ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - LIMNOLOGY doc

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LIMNOLOGY
INTRODUCTION
Limnology 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 ZONES
Lakes 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 Washington
watershed 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
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 sediments
© 2006 by Taylor & Francis Group, LLC
animals (Figure 1). The littoral zone is the portion of the

LIMNOLOGY 609
Marginal zone Littoral zone
Littoral
zone
Pelagic zone
Profundal zone
Pela
g
ic zone (open water) Benthic zone
FIGURE 1 The location and nature of typical lake communities, habitats, and organisms.
In addition to the lake’s watershed, all of these components are part of the lake ecosystem.
(U.S. EPA, 1990)
© 2006 by Taylor & Francis Group, LLC
610 LIMNOLOGY
relative to the volume of water, more complete wind mixing
of the lake water, and the large littoral zone along the lake
perimeter that can be colonized by plants. Shallow lakes often
have most of their plants in littoral areas and have little pelagic
habitat. On the other hand, deep lakes have fewer areas that
receive enough light for rooted aquatic plants to grow, and
therefore have a high proportion of pelagic habitat and less
littoral habitat.
HYDROLOGIC CYCLE AND WATER BUDGET
Since precipitation and surface water runoff have direct influ-
ences on lake ecosystems, understanding the hydrologic cycle
and water budget are key concepts in limnology. The hydro-
logic (water) cycle refers to the circulation of water between
the Earth’s surface and the atmosphere. This is powered by
the sun. Water falls to Earth as precipitation. About 75 percent
of the precipitation is returned to the atmosphere as vapor

through direct evaporation and transpiration from both ter-
restrial and aquatic plants during photosynthesis. The remain-
ing 25 percent of the precipitation is stored in ice caps, drains
directly off the land into lakes, streams, wetlands, rivers, and
oceans, or infiltrates the soil and underlying rock layers and
enters the groundwater system. Groundwater enters lakes
and streams through underwater seeps, springs, or surface
channels (Cooke et al. , 1986; U.S. EPA, 1990; Wetzel, 1983).
Drainage lakes are formed primarily by inflowing rivers
and streams. Therefore, their water levels vary with the sur-
face water runoff from their watersheds. On the other hand,
seepage lakes form where groundwater intersects with the
land surface. Since seepage lakes are maintained primarily
by groundwater inflow, their water levels fluctuate with sea-
sonal variations in the local water table. For both drainage
and seepage lakes, the balance between water inputs and
outputs influences the supply of plant nutrients (nitrogen
and phosphorus) to the lake and the lake’s hydraulic (water)
residence time, thereby influencing the lake’s water quality
and biological productivity (U.S. EPA, 1990).
The hydraulic (water) residence time is the amount of
time that water entering a lake will remain in it or the aver-
age amount of time required to completely renew a lake’s
water volume. The amount of water entering a lake from its
watershed controls the volume of the lake. The hydraulic
residence time is calculated by dividing the water volume of
a lake by its flow rate, and varies greatly among lakes. For
example, if a lake has a volume of 500 acre-feet and the out-
flow rate is 10 acre-feet per day, then the hydraulic residence
time would be 50 days. If the hydraulic residence time of a

lake is 100 days to several years, this means that plant nutri-
ents and pollutants remain in the water column long enough
to degrade water quality and to allow plants to accumulate
(U.S. EPA, 1990; Wetzel, 1983).
Each lake has a water balance, in which water input ϭ
water output ϩ the change in the amount of water stored in the
lake. If inputs are greater than outputs, lake levels rise as water
is stored in the lake. When outputs are greater than inputs, lake
levels fall. This happens during summer droughts.
A related concept is the lake water budget, which is a
measure of the sources of water entering and flowing out of a
lake over the course of a year. A lake’s water budget is affected
by the hydrologic cycle, and the quantity and timing of water
entering and leaving the lake. Types of data used in calculating
a water budget include precipitation, stream flow into and out
of the lake, and lake surface elevation (water level). Sources
of water input or inflow include the lake inlet(s), precipitation,
surface water runoff, point source discharges, and ground-
water. Sources of water output or outflow include the lake
outlet(s), evaporation, transpiration from lake plants, ground-
water seepage, and water withdrawals for domestic, agricul-
tural, and industrial purposes. The change in storage accounts
for changes in surface elevation over the year. This change is
positive if lake volume increases over the year, negative if lake
volume decreases (U.S. EPA, 1990; Wetzel, 1983).
Land use and geology of the surrounding watershed
affect the water budget. For example, lakes in areas with
permeable soils receive inflowing groundwater throughout
the year. Lakes in areas with impervious surfaces can receive
large volumes of stormwater runoff.

PHOSPHORUS BUDGET AND LOADING
Another important characteristic of lakes is the phosphorus
budget, a measure of the sources of phosphorus entering
and leaving the lake over the course of a year. Phosphorus
is a nutrient that is essential in plant growth. The amount of
phosphorus in a lake directly influences biological produc-
tivity. The phosphorus budget will indicate if the phosphorus
in the lake is coming from within the lake, from sources in
the watershed, or from both internal and external sources.
For a given lake, phosphorus inputs (inflow loading)
–phosphorus outputs (outflow loading) ϩ net sedimentation ϩ
change in storage. This means that phosphorus inputs to the
lake equal phosphorus losses from the lake plus or minus the
change in the total amount of phosphorus stored in the lake.
Change in phosphorus storage within the lake equals the
amount of phosphorus entering the lake minus the amount of
phosphorus leaving the lake minus the net loss of phosphorus
to the lake sediments. Sources of phosphorus inputs to a lake
are the lake inlet(s), point sources discharging directly to the
lake, precipitation, surface water runoff, leachate from mal-
functioning shoreline septic tanks, other groundwater inputs,
and migrant waterfowl wastes. Sources of phosphorus out-
puts from a lake are the lake outlet(s), groundwater seep-
age, and water withdrawals for domestic, agricultural, and
industrial purposes (Cooke et al. , 1993b; U.S. EPA, 1990;
Cottage Lake, near Seattle, Washington. As indicated in the
figure, most of the phosphorus in the lake comes from the
Daniels Creek inlet and from the lake sediments. Most of
the phosphorus that leaves the lake does so via the lake outlet
(KCM, 1994; Solomon et al. , 1996).

Net sedimentation refers to the amount of phosphorous
accumulated in lake bottom sediments, i.e., the difference in
the amount of phosphorus that binds to the sediments and
© 2006 by Taylor & Francis Group, LLC
Wetzel, 1983). Figure 2 illustrates the phosphorus budget for
LIMNOLOGY 611
the amount of phosphorus released from the sediments to the
water column. In general, lake water quality will improve as
the magnitude of sedimentation increases because higher sedi-
mentation means there is less phosphorus in the water column
to stimulate overgrowth of aquatic plants (for more details, see
changes in the total amount of phosphorus in the lake water
column between the beginning and end of the year.
Phosphorus concentration differs from phosphorus load-
ing. Phosphorus loading to a lake is calculated on the basis
of the water budget for the lake and measured phosphorus
concentrations in the lake, its inlets, its outlets, precipita-
tion, surface water runoff, and groundwater. Loadings are
based on concentrations and flow rates and most accurately
express the relative impacts of various watershed sources on
lake water quality. For example, a stream that is an inlet to a
lake may have a high concentration of phosphorus. This does
not necessarily mean that the stream is a major contributor to
the lake phosphorus budget. If the stream has a low flow, it
will contribute a relatively low annual phosphorus loading.
The concept of phosphorus loading can be illustrated with
Cottage Lake
Creek
11%
Internal

