United States
Environmental Protection
Agency
Environmental Research
Laboratory
Gulf Breeze FL 32561
Research and Development
EPA-600/3-83-098 Dec. 1983
oEPA Project Summary
Ecological Impact of Integrated
Chemical and Biological Aquatic
Weed Control
J. V. Shireman, W. T. Haller, D. E. Colle, C. E. Watkins, II, D. F. DuRant, and
D. E. Canfield
The final report summarized herein
presents results of a four-year study of
the ecological impacts of chemical,
biological, and integrated methods of
aquatic weed control. Biological and
water quality changes occurred as
abundance of macrophytic vegetation
was altered by natural factors or
managment practices. Macrophyte
abundance strongly influenced the
structure of communities, and it was
concluded that environmental effects
of plant management programs are
determined more by the amount of
vegetation controlled than by
management technique. Also, changes
in lake hydrology and rates of nutrient
loading appear to be more important as
determinants of lake water quality than
macrophytes. Research needs for
evaluation of effects of weed control on
aquatic systems are identified.
This Project Summary was developed
by EPA'S Environmental Research Lab-
oratory, Gulf Breeze, FL, to announce
key findings of the research project that
is fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Aquatic weed infestations in lakes of
the United States and other countries
have increased dramatically during the
last several decades because of cultural
eutrophication and the introduction of
exotic plants, such as hydrilla (Hydrilla
verticillata) and eurasian watermilfoil
(Myriophyllum spicatum). These weed
infestations have severely restricted
many domestic, agricultural, industrial,
and recreational water uses, thus
increasing demand for weed control.
Because water is becoming an
increasingly valuable resource, many
user groups, concerned about the impact
of aquatic plant control techniques on the
aquatic environment, are expressing
concern for development of effective, yet
environmentally safe, aquatic plant man-
agement programs.
Formulation of aquatic plant manage-
ment programs for different aquatic
systems, however, is extremely difficult.
While scientists believe aquatic plants
are important for healthy populations of
fish and wildlife, and for proper
functioning of aquatic ecosystems, little
quantitative data are available for
determining how many aquatic plants are
necessary. Also it is very difficult to
accurately assess the level of aquatic
plant abundance that constitutes a weed
problem. The short- and long-term
economic, sociological, and environmen-
tal impacts of different control techniques
are likewise difficult to assess. For
example, aquatic herbicides are used by
many local, state, and federal agencies to
control aquatic macrophytes. Although
the herbicides are tested and eventually
registered for use by the U.S.
Environmental Protection Agency, some
other agencies and some segments of the
public believe that the long-term impacts
of chemical control are not sufficiently
known to justify the use of herbicides.
Also, since many aquatic herbicides are
not effective over long periods of time,
chemical control materials costs are very
high. Florida alone spends over $20
million annually for chemical control
programs. The grass carp (Ctenopha-
ryngodon idella) is an effective and eco-
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nomic biological control organism, but
these fish consume all macrophytic
vegetation when stocked in sufficient
numbers. Research studies on grass carp
have provided conflicting and often
confusing results, and biologists disagree
about the grass carp's ultimate impact on
the aquatic environment. These research
conflicts have led to the banning of the
grass carp in many areas, including
Florida.
Because fresh waters are used for a
variety of purposes, the costs and
benefits to various user groups must be
considered. In Florida, freshwater sport-
fishing is a very large aquabusiness,
generating over $525 million annually.
While fishermen generally prefer
vegetation, homeowners generally want
weed-free shorelines, and, quite often,
weed-free lakes. Because of their
different desires, these groups are often
in direct conflict.
This study was initiated to provide
quantitative information on the influence
of the density of aquatic plants and the
impact of chemical and biological (grass
carp) management techniques on the
aquatic environment. The study
consisted of three separate projects in
Florida. Orange Lake, a large lake with
an abundance of macrophytes, was
studied to determine the effect of natu-
rally occurring fluctuations in vegetation
and the effect of different vegetation
types on the aquatic environment.
