LIMNOLOGY 613
Temperature-related characteristics of water have a
large effect on the water quality and ecology of lakes. Water
is at its densest at 4C or 39F, then expands (becomes less
dense) until it freezes at 0C (32F). This anomalous expan-
sion of water allows ice to float and form at the surface
of lakes at 0C (32F) or less, and thermal stratification to
occur during the warmer, summer weather. During spring
and early summer, energy from the sun heats the upper
water layer. The warmer, less-dense surface waters float
on top of the cooler, denser bottom waters. This results in
the upper layer, or epilimnion, becoming isolated from the
lower layer, or hypolimnion (Figure 3). The two layers are
separated by the middle layer, or metalimnion, where large
temperature changes occur with changes in depth. The
thermocline, which is located within the metalimnion, is a
horizontal plane of water across the lake through the point
of the greatest temperature change. The metalimnion pres-
ents a physical barrier to the mixing of the epilimnion and
hypolimnion. Since there is little or no exchange of water
between the epilimnion and the hypolimnion, water quality
can be quite different in each layer.
In the temperate regions of the world where there are
not strong contrasts in seasonal conditions (e.g., mild
winters and summers), this type of thermal stratification
is common during the summer and early fall. After the
summer, the epilimnion tends to cool, and by late fall or
early winter the temperature difference between the two
water layers is small enough that the winds will mix the
water throughout the lake, which will then remain fully
mixed until the onset of stratification in late spring. Lakes
that undergo this type of seasonal pattern (i.e., they stratify
once and re-mix or turn over once each year) are called
monomictic lakes. These include lakes in mountainous
regions of the temperate zones, warm regions of the tem-
perate zones, many coastal regions of North America and
Europe, and mountainous areas of subtropical latitudes
(U.S. EPA, 1990; Wetzel, 1983).
By contrast, in the temperate regions of the world with
strong contrasts in seasonal conditions (e.g., very cold win-
ters and very hot summers), lakes undergo complete turn-
over in the spring and fall separated by thermal stratification
in the summer (i.e., warmer surface waters float on top of
cooler bottom waters) and inverse thermal stratification in
the winter. Ice cover forms and floats on the surface of such
lakes under clam, cold conditions. Inverse stratification of
water temperatures occurs under the ice, in which colder,
less-dense water overlies warmer, more-dense water near the
temperature of maximum density at 4C. Some gradual heat-
ing of the water occurs during the winter under ice cover.
When the ice cover melts in the spring, the water column is
nearly uniform in temperature. If the lake receives sufficient
wind energy, as is usually the case, then the lake circulates
completely and undergoes spring turnover. Stratification
occurs during the warmer days of summer, with another
complete turnover in fall. Lakes that undergo this type of
seasonal pattern (i.e., they stratify twice and re-mix twice
each year) are called dimictic lakes and include most lakesof the cool temperate regions of the world (U.S. EPA, 1990;
Wetzel, 1983).LAKE BIOTAThe types of organisms found in a lake may include phyto-
plankton (algae), zooplankton, benthic infauna, fish, amphibia
(such as tadpoles, frogs, and salamanders), reptiles (such as
turtles and water snakes), and birds (waterfowl and shorebirds).
Lake plants and animals are interrelated via a food chain.
Algae are microscopic plants found in the lake water
column. Algal species may occur in many different forms
including filamentous, colonial, and single-celled. Algae
are easily carried by wind-generated currents and will often
accumulate in windward areas of the lake, forming surface
scums. When algae populations increase rapidly, the algae
can become a nuisance by forming high concentrations in
the water column, or even surface accumulations, called
algal blooms.
Several different algal species can usually be found in
a lake at any time of the year. A variety of environmental
factors including light, temperature, and nutrient levels,
affect phytoplankton production and the occurrence of algal
blooms. Diatoms are algae that are golden in color and con-
tain silica. They predominate in the spring and autumn due
to their ability to reproduce and grow in cooler temperatures
and less light. During the summer, increased water tempera-
tures and available light create conditions that favor green
algae or blue-green algae. Blue-green algae can form nui-
sance blooms; they are particularly problematic because
they will float to the surface, forming scums that affect the
recreational uses and aesthetic qualities of a lake. In some
lakes with high biological productivity, blue-greens domi-
nate in spring, summer, and fall.
In addition to algae, large vascular plants (plants with
roots, stems, and leaves) or macrophytes are found in lakes.
Macrophytes are classified as emergent, floating, or sub-
mersed. Emergent plants grow on the shoreline and include
cattails, irises, and purple loosestrife. Floating plants are
plants that float on the surface of the lake. They can be rooted
in the lake bottom such as water lilies or watershield or free-
floating such as duckweed. Submersed plants are rooted
plants that live below the lake surface and include pondweed
and water weed ( Elodea ). Figure 4 illustrates the community
types and common examples of plants associated with each
type (Washington State Department of Ecology, 1994).
Some macrophytes are native to the particular lake and
geographic region; others, called exotics, have been imported
or are transported to the lake from other lakes. For example,
native macrophytes in lakes of the Pacific Northwest region
of the United States include cattails, yellow water lilies, and
pondweed. Exotic or non-native plants in Pacific Northwest
lakes include purple loosestrife, white water lilies, and
Eurasian watermilfoil. Some non-native plants are invasive,
crowding out native plants and not providing useful habitat
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