29%
Daniels Creek
51%
Basin C5-
Subsurface
5%
Basin C5-Surface
2%
Precipitation
2%
Inputs
Outputs
Sedimentation
36%
Cottage Lake Cree
k
64%
FIGURE 2 Cottage Lake total phosphorus inputs and outputs.
© 2006 by Taylor & Francis Group, LLC
an analogy to a grocery bill as shown in Table 1. For each
section on eutrophication). The change in storage accounts for
612 LIMNOLOGY
grocery item, the cost is determined by multiplying the unit
cost and the number of items purchased. The total grocery
bill is the sum of the costs of all items. Likewise, for each
source of phosphorus input to a lake, the phosphorus load-
ing is determined by multiplying the flow rate for the source
(lake inlet, groundwater, etc.) and its phosphorus concentra-
tion over annual and seasonal periods. The total “phosphorus
bill” (total phosphorus loading from all sources) is the sum

of the loadings from each source.
Phosphorus loadings change in response to season,
storm events, upstream point sources, and land use changes.
For example, converting an acre of forest into residential
or commercial land typically increases the phosphorus
loading to a lake in that watershed fivefold to twentyfold.
This is because there will be increases in both water flow
(runoff from the newly created impervious surfaces) and
phosphorus concentration (deposition of phosphorus on
impervious surfaces). An evaluation of phosphorus load-
ings provides a basis for predicting lake responses to
changes in land use.
STRATIFICATION
Many swimmers notice that when they dive into a lake during
the summer, the deeper waters of the lake are much colder than
the surface waters. This is due to stratification, an interesting
temperature-related characteristic of most temperate climate
lakes.
TABLE 1
Phosphorus Loading Concept (U.S. EPA, 1990)
Grocery Bill
Phosphorus Loading
Item Source
Quantity Flow
Unit Cost Concentration
Cost of Item Loading from Source
Total Cost of All Items Total Loading from All Sources
EPILIMNION or mixed layer-warm (light) water
HYPOLIMNION
cool (heavy) water

Dissolved
Oxygen
Temperature
Profile
Low High
Low High
METALIMNION
THERMOCLINE
FIGURE 3 Thermal stratification.
© 2006 by Taylor & Francis Group, LLC
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 4ЊC or 39ЊF, then expands (becomes less
dense) until it freezes at 0ЊC (32ЊF). This anomalous expan-
sion of water allows ice to float and form at the surface
of lakes at 0ЊC (32ЊF) 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
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 4ЊC. 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 lakes
of the cool temperate regions of the world (U.S. EPA, 1990;
Wetzel, 1983).
LAKE BIOTA
The 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
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
for fish and wildlife.

© 2006 by Taylor & Francis Group, LLC
lower layer, or hypolimnion (Figure 3). The two layers are
and water weed ( Elodea ). Figure 4 illustrates the community
614 LIMNOLOGY
Aquatic plants provide many benefits, including sedi-
ment and shoreline stabilization; food source and habitat
for benthic invertebrates, fish, and wildlife; oxygenation of
the water column; and aesthetics. Most rooted macrophytes
obtain their nutrients from lake sediments rather than the
water column, and take up phosphorus that would otherwise
have been available for algal growth, thus preventing the
overgrowth of algae.
However, when there are too many aquatic plants, par-
ticularly non-native plants, the advantages turn into disad-
vantages. When a lake is shallow and nutrient-enriched, then
there can be too many macrophytes. Too many aquatic plants
can decrease the quality of fish and wildlife habitat, interfere
with beneficial uses of a lake such as swimming and boating,
and even create safety problems, i.e., swimmers can become
entangled in milfoil and other plants. When the plants decay,
they deplete the lake waters of oxygen and release nutrients
into the water column which can promote algal growth. Too
many macrophytes can make a lake look unsightly. The
advantages and disadvantages of aquatic plants are opposite
sides of the same coin; it’s a matter of degree and balance.
Many types of animals are found in lakes. Zooplankton
are microscopic animals found in the lake water column.
Examples are rotifers and water fleas, e.g., Daphnia. They
are visible to the naked eye on close inspection of a glass of
lake water. Zooplankton are important in the food web of a

lake because they eat algae and, in turn, are eaten by plank-
tivorous fish. The types and number of zooplankton pres-
ent are also indicative of lake water quality. Generally, large
grazing species improve water quality by eating algae. On
the other hand, a general decrease in the size of zooplank-
ton species, with their reduced capacity to graze the phyto-
plankton, is a response to the greater availability of bacterial
detritus resulting from the relatively ungrazed algae (Welch,
1992). Therefore, the presence of larger zooplankton in a
lake usually indicates good water quality, while the presence
of smaller zooplankton generally indicates more nutrient-
rich waters.
Benthic infauna are small invertebrate animals such as
molluscs, worms, and midges that live in the bottom sedi-
ments of lakes. They feed on detritus in the sediments and
recycle nutrients to the water. The species of benthic animals
found in a given area are usually indicative of the surround-
ing water quality. Some invertebrates, such as mayflies, are
intolerant of low dissolved oxygen conditions; their presence
in large numbers in lake ecosystems indicates good water
quality. Other invertebrates, such as oligochaetes and chi-
ronomids, are more tolerant of low dissolved oxygen condi-
tions; their presence in large numbers in a lake may indicate
the presence of pollutants or degraded water quality.
The greatest density and diversity of benthic inverte-
brates is usually found in the littoral zone of a lake, where
ample vegetation and oxygen are present. The benthic com-
munities, in turn, provide food for larger invertebrates, fish,
amphibians, and birds.
The types of fish found in a lake are influenced by water

temperature and dissolved oxygen levels. Fish such as perch,
bass, and smelt are warmwater fish and thrive in lakes where
the summer water temperature exceeds 65ЊF. The dissolved
oxygen level needs to be at least five parts per million (ppm)
in order for the fish to remain healthy. Coldwater fish such as
salmon and trout are found in lakes where the summer water
temperature is less than 65ЊF. The dissolved oxygen level
needs to be at least seven ppm for these fish. If the summer
water temperature is too high, the dissolved oxygen level is
often too low to support healthy fish populations.
Each organism in a lake is dependent on other organisms
for its food. Each lake has a natural food chain. Algae are
eaten by zooplankton. In some lakes, the efficient grazing
of zooplankton by algae can help to maintain water clarity.
Zooplankton are eaten by planktivorous fish such as long-
fin smelt and perch. Planktivorous fish are eaten by larger,
piscivorous fish such as northern squawfish and largemouth
bass. The larger fish are eaten by birds and by mammals,
found in many lakes.
Free-floating
Emergent
Planktonic algae
Submergents
Rooted, floating-leaved
FIGURE 4 Macrophyte community types.
© 2006 by Taylor & Francis Group, LLC
including humans. Figure 5 illustrates the aquatic food chain
LIMNOLOGY 615
The food chain concept involves the flow of energy
among the lake organisms and the recycling of nutrients.

Each trophic level (food chain level) transfers only 10 to
20 percent of the energy received up the chain to the next
trophic level (Kozlovsky, 1968; Gulland, 1970). This means
that a few large piscivorous fish depend on a large supply
of smaller planktivorous fish which depend on a very large
supply of zooplankton which depend on a successively
much larger base of photosynthetic production by phyto-
plankton and other aquatic plants. By constantly producing
wastes and eventually dying, all of these organisms provide
nourishment to detritus-eating organisms in the sediments,
which obtain their energy by decomposing organic matter.
Organic matter decomposition results in the recycling of
nutrients that are required for further plant production (U.S.
EPA, 1990).
PISCIVOROUS
FISH
EAT
EAT
EAT
USE
NUTRIENTS
RECYCLE
NUTRIENTS
ALGAE
PLANKTIVOROUS
FISH
ZOOPLANKTON
BENTHIC
ORGANISMS
MICROSCOPIC

