Deep lakes in temperate climates exhibit an interesting circulation pattern. Under
ordinary conditions there is a period of stratification during ice cover in the winter
and another period of stratification during the summer. There are also two periods
during which the water is completely mixed from top to bottom by the impact of
wind at the surface. This occurs when the water temperature is uniform and usually
occurs just before ice formation (fall turnover) and just after ice-out (spring turnover). Such a lake is called dimictic. This pattern is the result of the temperaturedensity relationship and the anomalous condition of water’s being most dense at 4
C. Thus in winter the bottom temperature is 4
while the ice on the surface is at zero.
During the summer the surface is warmed by the sun while the bottom may remain at
or near 4
C. During the summer thermal stratification usually occurs as a result of a
combination of solar heating of the surface, the impact of wind, and the temperatureviscosity relationship of the water. This summer stratification prevents surface
reaerated water from being carried to greater depths, a factor that also contributes
to the long-term demise of the lake.
The temperature succession in a lake may be shown starting with ice cover in
winter. The surface ice is at or below 0
C, while the bottom is at 4
C. There is no
mixing of this water, because the ice cover prevents any wind effects. Biological
activity is also at a minimum. As spring comes, the sun melts the ice and then begins
to warm the surface of the lake. As all of the water approaches 4
C, even a gentle
wind will mix this isothermal water from top to bottom, called the spring overturn.
As the sun warms the surface of the lake, the warmer water will tend to float on the
surface due to its lower density. If this heating occurs during a period of strong wind,
there may still be complete mixing and the entire lake will be heated to the
temperature of the surface. However, if warming occurs during a period of light or
no wind, a point is reached at which the wind does not have sufficient energy to mix
the upper warmer water with the cooler lower layer of water with greater density and
viscosity. This forms a period of summer stratification where there is circulation near
the surface, but none below a certain depth. Frequently in large deep temperate lakes,
the level of stratification occurs at about 10 m depth. Further, the shape and
orientation of the lake to the wind have an influence on the depth of the upper
mixed zone. During the summer in a typical temperate lake, there is a warm upper
layer that is equally mixed by the wind, then a zone in which there is a rapid decrease
in temperature with depth, and finally a layer of relatively cold uncirculating water
near the bottom. Thus the lake is divided into three layers in which the upper layer is
called the epilimnion, the middle layer the metalimnion or the thermocline, and the
bottom layer the hypolimnion, as shown in Fig. 7.1.
The summer stratification may last up to 5 months, during which time there is
little to no mixing in the hypolimnion and no opportunity for oxygen from surface
aeration to reach this area. So long as only a little decomposable organic matter is
present at the onset of stratification, the available oxygen present may not be entirely
consumed. The colder bottom water temperature also contributes to a slower biological activity, thereby conserving the oxygen supply. This condition is conducive
to supporting a cold-water fish habitat. However, if large amounts of decomposable
organic matter settle into the hypolimnion, the limited amount of oxygen available
7 Lake Restoration
261
ordinary conditions there is a period of stratification during ice cover in the winter
and another period of stratification during the summer. There are also two periods
during which the water is completely mixed from top to bottom by the impact of
wind at the surface. This occurs when the water temperature is uniform and usually
occurs just before ice formation (fall turnover) and just after ice-out (spring turnover). Such a lake is called dimictic. This pattern is the result of the temperaturedensity relationship and the anomalous condition of water’s being most dense at 4
C. Thus in winter the bottom temperature is 4
while the ice on the surface is at zero.
During the summer the surface is warmed by the sun while the bottom may remain at
or near 4
C. During the summer thermal stratification usually occurs as a result of a
combination of solar heating of the surface, the impact of wind, and the temperatureviscosity relationship of the water. This summer stratification prevents surface
reaerated water from being carried to greater depths, a factor that also contributes
to the long-term demise of the lake.
The temperature succession in a lake may be shown starting with ice cover in
winter. The surface ice is at or below 0
C, while the bottom is at 4
C. There is no
mixing of this water, because the ice cover prevents any wind effects. Biological
activity is also at a minimum. As spring comes, the sun melts the ice and then begins
to warm the surface of the lake. As all of the water approaches 4
C, even a gentle
wind will mix this isothermal water from top to bottom, called the spring overturn.
As the sun warms the surface of the lake, the warmer water will tend to float on the
surface due to its lower density. If this heating occurs during a period of strong wind,
there may still be complete mixing and the entire lake will be heated to the
temperature of the surface. However, if warming occurs during a period of light or
no wind, a point is reached at which the wind does not have sufficient energy to mix
the upper warmer water with the cooler lower layer of water with greater density and
viscosity. This forms a period of summer stratification where there is circulation near
the surface, but none below a certain depth. Frequently in large deep temperate lakes,
the level of stratification occurs at about 10 m depth. Further, the shape and
orientation of the lake to the wind have an influence on the depth of the upper
mixed zone. During the summer in a typical temperate lake, there is a warm upper
layer that is equally mixed by the wind, then a zone in which there is a rapid decrease
in temperature with depth, and finally a layer of relatively cold uncirculating water
near the bottom. Thus the lake is divided into three layers in which the upper layer is
called the epilimnion, the middle layer the metalimnion or the thermocline, and the
bottom layer the hypolimnion, as shown in Fig. 7.1.
The summer stratification may last up to 5 months, during which time there is
little to no mixing in the hypolimnion and no opportunity for oxygen from surface
aeration to reach this area. So long as only a little decomposable organic matter is
present at the onset of stratification, the available oxygen present may not be entirely
consumed. The colder bottom water temperature also contributes to a slower biological activity, thereby conserving the oxygen supply. This condition is conducive
to supporting a cold-water fish habitat. However, if large amounts of decomposable
organic matter settle into the hypolimnion, the limited amount of oxygen available
7 Lake Restoration
261