Physiography of Lakes and Reservoirs - Freshwater Ecology: Concepts and Environmental Applications - page 100

pinch off (see Chapter 5). A levee lake is another fluvial type that is formed
next to rivers where periodic floods scour and fill depressions parallel to
river channels. The lowland areas surrounding the Amazon River contain
many lakes of this sort. They are connected to the Amazon during times
of high flow. Fluvial lakes provide an important habitat for many organ-
isms and are involved intimately with the ecology of the river. Fluvial lakes
tend to be smaller than either glacial or tectonic lakes and are less impor-
tant globally than the other two lake types (Fig. 6.5).
Additional processes that can form lakes include erosion by wind (ae-
olian lakes), crater formation by meteoric impacts, formation of depres-
sions by alligators (Alligator mississippiensis) or bison (Bos bison), accre-
tion of corals leading to lakes in the centers of coral atolls, and dam
building by beavers (Castor).

LAKE HABITATS AND MORPHOMETRY

A lake can be divided into several subhabitats. Lake habitats in gen-
eral are referred to as lentic or lacustrine (i.e., habitats with deep, non-
flowing waters). The open water of a lake, particularly that above sedi-
ments that do not receive enough light to maintain photosynthetic
organisms, is the pelagic habitat. The profundal zone is the benthic habi-
tat below the pelagic waters. The profundal zone is influenced by materi-
als that settle from the pelagic waters and usually has sediment composed
of fine silt or mud. The shallow zone of a lake, where enough light reaches
the bottom to allow the growth of photosynthetic organisms, is referred to
as the littoral zone. The relative occurrence of these different subhabitats
is determined by the size and shape of the lake.
Morphometry, or the shape and size of lakes and their watersheds, is
one of the first ways that people classify lakes. The bathymetric map (a
depth-contour map of a lake bottom) of a lake provides important infor-
mation on geomorphologic properties (Fig. 6.9). Generally the first mea-
surement made is of the area of the lake ( A ) and the second is of depth ( z ).
The maximum depth ( z max ), mean depth ( z ), and volume ( v ) are also of in-

terest. The volume is the product of area and mean depth:

v A z

In general, lakes with a low mean depth are more productive than deeper
lakes. Greater productivity of shallow lakes is a consequence of wind mix-
ing the nutrients up from the bottom more readily, more extensive shallow
habitat for primary producers that use the lake bottom, and other mor-
phometric considerations.
If the volume of a lake and the amount of water entering and leaving
the lake are known, then the retention time or water residence time of the
water in the lake can be determined. The average retention time can be cal-
culated as follows:

Retention time volume/discharge into lake

The retention time can vary widely from several hours for a small pond with
a large inflow to thousands of years for very large lakes. The water residence
time is important in determining the residence time of pollutants in a lake,