Orange Lake was not treated for weed
control. Lake Pearl, a small lake (23.5 ha)
with an abundance of hydrilla, was
studied to determine if chemical and
biological control techniques could be
integrated to provide long-term vegeta-
tion management without removing all
vegetation. At Lake Pearl, herbicides and
grass carp were studied to determine the
impact of integrated management on the
aquatic environment. Finally, pond
studies were conducted to determine the
environmental impact of different aquatic
plant management techniques at
different degrees of weed management.
Effects of several herbicides and the
grass carp were determined.
Experimental Procedures
Limnetic and littoral stations were
established in Lake Pearl and Orange
Lake. Line transects were run monthly at
the lakes and at 24 artificial ponds at
Welaka to determine frequency of occur-
rence and the percentage of the cover for
weed species. Water quality was
monitored and related to density of weeds
and weed-control treatments. Water
quality parameters measured were:
dissolved oxygen, pH, carbon dioxide,
bicarbonate, carbonate, color, turbidity,
specific conductance, potassium,
calcium, magnesium, chlorophyll,
pheophytin, orthophosphate, total
phosphorus, and total nitrogen.
Periphyton was analyzed by use of glass-
slide samplers; benthos, by use of a Ponar
dredge; plankton, by vertical hauls of a
Wisconsin plankton net; and standing
crop and biomass of fishes, by blocknet
sampling. A pulsed DC current
electrofishing boat was used triannually
to collect bluegill (Lepomis machrochirus).
redear (Lepomis microlophus), warmouth
(Lepomis gulosis). largemouth bass
(Micropterus salmoides), and chain
pickerel (Esox niger). Stomachs of the
fishes were analyzed for food content.
Migration of largemouth bass and grass
carp in Lake Pearl was followed by
telemetry.
The Lake Pearl study was designed to
assess the feasibility of an integrated
approach to aquatic weed control utilizing
herbicides and grass carp. Where
herbicides are used extensively for
aquatic vegetation control, several
treatments are required annually. Grass
carp eliminate submersed macrophytes
when stocked in numbers large enough
for weed control. An attempt was made to
develop a cost-effective method for
vegetation management without elimin-
ation of all submersed macrophytes by
integrating biological and chemical
control methods.
Before initiation of control measures,
total weed biomass in Lake Pearl was
estimated with the U.S. Army Corps of
Engineers' biomass sampler. Because
hydrilla comprised over 99% of total
vegetation biomass, the lake was
considered to contain a monoculture of
that species. Herbicides used were a
mixture of diquat and copper, a mixture of
endothall and copper, and endothall
alone. Grass carp were stocked at the rate
of 12 fish/ha.
In Orange Lake, hydrilla was dominant
between July and November, while
coontail (Ceratophyllum demersum) and
southern naiad (Najas quadalupensis)
were dominant at other times.
Spatterdock (Nuphar luteum) and
maidencane (Panicum hemitomon) and
17 other macrophytic species were also
present, and natural variation in plant
numbers, water quality, and composition
of plankton, benthos, epiphyton, and
fishes were described.
Similar studies were done on 24 ponds
at Welaka, Florida. In addition to
untreated controls, the ponds wer<
treated with fertilizer, grass carp, or the
herbicides endothall, diquat, 2, 4-D, o
glyphosate.
Conclusions
Orange Lake, Lake Pearl, and Welakc
pond data indicate that hydrilla car
rapidly colonize both the littoral anc
pelagic lake regions under optima
limnological conditions. In shallow watei
systems, such as Lake Pearl, this car
eliminate the ecotone between oper
water and vegetated areas. Dense
vegetation, therefore, alters water qualit\
and native plant and anima
communities. Whether these changes
are good, bad, or indifferent depends or
the criteria used by various user groups tc
judge the quality of the lake.
Our study, like many others, demon-
strates that aquatic macrophytes may
alter water quality. In Lake Pearl, where
macrophyte abundance was initially
high, reduction in average pH and an
increase in bicarbonate, specific conduc-
tivity, calcium, magnesium, potassium,
total nitrogen, total phosphorus, color,
turbidity, and chlorophyll a, coincided
with major reduction in submersed
macrophyte abundance. No statistically
significant effects of macrophytes on
water quality, however, could be demon-
strated in Orange Lake, or Welaka ponds.
