608LIMNOLOGY
INTRODUCTIONLimnology is the scientific study of the physical, chemical,
and biological factors that affect aquatic productivity and
water quality in lakes. Lakes are important resources—much
more than places for groundwater, surface water, and pre-
cipitation to collect. They control flooding, provide water
for domestic and agricultural uses, and provide recreational
opportunities such as swimming, fishing, boating, and water-
skiing. Lakes also provide habitat for insects, fish, and wild-
life such as frogs, turtles, waterfowl, and shorebirds. Lakes’
commercial value in food supply, tourism, and transporta-
tion is worth many billions of dollars each year. Lakes also
offer opportunities for relaxation and appreciation of natural
beauty. According to the North American Lake Management
Society, this quality is not a minor asset; over 60 percent
of Wisconsin lake property owners who were asked what
they valued in lakes rated aesthetics as especially important.
(U.S. EPA, 1990).
However, a lake cannot be all things to all people. Desirable
uses, even obtainable ones, can conflict. For example, swim-
mers may want no plants but some plants are needed in order
to provide fish habitat. Lakeside property owners and lake
associations often want their lake to do everything; they want
aesthetic pleasure, great fishing, clean water, sandy shore-
lines and bottoms, and a healthy wildlife population—without
insects or weeds. No lake can meet all of these demands.
This article will provide an overview of the physical,
chemical, and biological components of lake ecosystems.
An ecosystem is a system of interrelated organisms and their
physical–chemical environment. It is impossible to alter one
characteristic of a lake ecosystem without affecting some
other characteristic of the ecosystem. The article will also
explain how lake ecosystems get out of balance, and what
can be also done to restore the balance.LAKE WATERSHEDS AND ZONESLakes are receiving bodies—constantly receiving water, dis-
solved materials, and particulates from their watersheds and
from the atmosphere, and energy from the sun and wind.
A watershed is the area which drains to a lake. Watersheds
come in all sizes. For example, the watershed that drains
to Beaver Lake in western Washington near Seattle is less
than two square miles in area, whereas the Lake Washingtonwatershed is 350 square miles. Lakes are sensitive to existing
conditions in the surrounding watershed and atmosphere.
Each lake has a unique watershed, size, and shape. The
size and shape are often determined by the origin of the lake
basin and, in turn, influence the lake’s productivity, water
quality, habitat, and lifespan.
The most common origin of lake basins in North America
has been glacial activity such as the erosion of bedrock and
deepening of valleys by expansion and recession of glaciers.
Glacial lakes of Canada and the upper midwestern United
States were formed about 8,000 to 12,000 years ago. For
example, the Finger Lakes of upper New York State were
formed when deep depressions left by receding glaciers
filled with meltwater (U.S. EPA, 1990).
The depressions left by melting ice blocks form kettle
or “pothole” lakes. This type of lake is common through-
out the upper midwestern United States, the eastern portion
of the state of Washington, and large portions of Canada.
Kettle lakes and their watersheds are popular home sites and
recreational areas. The size and shape of kettle lake basins
reflect the size of the original ice block and how deeply it
was buried in the glacial debris (U.S. EPA, 1990).
Some lakes are formed by volcanic activity; i.e., a volcano
erupts creating a huge depression or caldera which then fills
with water. Crater Lake in Oregon is an example of a volcanic
lake. Movements of large segments of the earth’s crust cre-
ated Reelfoot Lake in Tennessee, Lake Tahoe in California
and Nevada, and many other lakes (U.S. EPA, 1990).
Solution lakes are formed where groundwater has dis-
solved limestone; this is the case for many Florida lakes.
Other lakes originate from shifting of river channels. For
example, oxbow lakes are stranded segments of meander-
ing rivers. The persistence of dam-building beavers can also
create lakes (U.S. EPA, 1990).
A lake has four zones, each with different plants and
animals (Figure 1). The littoral zone is the portion of the
lake that extends from the shoreline lakeward to the great-
est depth occupied by rooted plants. By contrast, the pelagic
zone is the open area of a lake from the edge of the littoral
zone to the center of the lake. The benthic or profundal zone
refers to the deep waters at the bottom of a lake where pho-
tosynthesis does not occur because light does not penetrate.
The marginal zone refers to the margins of the lake on the
lake shoreline (U.S. EPA, 1990).
Shallow lakes tend to be more biologically productive
than deep lakes because of the large area of bottom sedimentsC012_002_r03.indd 608C012_002_r03.indd 608 11/18/2005 10:36:45 AM11/18/2005 10:36:45 AM