1/10 IN
6"-1 FT
1–2 FT
FIGURE 5 Aquatic food chain (U.S. EPA, 1990).
© 2006 by Taylor & Francis Group, LLC
616 LIMNOLOGY
When one level of the food chain of a lake is altered, it
affects all other levels, sometimes positively and sometimes
adversely. For example, in Lake Washington, Daphnia popu-
lations increased in the 1970s.Why? The longfin smelt popu-
lation increased in the 1960s when flood control activities in
the main inlet stopped and spawning beds were no longer
damaged. Longfin smelt feed on a large crustacean called
Neomysis which feeds on Daphnia. Predation on Daphnia
was thereby reduced. This had a positive effect on Lake
Washington because Daphnia grazed on the algae, resulting
in improved water clarity in the lake.
Another example of food chain manipulation is stock-
ing a lake with piscivorous fish. When the fish are removed
by anglers, there will be more planktivorous fish which will
result in a decreased zooplankton population. Fewer zoo-
plankton will mean more algae in the lake, which could have
adverse effects on water clarity in the lake. In sum, altering
one part of a lake’s ecosystem has repercussions throughout
the ecosystem.
TROPHIC STATUS/EUTROPHICATION
Lakes are characterized according to their level of biological
productivity, or trophic status. The trophic status of a lake
depends on the concentration of chlorophyll a (the pigment
found in green plants that traps energy from the sun to enable

the plants to produce their own food by the process of photo-
synthesis), frequency of algal blooms, the concentrations of
nutrients, particularly phosphorus, and water clarity (trans-
parency). The phosphorus concentration determines how
many algae and other plants will grow in the lake. The clar-
ity of the water is influenced by a variety of factors including
algae, turbidity from sediments or other suspended particles,
and the natural color of the water in the lake. Water clarity
is measured with the use of a Secchi disk, a 20-centimeter
plastic or metal disk that is divided into alternating black
and white quadrants. The disk is lowered into the water until
the observer can no longer see it. The distance between the
lake surface and the point at which the disk disappears from
view is called the “Secchi transparency” or “Secchi depth”
of the lake.
Three trophic classifications are commonly used for
lakes. An oligotrophic lake is one in which there is clear
water, low levels of chlorophyll a and nutrients, and hence
little aquatic life. Oligotrophic lakes tend to be found in
alpine and other wilderness areas. The lakes are beautiful to
look at and are fine swimming and boating lakes, but are not
good fishing lakes unless they are stocked with fish. There
are few naturally occurring fish in oligotrophic lakes because
there are few plants or insects for fish to eat.
At the other end of the scale are eutrophic lakes. A eutro-
phic lake has murky water, high levels of chlorophyll a and
nutrients, and is full of aquatic life. Many lakes in urban and
suburban areas are eutrophic, as evidenced by algal blooms.
A mesotrophic lake is in between, i.e., is moderately trans-
parent, with moderate levels of chlorophyll a and nutrients,

and some aquatic life.
Transparency, chlorophyll a and total phosphorus (both
organic and inorganic forms of phosphorus) are most fre-
quently used to assign trophic status to lakes. The general
relationship between these lake water quality parameters and
trophic status index (TSI) is summarized in Table 2.
A lake’s natural level of productivity is determined by
a combination of factors, including the geology and size of
the watershed, depth of the lake, climate, and water sources
entering and leaving the lake. Some lakes are naturally eutro-
phic based on their inherent physical attributes and watershed
characteristics.
Increases in a lake’s natural productivity over time, a pro-
cess called eutrophication, occurs naturally in some lakes, and
may be accelerated in others by human activities. For many
small lakes, natural eutrophication typically occurs over hun-
dreds or thousands of years, and is hence not observable in
a single lifetime. What is observable in a single lifetime is
the human-induced, or cultural eutrophication of lakes. Our
land-based activities, including home-building, agriculture,
forestry, resource extraction, landscaping, gardening, and
animal husbandry, all contribute nutrients and sediments to
surface waters, which in turn contribute to increasing a lake’s
biological productivity. Land erosion and forest clearcutting
contribute sediments to lakes. Surface water runoff from
impervious surfaces such as construction sites, parking lots,
and pavement contributes nutrients and pollutants to lakes.
Agricultural practices such as horses grazing near lakes, cows
wandering in streams, and extensive pesticide use contribute
nutrients and toxic pollutants to lakes. If oil or other toxic

chemicals are poured down storm drains, these end up in the
nearby lake, stream, or bay. Gardening chemicals such as fer-
tilizers and household toxic chemicals can end up in storm
TABLE 2
Trophic status and associated values (Carlson, 1977; Cooke et al., 1993b;
Porecella et al., 1980)
Trophic Status
Transparency
(meters)
Chl. a
(mg/L)
Total Phosphorus
(mg/L)
TSI
(average)
Oligotrophic Ͼ4 Ͻ3 Ͻ4 Ͻ40
Mesotrophic 2–4 3–9 14–25 40–50
Eutrophic Ͻ2 Ͼ9 Ͼ25 Ͼ50
mg /L ϭ micrograms per liter (parts per billion).
© 2006 by Taylor & Francis Group, LLC
LIMNOLOGY 617
drains and thus in lakes. Failing septic systems can discharge
nutrients from raw sewage to lakes.
The result of all these inputs of phosphorus, nitrogen,
sediment, and organic matter in large algal blooms which
are unsightly and can severely restrict lake beneficial uses
including swimming, fishing, boating, and aesthetic appre-
ciation. Beneficial uses of a lake may also be degraded by
other water quality problems related to eutrophic conditions,
including low dissolved oxygen levels, fish kills, algal toxic-

ity, and excessive aquatic macrophyte growth.
The level of dissolved oxygen in lakes is one determi-
nant of the habitat available to aquatic organisms. Oxygen is
added to a lake from exposure to the air, and by the contri-
bution or aquatic plants through photosynthesis. Oxygen is
removed from a lake by the respiration of aquatic organisms
and plants, and the bacterial decomposition of organic matter
in the water and sediments. Eutrophic lakes with large algal
blooms are characterized by high phosphorus concentrations
and low dissolved oxygen concentrations in the lake hypo-
limnion in the summer. This happens because decaying algae
and other plants fall to the bottom of the lake where they
contribute phosphorus and remove oxygen. Photosynthesis
does not take place in the hypolimnion of a eutrophic lake
because light does not penetrate to that depth; hence, the
oxygen that is being depleted is not replaced. Anoxic (lack
of oxygen) conditions at the water—sediment interface on
the lake bottom usually increase the potential for nutrient
release by converting iron phosphate in the sediments from a
water-insoluble to a water-soluble form.
The very low dissolved oxygen levels in the hypolim-
nion of eutrophic lakes during the summer months may be
too low to support coldwater fish such as salmon and trout.
The salmon and trout would then move to the lake epilim-
nion, but the water temperatures may be too high for them in
the surface waters. Eutrophic conditions in lakes often lead
to decreased quantity and quality of fish habitat and stressed
fish populations.
RESTORING BALANCE TO LAKE ECOSYSTEMS
Management of Eutrophic Lakes

When a lake is eutrophic with unsightly algal blooms, water
quality problems, and impaired beneficial uses, its ecosys-
tem is out of balance. Lake restoration involves reducing
the impact of human activities on lake water quality, with
the goal of decreasing biological productivity and improv-
ing water quality and associated beneficial uses of the lake.
Several methods are available to accomplish this goal. Each
method has its advantages and drawbacks.
In order to determine the most effective method(s) to
use in a given lake, it is first necessary to be knowledgeable
about the physical, chemical, and biological components of
the lake’s ecosystem. This can be accomplished through one
or two years of monitoring parameters such as transparency,
lake temperature, acidity, alkalinity, dissolved oxygen, lake
level, amount of precipitation, nutrient levels, chlorophyll a,
fecal coliform bacteria (a group of bacteria associated with
human, other mammal, and bird wastes), algae, zooplankton,
benthic infauna, an fish. Once monitoring data are obtained,
they need to be summarized and pollution sources priori-
tized for control.
Most lake water quality problems are associated with an
overabundance of nutrients, which results in excessive plant
growth. In managing such water quality problems, it is impor-
tant to assess what nutrient limits plant growth. In eutrophic
lakes, phosphorus is often the limiting nutrient; this means
that the amount of phosphorus in the lake will determine the
amount of plant growth. Therefore, most lake management
strategies focus on reducing phosphorus loading.
If the lake’s phosphorus budget shows that most of the
phosphorus is coming from within the lake, in-lake restora-

