44 
These scientists also note that the amount of dissolved oxygen dissolved declines as 
temperature and salinity increase. For example, fully saturated freshwater at 20°C 
holds 9.28 mgliter' 1 of oxygen, but fully saturated seawater at the same temperature 
only contains 7.58 mg liter" 1 of oxygen. Seawater at 1°C can hold 11.38 mg liter' 1 of 
oxygen; at 30°C it can hold only 6.37 mg liter" 1 oxygen. As for the aquatic organ¬ 
isms, research indicates that percent saturation drives the oxygen diffusion supplying 
their respiratory demands. 
Concentration-based, not percent-saturation-based, criteria were published given the 
lack of reporting dissolved oxygen concentrations in terms of percent saturation in the 
extensive effects database used to derive the Chesapeake Bay dissolved oxygen criteria 
(U.S. EPA 2000). In addition, the lack of salinity and temperature values for each data 
point in the laboratory-based low dissolved oxygen effects database prevented calcula¬ 
tion of the concentration-based effects data into percent saturation numbers. 
Following publication of the Ambient Aquatic Life Water Quality> Criteria for Dissolved 
Oxygen (Saltwater): Cape Cod to Cape Hatteras , EPA scientists evaluated the implica¬ 
tions of recommending dissolved oxygen criteria as percent saturation versus 
concentration (U.S. EPA 2000). In an addendum to the 2000 Virginian Province salt¬ 
water dissolved oxygen criteria document, the U.S. EPA (2003b) reported: 
A standard based on percent saturation has a wide range of differences in 
partial pressure (2.14-4.01 Torr), which decreases with increasing temper¬ 
ature. The opposite is more desirable, however, since respiratory’ demand 
increases with temperature. Thus standards based on percent saturation are 
likely to overprotect during winter and potentially underprotect in summer, 
when organisms need the most oxygen. A standard based on concentration 
provides a more uniform difference in partial pressure over the temperatures 
used (2.45-2.72 Torr). Even though the range of difference is smaller, it still 
increases with temperature. Thus a standard based on absolute concentra¬ 
tion is more likely to create stable physiological conditions for animals 
throughout the year. 
Scientists from the EPA have generated a version of the EPA Virginian Province salt¬ 
water dissolved oxygen criteria as percent saturation for the State of Maine (G. 
Thursby, personal communication). At this time, however, the EPA does not have the 
scientific basis to recommend a set of Chesapeake Bay dissolved oxygen criteria in 
terms of percent saturation. 
chapter iv 
Refinements to the Chesapeake Bay Dissolved Oxygen Criteria Assessment Procedures 
