Once this summer stratification is established, there is little to no mixing in the
hypolimnion. This also restricts the reaeration of the hypolimnion from the surface.
Thus if large amounts of organic material reach the hypolimnion, the oxygen
available will be utilized and the area will become anaerobic. If on the other hand
there is little productivity in the lake, or if the lake is so clear that photosynthesis can
occur in the hypolimnion, then there will not be oxygen depletion in the hypolimnion
and this area will support a fine crop of cold-water fish. Also keeping the hypolimnion aerobic helps to stabilize the nutrients that settle to the bottom and are normally
precipitated under aerobic conditions. It can be seen that this is a critical condition,
for once the hypolimnion becomes anaerobic the nutrients that are soluble under
anaerobic conditions are released to the water column, allowing for the growth of
more biological materials that will in turn die, settle to the bottom, and utilize more
dissolved oxygen. Thus, in order to maintain the lake in an oligotrophic condition,
management efforts should be made to maintain the hypolimnion in an aerobic state.
As fall approaches, the surface of the water is cooled and the cooler water
circulates to a depth of equal temperature and/or density. This tends to lower the
thermocline until the lake again becomes uniform in temperature and even a gentle
wind will mix the lake from top to bottom. This is called the period of fall turnover.
As the air temperature reaches 4
C and becomes colder, the surface of the lake will
approach 0
C, but the warmer 4
C water will remain on the bottom. After ice is
formed, there is no wind effect on the lake to cause mixing. Thus the bottom of a
deep lake never freezes. However, again during ice cover there is no opportunity for
reaeration of the lake from the surface, and thus the biological activity in the water
must be such that it does not utilize the available oxygen in the take. Because of the
cold temperature, this is usually fairly easy to maintain; however, in certain eutrophic lakes, oxygen depletion may occur under the ice. This period of stratification
under the ice is called the winter stratification period. In evaluating a lake it may be
seen that it is essential to evaluate it under all conditions of stratification and
nonstratification.
A lake contains many biological communities. Within the water column are
numerous organisms of microscopic size. These are generally termed plankton,
which are microscopic floating organisms. The plankton may be subdivided into
two general groups: the phytoplankton, which are the plant life and include the algae,
the fungi, and the pollen that fall into a lake, and the zooplankton, which represent
the animal forms. In another category, the plankton may be broken down into the
nekton or free swimming organisms and the benthon, which are the microscopic
organisms that exist on the bottom.
A prime concern is the algae, the microscopic green plants floating in the water
column. These organisms represent the base of the food chain in that they can
convert simple inorganic materials into organic materials with the aid of sunlight.
In addition, during the daytime or sunlight hours, these plants undergo photosynthesis, which is a process in which oxygen is liberated in the water. It has been
estimated that ¾ of the world’s supply of oxygen is generated by algae in the ocean.
In terms of the food chain, the algae are normally consumed by the zooplankton,
which are in turn consumed by larger animal forms, which may be consumed by
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
277
hypolimnion. This also restricts the reaeration of the hypolimnion from the surface.
Thus if large amounts of organic material reach the hypolimnion, the oxygen
available will be utilized and the area will become anaerobic. If on the other hand
there is little productivity in the lake, or if the lake is so clear that photosynthesis can
occur in the hypolimnion, then there will not be oxygen depletion in the hypolimnion
and this area will support a fine crop of cold-water fish. Also keeping the hypolimnion aerobic helps to stabilize the nutrients that settle to the bottom and are normally
precipitated under aerobic conditions. It can be seen that this is a critical condition,
for once the hypolimnion becomes anaerobic the nutrients that are soluble under
anaerobic conditions are released to the water column, allowing for the growth of
more biological materials that will in turn die, settle to the bottom, and utilize more
dissolved oxygen. Thus, in order to maintain the lake in an oligotrophic condition,
management efforts should be made to maintain the hypolimnion in an aerobic state.
As fall approaches, the surface of the water is cooled and the cooler water
circulates to a depth of equal temperature and/or density. This tends to lower the
thermocline until the lake again becomes uniform in temperature and even a gentle
wind will mix the lake from top to bottom. This is called the period of fall turnover.
As the air temperature reaches 4
C and becomes colder, the surface of the lake will
approach 0
C, but the warmer 4
C water will remain on the bottom. After ice is
formed, there is no wind effect on the lake to cause mixing. Thus the bottom of a
deep lake never freezes. However, again during ice cover there is no opportunity for
reaeration of the lake from the surface, and thus the biological activity in the water
must be such that it does not utilize the available oxygen in the take. Because of the
cold temperature, this is usually fairly easy to maintain; however, in certain eutrophic lakes, oxygen depletion may occur under the ice. This period of stratification
under the ice is called the winter stratification period. In evaluating a lake it may be
seen that it is essential to evaluate it under all conditions of stratification and
nonstratification.
A lake contains many biological communities. Within the water column are
numerous organisms of microscopic size. These are generally termed plankton,
which are microscopic floating organisms. The plankton may be subdivided into
two general groups: the phytoplankton, which are the plant life and include the algae,
the fungi, and the pollen that fall into a lake, and the zooplankton, which represent
the animal forms. In another category, the plankton may be broken down into the
nekton or free swimming organisms and the benthon, which are the microscopic
organisms that exist on the bottom.
A prime concern is the algae, the microscopic green plants floating in the water
column. These organisms represent the base of the food chain in that they can
convert simple inorganic materials into organic materials with the aid of sunlight.
In addition, during the daytime or sunlight hours, these plants undergo photosynthesis, which is a process in which oxygen is liberated in the water. It has been
estimated that ¾ of the world’s supply of oxygen is generated by algae in the ocean.
In terms of the food chain, the algae are normally consumed by the zooplankton,
which are in turn consumed by larger animal forms, which may be consumed by
6 Basic Hydrology, Water Resources, and DAF Boat Plant for Lake Restoration
277