tion techniques should help to reduce phosphorus levels and
make the lake less eutrophic. On the other hand, if most of
the phosphorus is coming from the watershed (this is often
true of small lakes with very large watersheds), then the focus
should be on watershed best management practices (BMPs)
to control sources of nutrients. In some lakes, phosphorus
comes from within the lake and the watershed, so both types
of actions are needed.
Watershed Best Management Practices Implementation
of watershed best management practices (BMPs) improves
water quality by reducing the quantity of pollutants enter-
ing the lake. Most pollutants within a watershed result from
human activities. Pollutants originating on each parcel of
land within a watershed can collectively become a serious
threat to the receiving water quality. BMPs are structural and
nonstructural methods, including common sense “house-
keeping measures,” used to prevent or reduce pollution by
controlling erosion, surface water runoff, sources of nutri-
ents, and sources of toxic chemicals. Watershed BMPs can
be basin-wide or can target management of developed prop-
erty. These measures can include native plant revegetation of
lake shorelines, retention/detention ponds and biofiltration
swales for stormwater treatment, and homeowner/business
owner BMPs to enhance water quality through better land-
scaping methods, alternative household and gardening prac-
tices, better animal-keeping practices, drainage controls,
and septic system maintenance and repairs. Local and state
agencies can work in partnership with lake associations and
other citizen groups in a watershed to educate residents and
business owners about BMPs that are inexpensive, easy to

implement, and make a difference in protecting lake water
quality and aquatic biota. Ideally, this environmental educa-
tion should include hands-on water quality activities (e.g.,
storm drain stenciling, lakeshore revegetation) and habitat
monitoring activities for community volunteers including
schoolchildren because these activities impart a sense of
lake stewardship to people who live on or upstream of a lake.
Following is a discussion of each type of BMP.
Many lakes have no native plants growing on the shore-
line; houses may have manicured lawns leading to the water’s
edge. Where shoreline vegetation is absent, surface water
runoff enters the lake directly, degrading lake water quality.
© 2006 by Taylor & Francis Group, LLC
618 LIMNOLOGY
Ducks and geese may also graze on the shoreline and affect
lake water quality via the nutrients and fecal coliform bac-
teria in their wastes. Native plantings along lake shorelines
and streambanks of creeks that are tributary to lakes serve
multiple functions: improving wildlife habitat; acting as a
physical barrier to intrusion by ducks and geese; increas-
ing shoreline soil stability thereby preventing erosion; and
moderating impacts of surface water runoff by filtering out
suspended solids, nutrients, and toxic chemicals.
Alternatives to standard lawn maintenance and landscap-
ing practices include minimal use of fertilizers, reduction in
lawn size, regular thatching and aeration, incorporation of
native plants in new landscaping, soil enhancement through
mulching and composting rather than chemical fertilizers,
and integrated pest management techniques rather than
chemical pesticides.

Household hazardous wastes should be properly disposed
of at collection sites and never in storm drains. Homeowners
should be educated about non-toxic alternatives to common
household cleaning products.
When cars are washed near storm drains, wash water car-
ries oils, greases, nutrients, heavy metals, suspended solids,
and soaps to local water bodies including lakes and their
tributaries. Residents of a lake watershed should be encour-
aged to wash their cars at commercial car wash facilities,
which discharge wash water to the sanitary sewer system.
People who do wash their cars at home should be informed
about draining wash water to vegetated areas such as lawns,
using a high pressure nozzle with trigger to minimize water
usage, and using commercial products that clean vehicles
without water.
The feeding of waterfowl by lakeside residents should
be discouraged. Pet and domestic animal waste should be
properly disposed of away from a lake and surface water
pathways that reach a lake.
Business owners should be educated about BMPs, such
as proper storage of toxic chemicals and proper mainte-
nance and repair of oil-water separators in order to prevent
the discharge of petroleum hydrocarbons, metals, and other
toxics to lakes and their tributaries. Many local and state
agencies provide technical assistance to businesses in pre-
venting or reducing the discharge of pollutants to lakes and
streams.
State and local agencies such as conservation districts
should also educate agricultural landowners about agricul-
tural BMPs to improve pastures, maintain healthy livestock,

dispose of or recycle livestock water, restrict livestock access
to lakes and their tributaries (e.g., building fences around
streams which are inlets to lakes), and prevent discharge of
pollutants from livestock waste and farm operations to lakes
and tributaries. Cost-sharing incentives and technical assis-
tance increase the success rate of these measures.
Maintaining on-site wastewater treatment systems (septic
systems) in good working order is another way to reduce phos-
phorus loading to a lake. Lakeside and watershed residents and
business owners should know about septic system operation
and maintenance practices, such as using no- or low-phos-
phate detergents, composting organic wastes rather than using
garbage disposals, selecting and maintaining optimal veg-
etative cover over drainfields, and inspecting and cleaning the
system on a regular basis to ensure proper system functioning.
This can be accomplished through articles in lake association
newsletters, through brochures, and through workshops con-
ducted by local and state agencies.
In-Lake Restoration Techniques In-lake restoration tech-
niques that can be used to control internal phosphorus
loading are phosphorus inactivation and precipitation (e.g.,
aluminum sulfate treatment), removing sediments from the
lake bottom (dredging), hypolimnetic aeration, hypolimnetic
withdrawal, dilution, and artificial circulation. Following is
a discussion of the principles, advantages and disadvantages
of each technique.
Adding aluminum sulfate (alum) to a lake reduces the
lake’s phosphorus content by precipitating phosphorus and
retarding its release from the sediments (Cooke et al. , 1993a).
When alum is added to the water column, a polymer forms

that binds phosphorus and organic matter. The aluminum
phosphate-hydroxide compound (commonly called alum
floc) is insoluble and settles to the lake bottom. Dramatic
increases in water clarity typically occur immediately fol-
lowing an alum treatment, as suspended and colloidal par-
ticles are removed from the water column by the floc.
Alum has been used extensively in the United States, with
general success in controlling phosphorus release from lake
sediments for several years (Cooke et al. , 1993a; Garrison
and Knauer, 1984). If external sources of phosphorus are
not controlled, the effectiveness of alum will decrease with
time as the alum layer on the sediments becomes covered by
nutrient-rich silt and organic material. The lake may there-
fore need to be treated again. Regular long-term monitoring
is required in an alum-treated lake to evaluate the effective-
ness of the treatment.
The alum dose should be based on the pH and alkalin-
ity of the lake, and the potential toxicity of aluminum to the
lake (Cooke et al. , 1986, 1993a; Kennedy and Cooke, 1982).
As alum is added to a lake, pH and alkalinity decrease and
dissolved aluminum concentrations increase; alkaline lakes
can tolerate higher alum doses than can softwater lakes.
Adding alum to a lake with low to moder ate alkalinity
requires careful planning to ensure that pH and alkalinity
are not lowered to levels that would stress aquatic biota.
The use of sodium aluminate as a buffer permits a greater
alum dose to be used. Such buffering agents have been
applied with alum in several northeastern lakes and high
success in maintaining normal lake pH and alkalinity levels
(Cobbossee Watershed District, 1988; Dominie, 1978). The

use of sodium carbonate in the alum treatment of Long Lake
in western Washington was also highly successful in main-
taining safe pH and alkalinity levels, as well as in improving
lake water quality (KCM, 1994).
Alum is a promising technique for reducing algae through
physical settling and removal during the application and
through the long-term control of internal nutrient loading. The
treatment does not kill the algae instantaneously in the water
column, but settles them on the lake bottom, where they die
© 2006 by Taylor & Francis Group, LLC
LIMNOLOGY 619
over a period of up to two weeks. This longer time period and
the location at the lake bottom greatly reduce the hazard from
toxins that might be released by the dying algal cells. Alum
can provide long-term reduction in the occurrence of algal
toxicity if internal phosphorus loading is reduced. Alum has
also been found to reduce the sediment-to-water migration of
blue-green algae in Green Lake in Seattle (KCM, 1994).
The use of alum salts may cause toxic conditions. Alum
causes zooplankton to flocculate and settle out of the water
column, along with sediment and phytoplankton, which can
stress the food chain of a lake. To date, alum treatments have
not resulted in adverse effects on fish and have not dam-
aged invertebrate populations in well-buffered lakes (Cooke
et al. , 1993a; Narf, 1990). Invertebrate populations may,
however, be more sensitive to alum application in softwater
lakes. For example, the alum/sodium aluminate treatment of
Lake Morey in Vermont (alkalinity ϭ 30 to 50 milligrams of
calcium carbonate per liter) resulted in a short-term decrease
in density and species diversity of benthic invertebrates

