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In Maine, we can observe two main types of standing water: lakes
and ponds. There is not a sharp distinction between them, but generally,
a pond is defined as a body of water in which light can reach the
bottom in all places. In contrast, light cannot reach the bottom in
deeper spots in a lake. Lakes are bodies ofwater greater than one
acre in size; ponds are smaller and shallower. Great Ponds are
lakes that are larger than ten acres.
Succession is the gradual replacement of one community of living things
to another. It occurs in all ecosystems. In a lake, since it is quite
deep, sufficient light cannot penetrate down to the bottom to promote
plant growth. The bottom, for the most part, is bare. With time runoff
from the land and decay from dead organisms adds to the sediments
at the bottom of the lake. The lake becomes shallower, eventually
allowing light to penetrate to depths that enable green plants to
grow on the bottom. The bottom soon becomes covered with vegetation,
which adds more organic matter to the bottom, making the lake even
shallower. Next, plants start to emerge above the surface of the water.
The decay of these emergent plants fills the pond even more. Gradually,
the open space of water closes. As the area dries, shrubs will begin
to grow, then trees. Soon the lake will be filled in completely.
A change in plant life brings a change in organisms inhabiting the
area. Eventually the entire ecosystem is changed.
Phytoplankton forms the base of most aquatic food chains. The phytoplankton
mainly live in the upper few meters of a lake. Phytoplankton are producers,
and require light for photosynthesis.
Consumers are found at all depths of a lake. Herbivores hang out at
the top layers, grazing on phytoplankton. Carnivores spend their time
in various depths, perhaps as much as several hundred meters deep.
These consumers, along with the decomposers at the bottom, need oxygen.
Yet photosynthesis does not occur at such depths. How does oxygen
get to the bottom of deep lakes? It does so by an interesting process
called overturn.
As ice on a lake melts in the spring, the cold water that is formed
sinks to the bottom. Gradually the bottom of the lake is filled with
cold water. This mixing of water carries oxygen to the bottom. Water
mixing from bottom to top also carries nutrients to the top which
supports the growth of phytoplankton. This process is called
spring overturn. For a time in late spring, due to wind and wave conditions,
the water will achieve a uniform density and temperature throughout.
As the summer approaches, the sun warms the upper layers of the water
faster than the wind can mix it. By mid summer, the lake usually has
three layers. The upper layer or epilimnion, contains warmer water
that circulates freely. The water becomes too warm for cold-water
fish to tolerate, but Algae and phytoplankton photosynthesize in the
top layers, adding nutrients and oxygen to the water. The lower layer,
or hypolimnion, contains colder water that does not circulate well.
In between is a layer of transition from warm to colder water called
the mesolimnion, or thermocline. Because of density differences, the
hypolimnion is cut off from circulation, causing the oxygen remaining
there to be used up. The lake will reach summer stagnation, when the
bottom layer becomes unsuitable for life that requires large amounts
of oxygen.
As the fall approaches, the top layer cools and gradually reaches
deeper. At a point in the fall, the water at the surface will rapidly
cool. The cooler, denser, more oxygenated water will sink, displacing
nutrient-rich water on the bottom. This is called fall overturn.
The fall winds are now able to form another mixing of water.
With the coming of winter, the upper part of the lake cools even more.
This cooling increases the density of water. The cool, denser water
sinks to the bottom, allowing the less dense ice to float on top.
Three layers form; frozen water at the top to warmer water at the
bottom. No additional oxygen will reach the bottom until spring turnover.
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