With the exception of chlorophyll a,
which was directly influenced by macro-
phyte abundance, water quality changes
in Lake Pearl were caused primarily by
hydrologic changes. This study demon-
strates that overall lake water quality
changes will occur only if macrophyte
abundance is high and generally above
levels acceptable to user groups. Alter-
ations in water quality, resulting from
aquatic plant management programs,
will generally be short-term, and lakes
will return to their limnological potential
based on material loading rates, lake
mean depth, sedimentation rates, and
hydrologic flushing rates. Overall, lake
hydrology and change in land use practic-
es within the watershed can have greater
long-term impacts on water quality than
the invasion by macrophytes or their
control.
In Orange Lake, Lake Pearl, and Welaka
ponds, native emergent and floating
leafed plants did not undergo any
reduction in coverage or frequency of
ocurrence as hydrilla expanded into these
vegetation communities. Changes in
water level seemed to affect those
communities more than the abundance
of hydrilla. Hydrilla, however, can elimi-{
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nate native submersed vegetation. This
apparently is caused by its greater ability
to utilize free CO2 at lower light intensi-
ties than native submersed vegetation.
The growth form of hydnlla, which is
characterized by dense continuous
surface mats, can also limit light
penetration, which also inhibits the
growth of other submersed species. With
increases in hydnlla, there is a trend
toward reduced algal cell numbers. This
trend is reversed when hydrilla
abundance is reduced In Lake Pearl,
following the major reduction of
macrophytes, there was a shift in
dominance from bluegreen to green
algae. Similar phytoplankton changes
were observed in Orange Lake as
bluegreen algae became dominant with
an increase in hydrilla. Whether these
changes in algal community composition
are directly related to hydrilla abundance
or indirectly to changes in other factors.
such as the abundance of specific plant
nutrients, is unknown.
In Lake Pearl, a decline in total benthic
macroinvertebrates and littoral zooplank-
ton, with simultaneous increase in
pelagic zooplankton, occurred as the
hydrilla volume decreased. In Orange
Lake, the greatest number and variety of
invertebrates were collected from the
vegetated regions, while the fewest were
collected from the nonvegetated lake
region. Among vegetative habitats,
spatterdock supported the greatest
number and variety of zooplankton
species and the largest number of benthic
macromvertebrate taxa, whereas hydrilla
supported a significantly greater number
of benthic macroinvertebrates. Results
for these lakes indicate that both the
amount and type of aquatic vegetation
influenced the abundance and species of
invertebrates collected. Within the
Welaka ponds, zooplankton and benthic
macroinvertebrate populations were not
significantly different among treatments
or among vegetation levels.
In Orange Lake and Lake Pearl, large
increases in total number and biomass of
fishes occurred when hydrilla coverage
exceeded 60%. These were due to
increases in the population densities of
small littoral fishes, such as bluefm
killifish, golden topminnows, and
bluespotted sunfish. At high hydrilla
density, these fishes became a major
biotic component. Sportfish populations
underwent shifts in length frequency
distributions, the distributions becoming
skewed toward small to intermediate-
sized individuals. Under these conditions,
there was little or no growth and very few
fish recruited into larger size classes,
which caused an overall stunting of the
sportfish populations. There also may
have been a shift in predator dominance.
For example, in Orange Lake, chain
pickerel, a fish not sought after by most
fishermen and a direct competitor of the
preferred largemouth bass, increased
greatly in abundance due to the estab-
lishment of their preferred habitat (vege-
tation). As hydrilla was removed from the
system, these trends were reversed. The
number and biomass of littoral species
dependent on submerged vegetation for
food and shelter were reduced but not
eliminated.
Numbers of young-of-the-year and
intermediate-sized sportfish declined
when no longer protected from predation
by dense vegetation cover. Concomitantly,
there was a reduction in total sportfish
biomass, but the average weight of
individual sportfish increased. These
changes in population occurred rapidly;
however, 3 to 5 years are probably
necessary for sportfish populations to
stabilize after hydrilla is removed from
the system.
Integrated plant management utilizing
grass carp and herbicides was not
successful for managing aquatic plantsto
prescribed densities in Lake Pearl.