(Smelzer, 1990).
Although most case studies of alum treatments dem-
onstrate multiple-year success, failures have also occurred.
These have been attributed to insufficient dose, lake mixing,
inadequate reduction in external nutrient inputs, and a high
coverage of macrophytes.
Other nutrient inactivation techniques have been used
with less success than alum. Calcium hydroxide (lime) has
recently been used in hardwater Alberta, Canada lakes to
control nutrient supply and algal growth (Murphy et al. ,
1990; Kenefick et al. , 1992). However, lime would not offer
the same phosphorus-binding benefit in softwater lakes
(Cooke et al. , 1993a).
The release of nutrients from lake sediments can also be
controlled by removing the layer of the most highly enriched
materials. This may result in significantly lower in-lake nutri-
ent concentrations and less algal production. Several types of
dredging equipment can be used to remove sediments from
lakes; a hydraulic dredge equipped with a cutterhead is the
most common choice. The cutter loosens sediments that are
then transported as a slurry of 80 to 90 percent water through
a pipeline that traverses the lake from the dredging site to a
remote disposal area. In the United States, a permit from the
U.S. Army Corps of Engineers is normally required before
sediments can be dredged from a lake (Cooke et al. , 1993b;
U.S. EPA, 1990).
Sediment removal to retard nutrient release can be
highly effective. For example, in Lake Trummen (Sweden),
the upper, nutrient-rich layer of sediments was removed,
increasing the lake depth from 3.6 feet to 5.8 feet. The sedi-

ment was disposed of in diked-off bays and upland ponds.
Return flow from the ponds was treated with alum to remove
phosphorus. The total phosphorus concentration in the lake
dropped sharply (U.S. EPA, 1990).
However, sediment removal has high potential for seri-
ous negative impacts on the treated lake and its surround-
ing watershed. The disposal area must be sufficiently large
to handle the high volume of turbid, nutrient-rich water
that accompanies the sediments. Unless the sediment-water
slurry can be retained long enough for settling to occur. The
turbid, nutrient-rich runoff water will enter the lake outlet
and end up in a tributary stream or another lake downstream
of the treated lake. Turbidity, algal blooms, and dissolved
oxygen depletion may result in the receiving waters (Cooke
et al. , 1993b; U.S. EPA, 1990).
Prior to dredging, the lake sediments must be analyzed
for heavy metals (especially copper and arsenic, which have
been extensively used as herbicides), chlorinated hydrocar-
bons (which have been used in pesticides), and other poten-
tially toxic chemicals. Special precautions will be required
if these substances are present in high concentrations (Cooke
et al. , 1993b; U.S. EPA, 1990).
Another technique for preventing the release of phospho-
rus from lake sediments to the water column is hypolimnetic
aeration. This technique involves oxygenating the bottom
waters of a lake without causing destratification. Air is used
to raise cold hyplimnetic water in a tube to the surface of deep
lakes, where the water is aerated through contact with the
atmosphere, loses gases such as carbon dioxide and methane,
and is then returned to the hypolimnion. Phosphorus release

from the sediments is limited by hypolimnetic aeration if
there is sufficient iron in solution to bind phosphorus in the
re-oxygenated waters. Aeration oxidizes the soluble ferrous
phosphate to insoluble ferric phosphate, which would then
precipitate out into the sediments and remain there. In addi-
tion, hypolimnetic aeration increases habitat and food supply
for coldwater fish species. The technique has been used with
varying levels of success. Unsuccessful treatments have been
attributed to inadequate oxygen supplies, disruption of lake
stratification, or lack of sufficient iron (Cooke et al. , 1993b;
KCM, 1994; U.S. EPA, 1990).
It is important that hypolimnetic aeration not destratify
the water column. Premature destratification (e.g., before
fall turnover of the lake) can be stressful and become toxic
to aquatic life when bottom waters with little dissolved
oxygen, low pH, and high concentrations of toxic gases
mix with surface waters. Destratification can also stimulate
algal growth by supplying hypolimnetic nutrients to sur-
face waters and mixing algae throughout the water column.
In shallow lakes, destratification could occur due to wind
mixing (KCM, 1994).
An alternative to aerating the hypolimnion is to remove
this nutrient-rich, anoxic water layer either through a deep
outlet in a dam or by a siphon, thereby accelerating a lake’s
phosphorus loss and perhaps producing a decrease in phos-
phorus concentration in surface waters. There are few docu-
mented case histories of hypolimnetic withdrawal (Cooke
et al. , 1993b; Nurnberg, 1987; U.S. EPA, 1990).
There are major disadvantages to hypolimnetic with-
drawal. The hypolimnion water that is discharged may be of

poor quality and therefore may require aeration or other treat-
ment. Federal, state or local regulatory agencies may require a
permit to discharge this water. Hypolimnetic withdrawal could
destratify the water column, thereby introducing nutrient-rich,
oxygen-poor water to the surface of the lake and triggering on
algal bloom (Cooke et al. , 1993b; Nurnberg, 1987; U.S. EPA,
1990).
© 2006 by Taylor & Francis Group, LLC
620 LIMNOLOGY
Another approach to reducing the phosphorus concentra-
tion in eutrophic lakes is to dilute the lake water with sufficient
quantities of another water source that is low in phosphorus;
algal cells will be flushed out of the lake at the same time.
When water low in phosphorus is added to the inflow, the
actual phosphorus loading will increase, but the mean phos-
phorus concentration will decrease, depending upon initial
flushing rate and inflow concentration. Concentration will
also be affected by the degree to which loss of phosphorus
to sediments decreases and counters the dilution. Lakes with
low initial flushing rates are poor candidates for this tech-
nique because in-lake concentration could increase unless the
dilution water is essentially devoid of phosphorus. Internal
phosphorus release could further complicate the effort (Cooke
et al. , 1993b; U.S. EPA, 1990).
Flushing can control algal biomass by cell washout;
however, the flushing rate must be near the cell growth rate
to be effective. Flushing rates of 10 to 15 percent of the lake
volume per day are believed to be sufficient (U.S. EPA,
1990).
There are very few documented case histories of dilu-

tion of flushing because additional water is seldom avail-
able, especially water that is low in nutrients. One successful
example is Moses Lake in eastern Washington. Low-nutrient
Columbia River water was diverted through the lake. Daily
water exchange rates of 10 to 20 percent were achieved in this
eutrophic lake. Lake transparency dramatically increased and
algal blooms dramatically decreased (Welch and Patmont,
1980).
For dilution and flushing to be successful, lake outlet
structures must be capable of handling the added discharge.
The increased volume of water released downstream could
have negative effects. Water used for dilution or flushing
must be tested to ensure that no toxics are present before the
water is introduced into the eutorphic lake.
Another in-lake restoration technique is artificial cir-
culation. This eliminates or prevents thermal stratification,
through the injection of compressed air into lake water
from a pipe or ceramic diffuser at the lake’s bottom. The
artificial circulation structure must be designed properly to
ensure an air flow of about 1/3 cubic foot per minute per acre
of lake surface; this is required to maintain oxygen within
the lake. Algal blooms may be controlled through one or
more of the following processes. First mixing of algae to
the lake’s bottom will decrease their time in full light, lead-
ing to reduced net photosynthesis. Introduction of dissolved
oxygen to the bottom of a lake may inhibit phosphorus
release from the sediments (i.e., have the same impact as
hypolimnetic aeration), hence curtailing internal phosphorus
loading. A third possible process is that rapid circulation and
contact of lake water with the air, as well as the introduction

