LAKES AND RESERVOIRS - Water Quality Engineering in Natural Systems - page 256

there are four well-defined seasons. In cool fall weather,
the surface layers of deep lakes begin to cool and can
become more dense than the underlying water, leading
to a gravity circulation , supplemented by the wind, that
causes the lake waters to overturn and become better
mixed. During winter, lakes are typically unstratified
except at higher latitudes, where further cooling of the
lake below 4°C * under cold winter conditions causes the
surface layers of the lake to become colder than 4°C,
making them less dense than the underlying water and
the lake is again stratified. As the temperature warms
during the spring, the surface waters warm to 4°C,
becoming more dense than the underlying water and
causing a turnover in the lake waters. As warm summer
weather returns, the lake tends to again become strati-
fied, and the seasonal cycle is complete. A typical
example of the lake stratification cycle in a temperate
lake is illustrated in Figure 7.8, where the lake is stably
stratified in the summer and winter, with seasonal over-
turning in between.
Lakes can be classified on the basis of their annual
pattern of overturning. These classifications are
described below.

ρ = ρ + ∆ ( ρ TSS ) = 999 1285 0 0013 999 1298 . + . = . kg/m

3

wt t t

ρ = ρ + ∆ ( ρ TSS

kg/m

) = 999 5261 0 0487 . + .

wb b b

= 999 5748 .

3

which indicates a density difference of 999.5748 kg/
m 3 − 999.1298 kg/m 3 = 0.4450 kg/m 3 . If only the tem-
perature difference is considered, the result would be
999.5261 kg/m 3 − 999.1285 kg/m 3 = 0.3976 kg/m 3 , and if
only the suspended solids is considered the difference
would be 0.0487 kg/m 3 − 0.0013 kg/m 3 = 0.0474 kg/m 3 .
Hence, most (89%) of the density difference is accounted
or by the temperature difference.

7.4.1 Layer Characteristics

In warm summer weather, heat added at the surface of
deep (>10 m [30 ft]) lakes is concentrated in the top few
meters, resulting in a warm, less dense layer that is well
mixed, overlaying a distinctly colder and weakly mixed
lower layer. The well-mixed surface layer is called the
epilimnion ; the weakly mixed lower layer is called the
hypolimnion . The warmer epilimnion is separated from
the colder hypolimnion by a thin layer with a sharp
temperature gradient called the metalimnion or meso-
limnion . The sharp temperature gradient in the meso-
limnion is called the thermocline , and in freshwater
lakes, the thermocline is defined as having a minimum
temperature gradient of 1°C/m (0.3°C/ft). When a ther-
mocline does not exist, the epilimnion and hypolimnion
are not defined. In shallow lakes, or shallow portions of
deep lakes, the thermocline eventually intercepts the
lake bottom so that no hypolimnion exists. The turbidity
of lake waters has a strong influence on the thickness of
the epilimnion since surface heat attenuates rapidly in
turbid waters. The depth of the epilimnion is related to
the size of the lake, where it can be as shallow as 1 m
(3 ft) in small lakes and as deep as 20 m (65 ft) or more
in large lakes. Waters in the epilimnion tend to be well
oxygenated, while waters in the hypolimnion tend to be
low in oxygen. Deep lakes typically have warm-water
fish in the epilimnion and coldwater fish in the hypolim-
nion. In a eutrophic lake that is stratified in the summer,
warm-water fish can live in the epilimnion, while low
DO can drive coldwater fish out of the hypolimnion and
perhaps result in death if there is not another suitable
habitat.

Amictic lakes never overturn and are permanently
covered with ice. These types of lakes are found in
the Antarctic and very high mountains.
Holomictic lakes mix from top to bottom as a result
of wind-driven circulation. Several subcategories
are defined:
(a) Oligomictic lakes are characterized by over-
turning that is unusual, irregular, and short in
duration. These types of lakes are generally
small to medium tropical lakes or very deep
lakes.
(b) Monomictic lakes undergo one regular over-
turn per year. Warm lakes in which the winter
temperature never drops below 4°C may be
monomictic, as may be cold lakes in which the
summer water temperature never rises above
4°C. Lakes in tropical regions and as far north
as 40° latitude are generally monomictic. Cold
monomictic lakes are connected with high lati-
tude and altitude and are frozen over most of
the year.
(c) Dimictic lakes overturn twice a year, in the
spring and fall, one of the most common types
of annual mixing in cool temperate regions,
such as central and eastern North America.
Most lakes in temperate climates in which the
summer water temperature is above 4°C and

7.4.2 Gravity Circulation

An annual cycle of gravity circulation is common in
large deepwater bodies at mid- and high latitudes, where

*Recall that the maximum density of water occurs at 3.98°C.