82 
dissolved oxygen in the system as documented previously. This results in relatively 
stable dissolved oxygen concentrations, which although sometimes below the 5 mg 
liter 1 30-day mean criterion level due to natural oxygen deficits, remain substantially 
above the instantaneous minimum criterion. The magnitude of the wetland-caused 
oxygen deficit is not enough to cause the calculated oxygen saturated concentrations 
to fall below the 3.2 mg liter -1 instantaneous minimum. Therefore any future observed 
exceedences of this criterion value are likely due to anthropogenic nutrient pollutant 
loadings, not natural wetland-caused oxygen deficits. 
At attainment levels sustained for long periods of time just above the 4 mg liter -1 
criterion concentration (e.g., very few observed concentrations above 4 mg liter -1 ), 
survival of open-water aquatic species in their larval, juvenile and adult lifestages 
will not be impaired but there is likely to be some unquantified level of growth- 
related impairments. However, the 18-year data record indicates a maximum of less 
than one-third of the segment-based dissolved oxygen concentrations would not 
attain a 5 mg liter -1 concentration (Table VI-4). Therefore, combined with imple¬ 
mentation of further nutrient reduction actions in the upstream watersheds yielding 
higher measured ambient dissolved oxygen concentrations in the future, the number 
of exceedences of the 5 mg liter -1 concentration will be even less, further limiting 
growth effects. 
With a 30-day mean criterion of 4 mg liter -1 , these segments are likely to pass or 
come close to passing a formal criteria assessment under current conditions. Given 
that some fraction of oxygen depletion in these segments is definitely caused by 
controllable nutrient inputs, tributary-based nutrient reduction strategies should be 
more than adequate to raise ambient oxygen levels above the 4 mg liter -1 
concentration. 
SITE-SPECIFIC CRITERIA BIOLOGICAL REFERENCE CURVE 
The criteria assessment protocol for all segments and designated uses employs moni¬ 
toring data to develop cumulative frequency distribution (CFD) curves of 
exceedance, which are compared to biological reference curves specific to desig¬ 
nated uses, salinity regimes, and seasons. Monitoring data are interpolated over a 
fixed three-dimensional grid to obtain dissolved oxygen concentrations for each grid 
cell. These are compared to appropriate criteria values and yield a grid-cell by grid¬ 
cell estimate of the volume or area of criteria exceedance. The percentages of a 
segment’s volume/area exceeding the criteria levels are accumulated over all obser¬ 
vation dates in the assessment period. The CFD generated from these data reflect 
exceedance (and by difference, attainment) in both space and time. (See Chapter 6 
of Ambient Water Quality Criteria for Dissolved Oyxgen, Water Clarity and Chloro¬ 
phyll a for the Chesapeake Bay and Its Tidal Tributaries (U.S. EPA 2003a) for more 
details on the criteria attainment assessment protocol.) The biological reference 
curve is the CFD of exceedances in segments or other areas that are determined to 
chapter Vi 
Guidance for Deriving Site Specific Dissolved Oxygen Criteria 
