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speeds, epilimnetic DO concentrations in the highly productive reservoir become supersatu-
rated, associated with strong vertical oxygen gradients. However, in the early morning hours with
increased wind speeds, mixing substantially reduces the gradient and results in decreased oxygen
concentrations (exceeding 4 mg L
−1
) in surface waters. This is not due to a change in productivity,
but rather to a change in the rates of vertical mixing. The impact of wind speed on vertical distribu-
tion of oxygen and pH is illustrated in Figure 14.12.
14.4.4.2 Seasonal Distribution: Vertical Distribution
Typically, oxygen concentrations vary over a yearly cycle in lakes and reservoirs as functions of
temperature, productivity, and decomposition. In a completely mixed lake with no production, the
oxygen concentration would be expected to be a constant value equal to the saturation concentra-
tion. That would be the case for a period of complete mixing for either of the vertical proiles for a
eutrophic or oligotrophic lake, as illustrated in Figure 14.13.
For an oligotrophic lake (little to no production or resulting decomposition), during periods of
summer stratiication, the temperature of the surface waters would be warmer resulting in, due to
lower saturation concentrations, the oxygen concentrations in the surface water being lower than in
the bottom waters, an
orthograde
proile (Wetzel 2001). With inverse stratiication during the winter
months, and with decreased surface temperatures, the oxygen concentrations in the surface waters
may increase, but will still commonly remain lower than that of the hypolimnion.
For a stratiied eutrophic lake during the summer months, with increased productivity (limited
to the photic zone) and reduced transparency, oxygen concentrations in the warmer epilimnion
would be greater than those in the cooler hypolimnion, where decomposition dominates and the
oxygen concentration would be reduced as a result. The resulting proile would be a
clinograde
Dissolved oxygen
(mg L
-1
)
pH
0
2
4
6
8
10
5
6
7
8
9
10
0
W
=
10 m s
-1
2
4
W
=
0.001 m s
-1
W
=
0.001 m s
-1
6
8
W
=
10 m s
-1
10
12
14
16
18
20
FIGURE 14.12
Predicted vertical variations in oxygen and pH as a function of wind speed. (From Martin,
J.L., Models of diel variations of water quality in a stratiied eutrophic reservoir (Lake Livingston, Texas),
PhD. dissertation, Texas A&M University, 1984.)
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