Environmental Engineering Reference
In-Depth Information
Summer stratification
75
°
68
°
Epilimnion
Metalimnion
Hypolimnion
50
°
46
°
43
°
43
°
Fall overturn
(September-October)
Spring overturn
(March-April)
Wind
Wind
39
°
39
°
39
°
39
°
39
°
39
°
39
°
41
°
Temperatures in °F
46
°
46
°
43
°
Winter stratification
Late fall
(November)
Ice
39
°
39
°
39
°
39
°
39
°
39
°
32
°
34
°
36
°
39
°
39
°
39
°
Figure 7.8.
Lake stratification cycle.
Source
: State of Wisconsin (2005).
the winter temperature is below 4°C are
dimictic.
(d)
Polymictic lakes
circulate frequently or con-
tinuously, cold lakes that are continually near
or slightly above 4°C, or warm equatorial
lakes where air temperature changes very
little.
Meromictic lakes
do not circulate throughout the
entire water column. The lower water stratum is
perennially stagnant.
7.4.3 Water-Quality Impacts
The primary water-quality parameter of concern in
lakes is DO, and since almost all aquatic organisms
require oxygen for respiration, it is generally considered
desirable for all parts of the water column to remain
oxygenated. Retardation of oxygen transport between
the epilimnion and the hypolimnion results in sharply
differentiated water quality and biology between the
lake strata. As oxygen is consumed within the hypolim-
nion without being replenished, life functions of many
organisms are impaired, and the biologically mediated
reactions fundamental to water quality are altered.
Since light is limited in the hypolimnion, algae that
settle into this zone can only respire, imposing an addi-
tional demand on the DO which, combined with the
benthic oxygen demand, causes the hypolimnion to
become
anoxic
(i.e., devoid of oxygen). Anoxic condi-
tions in the hypolimnion trigger reducing chemical reac-
tions, which convert some chemical compounds from
their more oxidized states into reduced ones. Typically,
reduced states of chemicals are more soluble in water.
Anoxic conditions in the bottom sediments also cause
dissolution of some metals, such as iron and manganese,
which interfere with potable uses of water. Some toxic
compounds, such as mercury, can undergo microbiologi-
cal methylation in the bottom sediment, which makes
them far more toxic than their other forms. Similarly,
It is important to keep in mind that lakes in the
tropics are characterized by a diurnal climate (day/night
variability), compared with a seasonal climate that is
characteristic of temperate zones. Consequently,
polymictic lakes are found more widely in tropical
regions. On a practical note, inaccurate conclusions are
sometimes made from water-quality measurements
taken in shallow tropical waters during morning hours
when the water is mixed, as it may become stratified and
anoxic during the day. Lake and reservoir water-quality
monitoring in tropical regions must consider these daily
differences much more strictly than in the temperate
regions, where differences in some variables such as
pH, oxygen, and nutrient concentrations are not very
pronounced over a period of a few hours (Jørgensen
et al., 2005).
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