygen throughout the Bay and its tidal tributaries and embay- 
ments. This system ensures at least one set of measurements for each month. 
The individual monthly estimates are considered accurate, although imprecise, since 
the sample sizes are small (n = 1 or 2). This imprecision is likely to be mitigated by 
the many estimates of monthly means (e.g., multiple months over the 3-year assess¬ 
ment period), which are combined into each single assessment of criteria attainment 
(see Chapter 2 and Appendix B for additional details). The monthly and twice- 
monthly fixed-station data are not adequate to assess attainment of the 7-day and 
1-day mean dissolved oxygen criteria directly because the sampling frequency rests 
outside the defined time intervals and is unable to capture the short-term variability. 
For the 2006 303(d) listing cycle, only three of the dissolved oxygen criteria compo¬ 
nents were assessed. The 30-day mean open-water criterion was determined in all of 
the assessment units of Chesapeake Bay using the fixed-station data and the CFD 
assessment methodology. In spatial assessment units where deep-water and/or deep- 
channel designated uses exist, the 30-day mean deep-water criterion and the 
chapter iv 
Refinements to the Chesapeake Bay Dissolved Oxygen Criteria Assessment Procedures 
