DESIGNATED USE ASSIGNMENTS 
Both the low frequency long-term fixed station and the continuous buoy data records 
were assessed relative to the published Chesapeake Bay dissolved oxygen criteria. 
The criteria are specific to different designated uses and, therefore, seasons (U.S. 
EPA 2003). With very few exceptions, the buoy data currently available were 
summer deployments (June-September). One exception begins at the end of April; 
this one and a couple of other deployments extend through October, and one extends 
to November. 
Each data record was assigned to a designated use within a Chesapeake Bay Program 
segment based on following method. Using the Chesapeake Bay Water Quality Moni¬ 
toring Program data, the depth of the upper and lower pycnoclines, if any, were 
calculated for each station for each cruise date and the segment averages for the 
month/year were determined. These segment-averaged pycnocline depths were then 
merged by corresponding dates with the buoy sensor depths in those segments where 
deep-water and deep-channel designated uses apply. It is important to remember that 
pycnocline depths may be fairly stable in some areas, but changeable and ephemeral 
in others, even within the same segment. An average pycnocline depth for the month 
may have a lot of variability around it, and thus the designated use assignments for 
some buoy data records may not be correct. Where the buoy dissolved oxygen 
concentrations suggested an incorrect assignment, the monitoring data at stations and 
dates nearest in time and space to the buoy deployment were examined in detail and 
any appropriate changes to the designated use assignment were made accordingly. 
FINDINGS 
Day/Night Differences In Dissolved Oxygen Concentration 
A commonly expressed concern about the Chesapeake Bay Water Quality Moni¬ 
toring Program’s dissolved oxygen data is that they reflect daytime dissolved oxygen 
levels, the time period when active photosynthesis by algae, and consequent gener¬ 
ation and introduction of new oxygen into the water column, may mask lower 
nighttime concentrations. To address this concern, the buoy data were partitioned 
into day (defined as 9:00 AM to 5:00 PM) and night (defined as after 5:00 PM to 
before 9:00 AM) periods and summarized. Table V-3 provides the following statis¬ 
tics for the day and night periods: minimum concentration, the concentration of the 
lowest 1 percent of measurements, the lowest 10 percent, the median, mean, standard 
deviation, and coefficient of variation, separately for day and night periods each 
month, and the number of measurements taken in that month. 
Table V-4 pools all the continuous buoy data for a station’s designated use to show 
average day/night differences at each site. The difference between the daytime mean, 
minimum, 1 percent, etc. and the equivalent nighttime statistic was computed for 
each date of deployment and the means of the daily day-night differences are shown 
in the table (difference = daytime concentration minus nighttime concentration). 
chapter v 
Guidance for Attainment Assessment of Instantaneous Minimum and 7-Day Mean Dissolved Oxygen Criteria 
