71 
DISSOLVED OXYGEN/TEMPERATURE RELATIONSHIPS 
Another natural feature of tidal systems strongly influenced by extensive adjacent 
tidal wetlands would be a strong relationship between the ambient dissolved oxygen 
concentrations (and dissolved oxygen deficit) and water temperature, useful for 
separating out the wetlands’ effect on dissolved oxygen versus an anthropogenic 
effect. Figure VI-2 shows dissolved oxygen concentration and dissolved oxygen 
deficit plotted versus water temperature for the tidal fresh and oligohaline segments 
of the Mattaponi and Pamunkey rivers and for the tidal fresh and oligohaline 
segments of the Rappahannock and Patuxent rivers for comparison. All the plots 
illustrated in Figure VI-2 show dissolved oxygen concentrations going down as 
water temperature rises due to decreasing saturation concentrations and likely 
increased biological/chemical demand. 
In the Rappahannock and Patuxent segments, however, dissolved oxygen concentra¬ 
tions begin to trend back upward (and the dissolved oxygen deficit levels out) as 
temperatures continue to increase. Presumably the generation of oxygen from plank¬ 
tonic algal photosynthesis at these increasing temperatures provides the beneficial 
boost during the daytime when these measurements were collected. 
This trend effect in which dissolved oxygen concentrations increase as temperatures 
continue to increase is not evident in the Mattaponi and Pamunkey segments. Based 
on a comparison of the values in Table VI-2 and Appendix A, the difference in 
chlorophyll a concentrations in Rappahannock and Patuxent (higher concentrations) 
versus Mattaponi and Pamunkey river segments (lower concentrations) supports this 
hypothesis. These findings lend further evidence of the lack of a strong influence of 
planktonic algal photosynthesis on dissolved oxygen concentrations with the 
Mattaponi and Pamunkey rivers. 
LOW VARIABILITY IN DISSOLVED OXYGEN CONCENTRATIONS 
One could also hypothesize that, within the temperature trend described above and 
illustrated in Figure VI-2, there should be less scatter in the data points in a system 
whose ‘stressor’ exerted its effect in a relatively constant manner, as the wetlands 
might. While this hypothesis may be true and is suggested in the plots provided in 
Figure VI-2, the differences among the segments in the number and diversity of 
stations contributing data points is confounding a clearer conclusion. Table VI-3, 
however, provides further quantitative information on dissolved oxygen concentra¬ 
tion variability in the Mattaponi and Pamunkey segments which does support that 
hypothesis. 
Through the long-term Chesapeake Bay Water Quality Monitoring Program, 
Virginia has been collecting monthly or twice monthly dissolved oxygen measure¬ 
ments (surface and bottom) at fixed stations in the Mattaponi and Pamunkey tidal 
fresh and oligohaline segments since 1985. The data are collected in the daytime and 
each measurement represents one point in time in the month or two-week interval. 
chapter v 
Guidance for Deriving Site Specific Dissolved Oxygen Criteria 