of carbon dioxide-rich bottom water during the initial period
of mixing, can increase the water’s carbon dioxide content
and lower the pH, leading to a shift from blue-green algae
to less noxious green algae. Finally, when zooplankton are
mixed to the lake’s bottom, they are less vulnerable to plank-
tivorous fish. If more of the zooplankton survive, then they
may eat more algae (Cooke et al. , 1993b; U.S. EPA, 1990).
Results of artificial circulation have been highly vari-
able. In about half the lakes where this technique has been
attempted and where temperature differences are small
between surface and bottom waters during the summer,
algal blooms have been reduced. In other cases, phospho-
rus and turbidity have increased and water transparency has
decreased (U.S. EPA, 1990).
Management of Aquatic Macrophytes
The watershed BMPs outlined above for reducing the quan-
tity of algae in a lake are also effective in reducing the
quantity of aquatic macrophytes in a lake. Watershed BMPs
involve voluntary changes in behavior and are easy and
inexpensive to implement. Any reduction in nutrient load-
ing to a lake as a result of BMPs can maintain or extend
the effectiveness of in-lake methods for managing aquatic
macrophytes. The disadvantage of watershed BMPs is that
they will not result in immediate, substantial reduction in
nuisance aquatic plant growth because habitat has already
been created in the lake that supports aquatic plant growth.
Therefore, watershed BMPs usually must be combined with
other methods to reduce aquatic plant growth.
There are physical, chemical, and biological methods for
reducing aquatic plant growth in lakes and restoring balance

to the lake ecosystem so that the aquatic plants are beneficial
rather than harmful. Following is a description of examples,
principles, advantages, and disadvantages of each method,
summarized from Aquatic Plant Control (Washington State
Department of Ecology, 1994), A Citizens ’ Manual for
Developing Integrated Aquatic Vegetation Management
Plans (Gibbons, Gibbons, and Systma, 1994), and Crary
WeedRoller Pilot Project Report (Cooke, 1996).
Physical Methods Physical methods of reducing the amount
of aquatic plants in a lake include hand-pulling, hand and
mechanical cutting, mechanical harvesting, bottom barriers
(sediment covers), water level drawdown, water column dyes,
rotovating, diver-operated suction dredging, and weed
rolling.
Hand-pulling aquatic plants is similar to pulling weeds
out of a garden. This method involves digging out the entire
plant with a spade or long knife and disposing of the residue
onshore. In waters deeper than three feet, hand removal can
best be accomplished by snorkelers or scuba divers carrying
collection bags for plant disposal. The technique results in
immediate clearing of the water column of nuisance plants
and is most appropriate for small-area, low-plant density
treatment, e.g., clearing pondweed from areas around docks
and beaches.
Hand and mechanical cutting differ from hand-pulling
in that plants are cut below the water surface (roots are usu-
ally not removed) with scythes, rakes or other specialized
devices that can be pulled through the weed beds by boat or
people. Rakes can be equipped with floats to allow easier
plant and fragment collection. Mechanical cutters can be

battery-operated and hand-held, portable and mounted on
boats, or specialized underwater cutters using a sickle to cut
© 2006 by Taylor & Francis Group, LLC
LIMNOLOGY 621
weeds in water as shallow as ten inches and as deep as five
feet. Cutting results in immediate removal of nuisance sub-
merged plants.
Hand-pulling, hand-cutting, and mechanical cutting are
inexpensive, easy to implement around docks and swim-
ming areas, environmentally safe, and allow removal of
undesirable aquatic plants while leaving desirable plants.
On the other hand, these methods are labor-intensive, time-
consuming, and may need to be repeated several times each
summer. It may be difficult for the laborer to see and dig out
all plant roots. Some plants, such as water lilies, are difficult
to cut or pull. Visibility may become obscured by turbidity
generated by cutting activities. All plant fragments must be
removed from the lake to prevent them from rerooting or
drifting onshore. This is particularly important in the case
of invasive, non-native plants such as Eurasian watermilfoil.
Environmental impacts of hand-pulling and cutting include
short-term, localized increases in water turbidity and some
disruption of benthic infauna.
Mechanical harvesting is a short-term technique to tem-
porarily remove plants that interfere with recreational uses
or aesthetic enjoyment of a lake. Mechanical harvesters are
large machines which cut plants below the water surface and
collect the plants and plant fragments for disposal. Harvested
plants are removed from the water by a conveyor belt system
and stored on the harvester until disposal. A barge stationed

near the harvesting site for temporary plant storage in an effi-
cient disposal method. Alternatively, the harvester can carry
cut weeds to shore for disposal in landfills or for use as com-
post. Harvesting is usually performed in late spring, summer,
and early fall when submersed and floating-leafed plants have
reached or are close to the surface of the lake. Harvesters
can cut and collect several acres per day depending on plant
type, plant density, and the storage capacity of the equipment.
Depending on the equipment used, the plants are cut from 5
to 10 feet below the water’s surface in a swatch that is 6 to
20 feet wide. Harvesting is most appropriately used for large,
open areas with few surface obstructions.
Like mechanical cutting, harvesting results in imme-
diate open areas of water and can be targeted to specific
locations. The lake can continue to be used for recreational
purposes while harvesting is underway. Another advantage
of mechanical harvesting is that removing plants from the
water also eliminates a possible source of nutrients often
released during fill dieback and decay. Furthermore, harvest-
ing can reduce sediment accumulation by removing organic
matter that normally decays and adds to the bottom sedi-
ments. Harvested vegetation can often be easily composted
and used to enrich soil.
Mechanical harvesters share the previously mentioned dis-
advantages of creating plant fragments and requiring repeated
application. Several disadvantages are unique to mechani-
cal harvesting. Off-loading sites and disposal areas for cut
plants must be available. On heavily developed shorelines,
suitable off-loading sites may be few and require long trips
by the harvester. Some large harvesters are not easily maneu-

verable in shallow water or around docks or other obstruc-
tions. Furthermore, harvesting can be detrimental to non-target
plants, insects, and small fish; these are often removed from
the lake along with target plants.
The use of bottom barriers (sediment covers) is an
effective physical method of plant control. Barrier mate-
rial is placed like a blanket over the lake bottom to prevent
plants from growing. Applications of bottom barriers can be
made up to any depth, with divers often utilized for deeper
water placement. Readily available materials such as burlap,
plastics, synthetic rubber, polypropylene, perforated black
mylar, fiberglass screens, woven polyester, and nylon film
can all be used as bottom barriers. There are also commercial
bottom barriers which are specifically designed for aquatic
plant control. These include Texel, a heavy, felt-like poly-
ester material, and Aquascreen, a polyvinylchloride-coated
fiberglass mesh which resembles a window screen.
The ideal bottom barrier should be durable, reduce or
block light, prevent plants from growing into and under the
fabric, be easy to install and maintain, and should readily
allow gases produced by rotting weeds to escape without
“ballooning” the fabric upwards. Even the most porous
materials, such as window screen, will billow due to gas
buildup. Therefore, it is very important to anchor the bottom
barrier securely to the bottom of the lake. Unsecured barriers
can create safety hazards for swimmers and boaters.
The duration of plant control depends on the type of
material used, application techniques, sediment composi-
tion (bottom barriers are difficult to place on deep muck
sediments), the rate that plants can grow through or on

top of the bottom barrier, and the rate that new sediment is
deposited on the barrier. Installation of bottom barriers is
easiest in winter or early spring when plants have died back.
In summer, plants should be hand-pulled or cut first in order
to facilitate bottom barrier installation. Bottom barriers may
also be attached to frames rather than placed directly onto
the sediment. The frames may then be moved for control of
a larger area.
Bottom barriers can provide immediate removal of nui-
sance plant conditions upon placement. It is easy to install
bottom barriers in small, confined areas such as around
docks, moorages or beaches. Other advantages of bottom
barriers are that they are hidden from view, do not interfere
with shoreline use, and do not result in significant produc-
tion of plant fragments.
Disadvantages of bottom barriers include high cost of
some materials, suitability only for localized plant control,
possible regrowth of plants from above or below the bar-
rier, and the need for regular inspection and maintenance to
remove accumulations of sediment and any rooting plant frag-
ments. Bottom barriers can also cause localized decreases in
the populations of benthic infauna such as aquatic insects.
A fifth type of physical plant control is water level draw-
down. This involves exposing plants and root systems to
prolonged freezing and drying or hot, dry conditions to kill
the plants. Drawdown is usually performed during the winter
and is more common in management of aquatic macrophytes
in reservoirs and ponds than in natural lakes. Accurate iden-
tification of target plant species is important because aquatic
plants vary greatly in terms of susceptibility to drawdown.

