29 
are fixed, but the water mass moves past, back and forth with the tide and the various 
complexities of the local riverine and estuarine circulation. The majority of the avail¬ 
able buoy data were collected through buoy deployments that were sited using 
stratified random design considerations or to answer location-specific questions, but 
not directly to address the relationship between instantaneous minimum and monthly 
mean concentrations. 
In contrast, the long term monitoring program includes a vast network of stations 
sited specifically to represent overall water quality conditions of the 78 Chesapeake 
Bay Program segments. The low-frequency monitoring record captures a snapshot of 
conditions only once or twice a month, but that series of snapshots now extends over 
an 19-year period and is ongoing. Each snapshot consists of synoptic measurements 
forming a relatively dense three-dimensional spatial data grid. The grid is formed 
horizontally by the network of mainstem and tidal tributary monitoring stations and 
vertically by the dissolved oxygen profiles measured at 1- to 2-meter intervals from 
water column surface to bottom water-sediment interface. A single summer ‘snap¬ 
shot cruise' typically includes over a thousand individual dissolved oxygen 
concentration measurements. 
REFERENCE POINTS WITH RESPECT TO DEPTH 
Dissolved oxygen levels are strongly related to depth, bathymetry, and flow and 
circulation patterns. Table V-l provides information that helps to decide how repre¬ 
sentative the long-term fixed-station monitoring data and the continuous buoy data 
records are of their respective Chesapeake Bay Program segment. Table V-l presents 
segment volume, the depth of the Chesapeake Bay Water Quality Program moni¬ 
toring station(s) in the segment, and the segment-wide bottom depth distribution i.e., 
maximum depth, the depth encompassing 90 percent, 75 percent, 50 percent (the 
median) and 25 percent of the bottom depths, as well as the minimum depth. 
DATA ASSEMBLAGE AND MANIPULATION 
Table V-2 lists the 147 continuous buoy data sets available for analysis through the 
Chesapeake Information Management System (partner network of Chesapeake Bay 
data and information servers), latitude/longitude location information, the time interval 
between measurements, the total duration of deployment, water depth and depth of the 
sensor at the site and in what depth category the sensor depth falls, based on the depth 
distributions listed in Table V-l. The list of data sets has been categorized according to 
Chesapeake Bay Program segment so that it is obvious which segments have or do not 
have such high frequency information available for evaluating and establishing the 30- 
day mean and instantaneous minimum concentration relationship. 
chapter v 
Guidance for Attainment Assessment of Instantaneous Minimum and 7-Day Mean Dissolved Oxygen Criteria 
