Environmental Engineering Reference
In-Depth Information
Winter
stratification
Oligotrophic
Summer stratification
Oligotrophic
mg O
2
/L
04812
04812
Fall turnover
T
DO
T
DO
Spring turnover
04812
04812
0 020 0°C
0
10
20
30 °C
T
DO
T
DO
04812
048 2
0 020 0°C
0 0 0 0°C
DO
DO
T
T
0
10
20
30 °C
0 020 0°C
Eutrophic
Eutrophic
FIGURE 14.13
Idealized vertical distribution of oxygen concentrations and temperature for an oligotrophic
and eutrophic dimictic lake. (From Water on the Web, Monitoring Minnesota Lakes on the Internet and training
water science technicians for the future—A national on-line curriculum using advanced technologies and real-
time data. University of Minnesota, Duluth, MN, 2004. With permission. Reprinted from
Limnology: Lake and
River Ecosystems
, 3rd ed., Wetzel, R.G., Copyright 2001, with permission from Elsevier.)
proile (Wetzel 2001; Figure 14.13). During the winter months, in the presence of ice cover, oxygen
concentrations may again be reduced with depth as a function of decomposition, reduced surface
exchange, and light penetration through the snow and ice.
The amount or proportion of the reservoir that is hypoxic or anoxic, and the rate at which it
becomes anoxic, is a commonly used limnological parameter in lake management and in assessing
the trophic status of a lake or a reservoir. It may be expressed in various ways, perhaps the most
common of which is the areal hypolimnetic oxygen deicit (AHOD). The AHOD is deined as the
change in DO per unit area (e.g., square centimeter) of hypolimnetic surface per day (e.g., milli-
grams of dissolved oxygen per square centimeter per day) over the stratiication season. The AHOD
was initially proposed by Strom (1931) and Hutchinson (1938) and is intended to remove the inlu-
ence of lake morphometry, making this metric more comparable between reservoirs.
Another common feature of the vertical structure of many reservoirs is oxygen minima and
maxima. That is, the maximum or minimum concentrations occur at some intermediate depth,
rather than at the surface or at the bottom. An example of DO maxima is illustrated in Figure 14.14
for Bulls Shoals Reservoir in Arkansas and an example of DO minima and maxima is illustrated
in Figure 14.15 for Lake Mead, Arizona and Nevada. Oxygen maxima and minima may be due to
a number of factors, such as interlows bringing high or low DO water from upper reservoir regions
to material accumulating at the density gradient represented by the thermocline.
14.4.4.3 Seasonal Variations in Longitudinal Distribution
Seasonal variations in the longitudinal distribution of oxygen also typically occur, particularly in
deep storage reservoirs, as described by Cole and Hannan (1990). Sedimentation in the transition
zone and spring warming often result in hypoxia irst developing in this upper zone of a reservoir.
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