termination naturally, the rate at which this process proceeds is often influenced by
humans, whose activities frequently result in increasing the erosion process that
speeds the filling of the lake with sediment. Also, the discharge of sewage effluent
and runoff from fertilized land may add to the nutrients in the lake, thereby
increasing the rate of biological growth. This also adds to the increasing rate of
sedimentation. Whereas little can be done to control the natural cycle of a lake, much
can be done to prevent the increased rate of eutrophication that occurs from
anthropogenic sources.
In terms of the natural rate of eutrophication in lakes, the morphology of the lake
frequently has the greatest controlling influence. The morphology may even limit or
restrict the impact of human activities until the morphology has been changed. As a
general rule, a deep lake with steep sloping sides will have a much slower rate of
eutrophication from all causes than a lake of similar volume that is shallow and has
large shallow shoreline areas. Also the larger the total volume of the lake, the slower
will be the rate of eutrophication.
Lakes in a temperate climate normally go through a stratification pattern that
results in dimictic conditions (equal mixing throughout the lake twice a year). This is
primarily caused by the anomalous temperature-density relation of water. It may be
recalled that the most dense phase of water is 4
C; thus, water at this temperature
will tend to settle on the bottom of the lake, assuming that the external air temperature reaches 4
C or lower sometime during the year. During the winter, ice will
form on the surface of the lake, with the temperature of the water immediately below
the ice at or near 0
C. However, with a deep lake the temperature at the bottom
approaches 4
C. As spring comes and the sun warms the water, the ice melts and
then begins to warm the surface of the lake. When all of the lake is warmed to 4
C,
even a gentle wind will circulate the water from top to bottom of the lake creating the
spring overturn. As spring continues and the sun warms the surface of the lake, the
warmer surface waters will tend to float on the colder lower-temperature water. If
this heating occurs during a period of strong wind, there may still be complete
mixing of the lake and the entire lake is warmed to the temperature of the surface.
However, if warming occurs on a calm day, the surface of the lake will become
significantly warmer than the lower level. With several days of warming and little
wind, a point is reached at which the wind does not have sufficient energy to mix the
upper warmer layers of lower-density and lower-viscosity water with the colder
lower levels of higher-density and greater-viscosity water. Thus a period of stratification begins. Frequently, in large temperate lakes, the level of stratification is
established near 10-m depth. In addition to the combination of warming and wind
during the establishment of stratification, the shape and orientation of the lake with
the wind have a great influence on the depth of the upper mixed zone. During the
summer, there is an upper layer that is equally mixed by the wind, then a zone of
rapidly decreasing temperature with depth, and then a third layer at the bottom with
relatively constant and cold temperatures. The upper layer is called the epilimnion,
the zone of great temperature vs. depth change is called the thermocline or
metalimnion, and the lower layer is called the hypolimnion.
276
D. B. Aulenbach et al.
humans, whose activities frequently result in increasing the erosion process that
speeds the filling of the lake with sediment. Also, the discharge of sewage effluent
and runoff from fertilized land may add to the nutrients in the lake, thereby
increasing the rate of biological growth. This also adds to the increasing rate of
sedimentation. Whereas little can be done to control the natural cycle of a lake, much
can be done to prevent the increased rate of eutrophication that occurs from
anthropogenic sources.
In terms of the natural rate of eutrophication in lakes, the morphology of the lake
frequently has the greatest controlling influence. The morphology may even limit or
restrict the impact of human activities until the morphology has been changed. As a
general rule, a deep lake with steep sloping sides will have a much slower rate of
eutrophication from all causes than a lake of similar volume that is shallow and has
large shallow shoreline areas. Also the larger the total volume of the lake, the slower
will be the rate of eutrophication.
Lakes in a temperate climate normally go through a stratification pattern that
results in dimictic conditions (equal mixing throughout the lake twice a year). This is
primarily caused by the anomalous temperature-density relation of water. It may be
recalled that the most dense phase of water is 4
C; thus, water at this temperature
will tend to settle on the bottom of the lake, assuming that the external air temperature reaches 4
C or lower sometime during the year. During the winter, ice will
form on the surface of the lake, with the temperature of the water immediately below
the ice at or near 0
C. However, with a deep lake the temperature at the bottom
approaches 4
C. As spring comes and the sun warms the water, the ice melts and
then begins to warm the surface of the lake. When all of the lake is warmed to 4
C,
even a gentle wind will circulate the water from top to bottom of the lake creating the
spring overturn. As spring continues and the sun warms the surface of the lake, the
warmer surface waters will tend to float on the colder lower-temperature water. If
this heating occurs during a period of strong wind, there may still be complete
mixing of the lake and the entire lake is warmed to the temperature of the surface.
However, if warming occurs on a calm day, the surface of the lake will become
significantly warmer than the lower level. With several days of warming and little
wind, a point is reached at which the wind does not have sufficient energy to mix the
upper warmer layers of lower-density and lower-viscosity water with the colder
lower levels of higher-density and greater-viscosity water. Thus a period of stratification begins. Frequently, in large temperate lakes, the level of stratification is
established near 10-m depth. In addition to the combination of warming and wind
during the establishment of stratification, the shape and orientation of the lake with
the wind have a great influence on the depth of the upper mixed zone. During the
summer, there is an upper layer that is equally mixed by the wind, then a zone of
rapidly decreasing temperature with depth, and then a third layer at the bottom with
relatively constant and cold temperatures. The upper layer is called the epilimnion,
the zone of great temperature vs. depth change is called the thermocline or
metalimnion, and the lower layer is called the hypolimnion.
276
D. B. Aulenbach et al.