cha pter ill 
Refinements to Procedures 
for Assessing Chesapeake Bay 
Dissolved Oxygen Criteria 
BACKGROUND 
In 2003, the EPA published detailed criteria for dissolved oxygen tailored to 
different habitats within the Chesapeake Bay and its tidal tributaries (U.S. EPA 
2003a). Oxygen is critical to most forms of life in the Bay; it must be available in 
adequate concentrations to support overall ecosystem health. Minimum concentra¬ 
tions of dissolved oxygen must be present to support the diversity of species and 
their various life stages requiring protection. 
Dissolved oxygen criteria were established for Chesapeake Bay that varied in space 
(e.g., designated uses) and time (e.g., summer) to provide protection for different 
species and communities. The criteria were also designed around several durations 
(e.g., 30-day, 1-day) to reflect the varying oxygen tolerances for different life stages 
(e.g., larval, juvenile, adult) and effects (e.g., mortality, growth, behavior). Thus, the 
dissolved oxygen criteria include multiple components. Each component includes a 
target of dissolved oxygen concentration, the duration over which the concentration 
is averaged, the space (designated-use area) where the criterion applies, and a time 
(season, months) when the criterion applies. EPA has published, and the States 
adopted into their water quality standards regulations, dissolved oxygen criteria 
protective of migratory spawning, open-water, deep-water, and deep-channel desig- 
nated-use habitats (U.S. EPA 2003a). These dissolved oxygen criteria include 
30-day, 7-day, and 1-day means along with instantaneous minima. 
Since the Chesapeake Bay dissolved oxygen criteria were published in 2003, the 
capability of fully assessing all the dissolved oxygen criteria for all four designated 
uses over all applicable time periods has progressed, however, some limitations 
remain. The refined and expanded dissolved oxygen criteria assessment methodolo¬ 
gies documented in this chapter replace the methodologies previous published by 
EPA. Work by EPA and its partners will continue to refine these methodologies to 
reduce uncertainty further and to increase confidence in the resulting assessments. 
chapter iii 
Refinements to Procedures for Assessing Chesapeake Bay Dissolved Oxygen Criteria 