© 2006 by Taylor & Francis Group, LLC
622 LIMNOLOGY
In addition to controlling aquatic plant biomass, drawing
down the water level makes it possible to use several other
lake restoration or improvement procedures. For example,
water level drawdown can be used for fish management to
repair structures such as docks or dams, to facilitate local-
ized dredging or bottom barrier placement, or to remove
stumps or debris. This technique can result in compaction
of certain types of sediments, such as mucky substrates and
thus improve shoreline use. Drawdown can reduce nearshore
vegetation, thereby reducing potential inputs of nutrients to
the water from seasonal dieback of aquatic plants. Drawdown
can also be used to attract waterfowl by enhancing growth of
emergent plants such as cattails and bulrushes.
A disadvantage of water level drawdown is that it is not
species-selective; hence beneficial plants may be removed
along with nuisance plants. Wetlands adjacent to the lake can
be exposed with adverse impacts on their plant and asso-
ciated animal communities. Prolonged freezing and drying
can kill benthic infauna that are important food sources for
planktivorous fish. Lowering the water level in a lake may
result in decreased levels of dissolved oxygen, with result-
ing negative impacts on fish and other aquatic biota. During
the period of drawdown, recreational use of the lake may be
limited or unavailable.
A sixth physical plant-control method is the application
of dark-colored dyes to reduce the amount of light reaching
the submersed plants, thereby shading the plants from the
sunlight needed for photosynthesis. Several commercial dye

products are available; they impart a blue color to the water.
Best results are obtained when the dye is used early in the
growing season.
Advantages of using water column dyes for aquatic
plant control are that no special equipment is needed for
application (the dye can be poured into the water by hand
from shoreline or boat) and the dyes are non-toxic to aquatic
organisms, livestock, and humans. However, there are sev-
eral drawbacks. Water column dyes can be used only in
shallow water bodies with no outlet and are less effective
when aquatic plant growth is within two feet of the lake sur-
face. Repeat dye treatments may be necessary throughout
the plant-growing season. Water column dyes should not be
applied to lakes used for drinking water.
Another method for reducing aquatic plant biomass in a
lake is rotovation (bottom derooting or underwater bottom till-
age). Rotovators use underwater rototiller-like blades to uproot
aquatic plants. The rotating blades turn seven to nine inches
deep into the lake bottom to dislodge plant roots. Plants and
roots may then be removed from the water using a weed rake
attachment to the rototiller head, a harvester or manual col-
lection. This technique is most suitable for use in larger lakes
because of the larger size and high costs of the equipment.
Rotovation can be used year-round but is most effective
in the winter and early spring when plants have died back.
Depending on plant density and sediment type, two to three
acres per day can be rotovated. Rotovation is particularly
effective in controlling Eurasian watermilfoil and can pro-
duce a high level of milfoil control for two to three growing
seasons.

Advantages of rotovation are that it can remove the
entire plant, can decrease density of undesirable plants, and
may stimulate the growth of some desirable native plants.
Rotovated areas in the state of Washington and the province
of British Columbia have shown increases in species diver-
sity of native plants, with resulting benefits to fisheries. Fish
are not removed through rototilling as they are by mechani-
cal harvesters. Since rotovating takes place during winter
and early spring, there is no interference with peak summer-
time lake recreational activities.
On the other hand, rotovation is limited to areas with
few bottom obstructions and should not be used where water
intakes are located. Bottom sediments are disturbed which
can result in short-term impacts on water quality and ben-
thic infauna. Plant nutrients or toxic chemicals in the sedi-
ments may be released into the water. Since rotovation is
not species-selective, beneficial plants may inadvertently
be removed. Rotovation may also interfere with fish spawn-
ing or migration. Some rotovators are difficult to maneuver
around docks and in shallow water. Plant fragmentation
resulting from rotovation may increase the spread of inva-
sive weeds like milfoil.
Another physical method for controlling aquatic plant
growth in lakes is diver-operated suction dredging. This
method was used in the late 1970s in British Columbia as an
improvement to hand removal of sparse colonies of Eurasian
watermilfoil. Scuba divers operate portable dredges with
suction heads to uproot and remove individual plants from
the lake sediment. After the divers physically remove the
plants with sharp tools, the plant/sediment slurry is then suc-

tioned up and carried back to a barge through hoses oper-
ated by the diver. Plants parts are sieved out on the barge
and retained for later off-site disposal. The water-sediment
slurry can be discharged back to the water or piped off-site
for upland disposal.
Efficiency of plant removal is dependent on sediment
condition, density of aquatic plants, and underwater visibil-
ity. Diver-operated suction dredging is best used for localized
infestations of low plant density where plant fragmenta-
tion must be minimized. Therefore, this technique has great
potential for milfoil control and can remove 85 to 97 percent
of milfoil from a lake.
Advantages of diver-operated suction dredging are that
it is species-selective and site-specific. Disruption of sedi-
ments and plant fragmentation are both minimized. The
method can be used to cover areas larger than practicable
for hand pulling or cutting, and can be conducted in tight
places or around obstacles that would preclude use of larger
machinery.
Drawbacks to this method are that it is labor-intensive
and expensive. The usefulness of this method may be reduced
in dense plant beds. Returning the water-sediment slurry
directly to the lake may result in some loss of plant fragments.
If the dredged slurry is disposed of upland, more specialized
equipment and materials are required and the process is much
more costly. Short-term environmental impacts include local-
ized increases in turbidity and release of nutrients and other
contaminants from the sediments. Some sediments, benthic
© 2006 by Taylor & Francis Group, LLC
LIMNOLOGY 623

infauna, and non-target plants may also be inadvertently
removed during this process.
A new mechanical method of controlling aquatic plant
growth is weed rolling. The method uses a commercially
available, low-voltage power unit that drives an up-to-25-foot
long roller set on the lake bottom through an adjustable arc of
up-to 270Њ. A vertical drive head mounts to a dock. A reversing
action built into the drive automatically brings the roller back
to complete the cycle. The 25 feet of roller includes flexi-
ble couplers to follow the contour of the lake bottom. The
device operates on the principle of the “well-worn path.”
Whole plants or plant stems and leaves can be removed.
Fins on the rollers detach some plants from the soil, while
the rollers force other plants flat, gradually inhibiting growth.
Detached plants and plant fragments need to be removed from
the water with a rake or net.
Once plants are cleared from the area, the weed rolling
device can be used weekly or less often to prevent regrowth.
The device requires little maintenance, lasts for at least five
years, and can be moved from dock to dock which allows
sharing by a lake association.
A weed rolling device was tested on three lakes in King
County, Washington, during the summer of 1995 and was
found to be easy to operate and effective in reducing the den-
sity of both milfoil and water lilies. Weed rolling has also
been effective in removing unwanted aquatic plants from
many Midwestern lakes.
Advantages of weed rolling are that it creates and main-
tains areas of open water adjacent to docks, installation is
simple, operating costs are low (similar to the costs of using

