73 
In 2003, in-situ, continuous monitoring devices were deployed by the Virginia Insti¬ 
tute of Marine Science at a number of sites within both tidal rivers and all four 
salinity-based segments. These ‘buoys’ were deployed to collect data at time-scales 
more relevant to the Chesapeake Bay dissolved oxygen criteria, which have 7-day 
mean and instantaneous minimum as well as the 30-day mean averaging periods 
(U.S. EPA 2003a). These buoys collect dissolved oxygen concentration and other 
physical data continuously at 15-minute intervals. 
For the comparisons in Table VI-3, the mean and other statistics of the long-term 
daytime Chesapeake Bay Water Quality Monitoring Program measurements were 
computed for each month over the 18-year record, separately for surface (water 
column depth = 1 meter) and bottom (where the water column depth was >1 meter) 
waters. The continuous buoy data were divided into day (6:00 AM-5:59 PM) and 
night (6:00 PM-5:59 AM) periods. All the buoys were deployed at the fixed depths 
listed in Table VI-3. 
The low variability in dissolved oxygen concentrations measured in the Mattaponi 
and Pamunkey segments are documented by four separate measures: 1) the small 
within-month range of concentrations measured in the Chesapeake Bay Water 
Quality Monitoring Program over the 18-year data record; 2) the small dissolved 
oxygen concentration differences between surface and deeper waters (long-term 
water quality monitoring program data station); 3) the good agreement between 
dissolved oxygen concentrations measured at the long-term water quality monitoring 
program stations and the continuous buoy sites; and 4) the small differences between 
day and night concentrations recorded in the continuous buoy data. Similar compar¬ 
isons are becoming possible in other Chesapeake Bay and tidal tributary segments 
with expanded implementation of shallow water and continuous buoy deployment 
monitoring programs. This expanding data record will be evaluated in the future to 
further confirm low-variability in dissolved oxygen concentrations are an important 
characteristic of segments where extensive tidal wetlands are directly influencing 
ambient dissolved oxygen concentrations. 
APPROACHES FOR ADDRESSING NATURALLY 
LOW DISSOLVED OXYGEN CONDITIONS 
DUE TO TIDAL WETLANDS 
Four approaches for addressing naturally low ambient dissolved oxygen concentra¬ 
tions due to adjacent extensive tidal wetlands within the context of state water 
quality standards were considered: 
1. Define a completely new designated use with the appropriate dissolved oxygen 
criteria. 
2. Develop a separate biological reference curve that would account for lower 
dissolved oxygen values in wetland-dominated tidal water segments. 
chapter vi 
Guidance for Deriving Site Specific Dissolved Oxygen Criteria 
