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%rophicaon process
The
of »; body of water does not exempt it from the inuence of added
waste. Two well—known cases of the massive dismption of an aquatic environ—
ment have occurred in Lake Brie and. in the Baltic Sea {21}. In both instances,
natural conditions—the shallowness of Lake Erie and the Baltic’s long periods
of stagnation—make excessive inputs of nutrients extremely hazardous to the
biological health of the environment.
To uncover the basic causes of eutrophication requires a consideration of
the factors involved in disturbing the equilibrium of the ecosystem. First of
all, certain lakes, because of their physical or chemical characteristics, are
much more likely to be adversely affected by overfertilization. In deep lakes,
for instance, a barrier develops between top and bottom waters because of
thermal stratication: Surface water heated by the sun is less dense than the
cold layers below, so the less dense layer oats on top of the denser layers in
the same way that oil oats on water. This division creates a denite barrier
against the transport of dissolved materials from one layer to the other. If no
mixing ever occurred, the biological activity necessary to break down the
dead plants and animals that sink to the cold bottom layer would completely
deplete the oxygen there. Nutrients trapped in this hypolimnion, if never
mixed with upper layers, would be unavailable for the growth of oxygen-producing plants at the surface Which in turn help to renew the reduced dis—
solved oxygen supply. This potential barrier to biological activity is usually
overcome naturally in the spring and fall each year. At those times, essentially complete vertical mixing occurs, redistributing oxygen and nutrients
and permitting normal biological cycles to continue. However, in a shallow
lake, such as the western basin of Lake Erie which is only 6.9 meters deep,
there is less thermal stratication, nutrients are always available, and algal
blooms may become much more frequent. As more and more algae die and
sink to the bottom, the lake becomes more and more shallow. The rate of
eutrophication will increase signicantly when the lake becomes shallow
132
Overfed
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enre:enfs
nr:
3î” ,lîäiî‘.”ä”îrîEÎ/“ :::nt:‘cile.
»:?efiñc
Witch
iemonsî‘rates
;“zuman
_nvclvemenr 11 je eu.—
%rophicaon process
The
of »; body of water does not exempt it from the inuence of added
waste. Two well—known cases of the massive dismption of an aquatic environ—
ment have occurred in Lake Brie and. in the Baltic Sea {21}. In both instances,
natural conditions—the shallowness of Lake Erie and the Baltic’s long periods
of stagnation—make excessive inputs of nutrients extremely hazardous to the
biological health of the environment.
To uncover the basic causes of eutrophication requires a consideration of
the factors involved in disturbing the equilibrium of the ecosystem. First of
all, certain lakes, because of their physical or chemical characteristics, are
much more likely to be adversely affected by overfertilization. In deep lakes,
for instance, a barrier develops between top and bottom waters because of
thermal stratication: Surface water heated by the sun is less dense than the
cold layers below, so the less dense layer oats on top of the denser layers in
the same way that oil oats on water. This division creates a denite barrier
against the transport of dissolved materials from one layer to the other. If no
mixing ever occurred, the biological activity necessary to break down the
dead plants and animals that sink to the cold bottom layer would completely
deplete the oxygen there. Nutrients trapped in this hypolimnion, if never
mixed with upper layers, would be unavailable for the growth of oxygen-producing plants at the surface Which in turn help to renew the reduced dis—
solved oxygen supply. This potential barrier to biological activity is usually
overcome naturally in the spring and fall each year. At those times, essentially complete vertical mixing occurs, redistributing oxygen and nutrients
and permitting normal biological cycles to continue. However, in a shallow
lake, such as the western basin of Lake Erie which is only 6.9 meters deep,
there is less thermal stratication, nutrients are always available, and algal
blooms may become much more frequent. As more and more algae die and
sink to the bottom, the lake becomes more and more shallow. The rate of
eutrophication will increase signicantly when the lake becomes shallow
132
Overfed