a 75 watt lightbulb), and the treatment area can be modi-
fied by varying the number of roller tube sections used and
adjusting the roller tube travel arc.
There are several disadvantages. Although it is easy to
operate a weed rolling device, collection of the resulting
plant fragments is labor-intensive. If the plant fragments are
not collected, invasive plants may be spread from one area of
the lake to the other. Weed rolling may disturb some bottom-
dwelling animals and may interfere with fish spawning.
There are also some safety considerations. People cannot
be allowed in the water near where the weed rolling device
is operating. After each use, the rollers must be unplugged
from the power source, moved, and stored under or along-
side a dock.
Chemical Methods Chemical control of aquatic plants
in lakes is accomplished through the use of aquatic herbi-
cides. Systemic herbicides are absorbed by and translocated
throughout the entire plant, and kill the entire plant. Contact
herbicides kill the parts of the plant with which they come in
contact, leaving roots alive and capable of regrowth. To be
most effective, herbicides must be applied to plants during
the period when they are growing most rapidly. Because of
environmental risks from improper application, aquatic her-
bicide use is regulated. Some expertise in using herbicides
is necessary in order to be successful and avoid unwanted
impacts. Generally, applicators must be licensed by and
obtain permits from a local, state or federal agency. A certain
percentage of all aquatic plants in each lake must usually
remain untreated in order to provide food and habitat for fish
and wildlife.

Some aquatic herbicides that are registered with the U.S.
Environmental Protection Agency include glyphosate, fluri-
done, endothall, and copper compounds. Glyphosate (com-
mercial name Rodeo) is a non-selective, broad-spectrum
herbicide used to control floating-leafed plants like water
lilies and emergent plants like purple loosestrife and cat-
tails. It is generally sprayed or painted in liquid form on plant
leaves. Symptoms of herbicidal activity are apparent after
seven days and include wilting and yellowing of plants, fol-
lowed by complete browning and death. Since glyphosate is
a systemic herbicide, it is effective in long-term plant control.
It dissipates quickly from natural waters, is low in toxicity to
benthic infauna, fish, birds, and mammals, and does not
prohibit use of a lake for swimming, fishing, and irrigation
while treatment of plants is underway. The major draw-
back of glyphosate is that it can affect non-target plant
species. Careful application to prevent drift will minimize
this possibility.
Fluridone (commercial name Sonar) is a slow-acting
systemic herbicide used to control Eurasian watermilfoil and
other submersed plants. Applied to the lake water as either
a liquid or pellet, this chemical begins to show effects 7–10
days after application, with full control of target plants often
requiring 6–12 weeks. Fluridone acts by damaging plant
chlorophyll and preventing photo-synthesis; hence dying
plants exhibit retarded growth and bleached out leaves before
falling to the lake sediments and decomposing. The best time
for fluridone application is early in the growing season for
the target plant, usually spring or early summer, because of
the amount of time required for full plant control.

As is the case with glyphosate, long-term plant control
can be achieved with the use of fluridone. Fluridone has a
very low order of toxicity of aquatic animals and humans.
Disadvantages of its use are that it is very slow-acting and
therefore not effective in flowing water. Fluridone can drift
out of the treatment zone, thereby affecting non-target plants.
Consequently, it is most suitable for whole-lake treatments,
not for treating a defined area within a large, open lake. As the
affected plants decay, they may consume dissolved oxygen
from and release nutrients to the water column. Furthermore,
fluridone-treated water may result in injury to irrigated veg-
etation. Therefore, use of lake water for irrigation is delayed
following treatment. To protect drinking water sources, flu-
ridone should not be applied within 1/4 mile of a lake water
intake.
Endothall (commercial name Aquathol) is a fast-acting
contact herbicide which is applied in either a granular or
liquid form and destroys the plant stems and leaves but does
not kill the roots. It is used for short-term control of aquatic
plants, i.e., a few weeks to a few months, with no carryover
to the next growth season. Advantages of endothall are that
plant death occurs in one to two weeks and there is little or
no drift impact from proper application of this chemical. On
the other hand, non-target plant impacts are possible because
many aquatic plants are susceptible to endothall. Following
© 2006 by Taylor & Francis Group, LLC
624 LIMNOLOGY
treatment, there is often a period of time when lake waters
cannot be used for swimming, fishing, or crop irrigation.
Copper sulfate and chelated copper compounds are gen-

erally used to control algal blooms. Copper is an essential
element, required in small amounts, for plant growth. High
concentrations of this metal can lead to inhibition of photosyn-
thesis and to plant death. The effectiveness of copper chelates
is enhanced by warm temperatures and sunlight, conditions
that stimulate copper uptake by sensitive plants. The effect of
copper treatment can be observed within 10 days, with full
effects manifested in four to six weeks. Several treatments may
be needed each season. An advantage of using copper com-
pounds for aquatic plant control is that there are no restrictions
in lake uses following treatment; copper compounds can even
be used in lakes that are drinking water supplies. However,
copper compounds must be used with extreme care because
copper is persistent in the environment. Yearly application
of copper to lakes can result in elevated copper concentrations
in sediments, where the copper can then be taken up by ben-
thic infauna and fish. The toxicity of copper to fish is higher in
soft than in hard water.
Biological Methods The development and use of biologi-
cal methods of aquatic plant control is in its infancy. Interest
in biological control agents was stimulated by a desire to
find more “natural” means of long-term control of nuisance
aquatic plants as well as reduce the use of expensive equip-
ment or chemicals. The biological control method that is
used in many states is stocking the lake with sterile grass
carp (white amur), a vegetarian fish native to large rivers
of China and Russia. The fish are sterile because they are
triploid, i.e., they have an extra set of chromosomes. Grass
carp were first introduced into the United States in 1963 by
the state of Arkansas and are now legal in most states. The

objective of grass carp use is to end up with a lake that has
20 to 40 percent plant cover, not a lake that is totally devoid
of plants.
Grass carp are rapidly growing fish that live for at least
10 years. In general, they reach at least 10 pounds in weight
and have been known to reach 40 pounds in the southern
U.S. They feed from the top of the plant down and therefore
do not stir up lake sediments. They are most appropriately
used for lakewide, low-intensity control of submersed plants
such as water weed, water celery, and certain pondweeds.
Milfoil is less preferred and water lilies and watershield are
not eaten at all. Grass carp are dormant during the winter and
start intensive feeding when water temperatures reach 68ЊF.
The appropriate stocking rate is usually determined by the
fisheries department of the particular state in which the lake
is located and will depend on the amount and type of plants
in the lake as well as spring and summer water temperatures.
Survival rates of the fish will vary depending on fish disease
and presence of predators such as ospreys and otters.
Grass carp are inexpensive compared to some other
aquatic plant control methods and offer long-term control.
However, there are numerous disadvantages to their use.
There is no control over how much the grass carp will eat.
Overstocking of grass carp could result in eradication of
beneficial as well as nuisance plants or eradication of all
plants in the lake. Removing excess fish is difficult and
expensive. If native plants are not growing in a lake, other
plants such as invasive, non-native plants or algae will move
in to fill the void. Furthermore, the grass carp will add nutri-
ents to the lake through their wastes and through the decay of

their bodies when they die; this will contribute to the growth
of other plants in the lake.
In addition to no control over how much the grass carp will
eat, there is no control over where they will graze. The fish
may avoid areas of the lake experiencing heavy recreational
use (e.g., swimming docks), resulting in less plant removal
than desired for those areas. Substantial removal of vegeta-
tion by grass carp may not become apparent until three to five
years after introduction. Furthermore, all inlets and outlets to
the lake must be screened to prevent grass carp from migrat-
ing out of the lake into streams, rivers, and other lakes with
potential impacts on downstream, non-target plants.
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© 2006 by Taylor & Francis Group, LLC