37 
water dissolved oxygen criteria are designed to provide protection of open-water 
habitat fish and shellfish communities at all times of the year. In spite of the year- 
round application of these criteria, natural processes complicate the use of a single, 
year-round assessment. Cooler temperatures affect the solubility of oxygen and allow 
higher concentrations compared to similar organic loading conditions in warmer 
months. Consequently, dissolved oxygen concentrations have a large natural vari¬ 
ability range. Detecting human effects in the presence of that greater variability often 
proves difficult. For this reason, as part of the dissolved oxygen criteria development 
process, the EPA originally intended that the year-round open-water dissolved oxygen 
criteria (see Table III-10, page 66 in U.S. EPA 2003a) be assessed in each season (see 
pages 150-151 in U.S. EPA 2003a). During the 2006 303(d) listing cycle, confusion 
arose as to the appropriate time period for open-water dissolved oxygen assessment. 
The criteria were clearly defined over the full annual cycle, but the stated intent was 
to assess them on a seasonal basis. Furthermore, the 2003 EPA Chesapeake Bay 
criteria document itself did not provide consistent guidance; it referred to assessment 
on both an annual basis and a seasonal one (U.S. EPA 2003a). 
Based on a re-evaluation of the underlying scientific basis for Chesapeake Bay 
dissolved oxygen criteria, the EPA recommends that jurisdictions assess attainment 
of the open-water dissolved oxygen criteria separately over two time periods: 
summer (June 1 through September 30) and non-summer (January 1 through May 31 
and October 1 through December 31). The open-water dissolved oxygen criteria 
were largely derived to protect open-water species during the summer when elevated 
temperatures, higher salinities, and naturally low dissolved oxygen levels occur 
(U.S. EPA 2003a). Given that summer is a critical period for many species, it should 
be assessed separately. The potential for dissolved oxygen impairments are lower in 
the non-summer period due to greater natural dissolved oxygen solubility and lower 
biological oxygen consumption—both due to lower water column temperatures. 
Nevertheless, low dissolved oxygen levels sometimes occur during other times of the 
year making a separate dissolved oxygen criteria assessment necessary for the non¬ 
summer period. The separate criteria assessments for summer and non-summer 
seasons will support year-round protective dissolved oxygen concentrations in the 
open-water designated-use habitats. 
DISSOLVED OXYGEN CRITERIA ASSESSMENTS 
IN SHALLOW VERSUS OPEN WATERS 
The open-water designated-use boundary is explicitly defined as including “tidally 
influenced waters extending horizontally from the shoreline to the adjacent shore¬ 
line” (see page 71 in U.S. EPA 2003c). Further, on page 68, the U.S. EPA (2003c) 
states that: 
The shallow-water bay grass designated use is intended specifically to delin¬ 
eate the habitats where the water clarity criteria would apply. The 
chapter iv 
Refinements to the Chesapeake Bay Dissolved Oxygen Criteria Assessment Procedures 
