43 
applied in assessing attainment of the respective instantaneous minimum criteria. 
The reader is left with the sense that the published reference curves should be 
applied to all the dissolved oxygen criteria, regardless of the stated duration. 
All four instantaneous minimum criteria for protection of the four designated uses— 
migratory spawning and nursery, open-water, deep-water, and deep-channel—protect 
against mortality from very short-term exposure to low dissolved oxygen concentra¬ 
tions (U.S. EPA 2003a). The other dissolved oxygen criteria with specific averaging 
periods (30-day, 7-day, and 1-day means) protect against impairments—including 
growth, respiration, and behavioral/avoidance—for which the impairments will not 
impact the designated use. The 2003 EPA criteria guidance stated that there were no 
“biologically acceptable exceedances of the applicable criteria” for the instantaneous 
minimum criteria, given that the impairment is death (page 151 in U.S. EPA 2003a). 
While updating the methodology for deriving the open-water and deep-water desig- 
nated-use dissolved oxygen criteria reference curves for the 30-day mean criteria 
(described above), there were times and locations in the Chesapeake Bay for which 
healthy benthic infaunal communities still existed despite exceedance of the 1 
mg liter' 1 instantaneous minimum criterion. The EPA recommends, therefore, that 
attainment assessment of the instantaneous minimum deep-channel dissolved 
oxygen criteria be conducted with the CFD methodology using the deep-channel 
biological reference curve (Figure IV-4; Appendices F and G). 
USE OF PERCENT SATURATION AS DISSOLVED 
OXYGEN CRITERIA 
Several Chesapeake Bay scientists have called for future publication of dissolved 
oxygen criteria based on percent saturation (not concentration) and for state adop¬ 
tion of such percent-saturation-based criteria into the states’ water quality standards 
regulations. They cite fisheries physiology research showing that the pressure 
gradient between the surrounding water and the blood running through the fishes’ 
gills that truly determines whether sufficient oxygen exists in the water to support 
aquatic life. For example, Dutil and Chabot (2001) reported: 
Fishes have developed several mechanisms to secure more oxygen from their 
environment in critical situations such as low oxygen availability (Hoar and 
Randall 1984). When the partial pressure of oxygen in the environment 
drops below some critical limit, however, the pressure gradient between 
blood and water may not allow the fish to deliver as much oxygen to its 
tissues as needed to meet metabolic requirements associated with ingestion, 
digestion, growth and activity. Thus critical thresholds may vary> through 
time in demersal fish species and are best described in terms of partial pres¬ 
sure of oxygen or percent saturation. 
chapter iv 
Refinements to the Chesapeake Bay Dissolved Oxygen Criteria Assessment Procedures 
