showed that low oxygen conditions, although not nearly as severe as today, may 
have been a historical feature in the deep channel of the bay (Cooper and 
Brush, 1991; Karlsen et al., 2000; Adelson et ah, 2001; Zimmerman and Canuel, 
2002; Bratton et ah, 2003; Colman and Bratton, 2003; Cronin and Vann, 2003; 
Zheng et ah, 2003).” 
In support of the deep-channel instantaneous minimum criterion of 1 mg/L, U.S. EPA 2003 
summarized findings published in peer-reviewed literature sources indicating that several 
keystone benthic species “are resistant to dissolved oxygen concentrations as low as 0.6 mg/L,” 
and that “extensive mortality is likely only under persistent exposure to very low dissolved 
oxygen concentrations at high summer temperatures” (p. 61). 
In light of both (1) the recognition that low dissolved oxygen conditions are a ‘pre-historical’ 
feature of these deep channel habitats, and (2) the observation that keystone benthic species of 
these deep channel habitats can tolerate small-scale occurrences of severe hypoxia (DO 
concentrations below 1 mg/L), EPA believes that an allowance for a small, limited set of 
exceedances in time and space is acceptable in assessment of the deep-channel designated use 
dissolved oxygen criterion. 
ASSESSMENT OF SUMMER SEASON DISSOLVED OXYGEN CRITERIA 
EPA recommends revising previously published dissolved oxygen criteria assessment guidance 
as described in Table 111-2, including applying the default 10% reference curve for assessment of 
summer season (June 1- September 30) 30-day mean open-water and instantaneous minimum 
deep-channel dissolved oxygen criteria. The 30-day mean deep-water dissolved oxygen criterion 
biologically-based reference curve, as described in this addendum, is recommended for use 
assessing attainment of this criterion. Until EPA publishes methodologies for assessing the 7-day 
mean, 1-day mean and instantaneous minimum open-water and deep-water dissolved oxygen 
criteria, respectively, the Agency recommends that the states and the District of Columbia rely 
strictly on the assessment of the 30-day mean open-water and deep-water dissolved oxygen 
criteria for listing decisions (U.S. EPA 2007). The previously published non-summer open-water 
dissolved oxygen criteria reference curve remains unchanged as the 10% default reference curve 
(U.S. EPA 2007, p.42). 
27 