Herbicide treatments were required
frequently, and control was not always
precise. Introduction of grass carp did not
control regrowth in treated areas; thus
herbicide usage in the treatment area
was not reduced. In the untreated area,
grass carp reduced biomass of hydrilla
only in small areas until stocking rates
reached 16 fish/ha. This stocking rate,
however, eliminated all hydrilla from the
system, suggesting that without very
intensive grass carp management, as in
the relatively small Welaka ponds, it is
unlikely that submersed vegetation can
be managed for and maintained at a
specific density. Floating leafed plants,
however, increased in Lake Pearl,
suggesting that large numbers of grass
carp will not necessarily reduce or
eliminate floating leafed vegetation.
However, the long-term impact of grass
carp on this type of vegetation is
unknown.
The calculated monetary values for the
Orange Lake fishery, as determined from
creel survey data, indicate that a large
economic loss can result from hydrilla
infestation. Hydrilla infestation was low
in 1979, and when creel survey data for
that year are used as baseline to estimate
fishery values, a 45% reduction m
monetary value occurred in spring 1978,
and a 38% reduction in spring 1982 when
infestation was high. Although the fall
fishery provides only 30% of the total
revenue, it contributes almost a quarter
of a million dollars to the local economy
each year. Hydrilla, during its peak
abundance in 1977, indirectly reduced
the income derived from sportfishing by
90% in that year. Reduced revenues due
to the presence of hydrilla in Orange Lake
were not caused by decrease in angler
success, but were the result of dramatic
reductions in total angler usage. When
hydrilla covered the entire lake, only indi-
viduals who lived close to Orange Lake
fished, which resulted in the closure of
several fish camps during 1977.
This study was not able to demonstrate
definitively any direct impact of herbi-
cides or grass carp on water quality or on
invertebrate and fish populations.
However, the data clearly demonstrate
that biological and water quality changes
occur as abundance of vegetation is
altered by natural and anthropogenic
factors. This suggests that the potential
environmental impact of various aquatic
plant management programs will be
determined more by the amount of vege-
tation controlled than by the control
method used, whether it be herbicides
(used according to label instructions) or
grass carp.
Recommendations
(1) The study data clearly indicate that
grass carp can eradicate submersed
vegetation from lake ecosystems at a
fraction of the cost of herbicide methods.
The long-term impact of complete
removal of submersed vegetation on the
aquatic environment is not known and
should be determined.
At least 3 to 5 years of study are
necessary to determine the ultimate
impact of vegetation removal because
many fish species are long-lived and
water quality is significantly influenced
by long-term lake hydrology.
Consequently, research funds are
needed for long-term research projects
where aquatic vegetation has been
eradicated by grass carp. If macrophytic
vegetation is not necessary for the
functioning of the ecosystem, grass carp
could be used as a cost-effective biologi-
cal control for aquatic weeds.
(2) Continua of lake types with regard
to trophic conditions and biological
communities exist in lakes in the United
States. In the last decade, limnologists
and fishery biologists have developed
empirical models based on regression
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analysis between important state
variables to describe the various lake
types. These models have become
increasingly important to researchers
and those charged with managing lakes
because they allow quantitative
predictions of change in important
variables, given changes in other
variables, and provide the basis for the
development of further hypotheses
concerning the functioning of lake
ecosystems. Most empirical relation-
ships, however, have concentrated on
prediction of parameters associated with
phytoplankton or fish communities, given
measurements of nutrient supply or
whole lake characteristics. It is difficult to
predict the impact of aquatic plant
management en the limnology of lakes of
different trophic status. Empirical models
are needed to predict the impact of
different densities of aquatic
macrophytes on lake limnology and
fisheries.
A survey of a large number of lakes,
ranging from oligotrophic to eutrophic,
should be conducted. Quantitative data
pertaining to nutrient loading rates, lake
nutrient concentrations, chlorophyll a
concentrations, and zooplankton and fish
biomass, as they relate to macrophyte
abundance, should be collected and
incorporated into empirical models that
can be used to predict and manage weed
growth. This type of information would
permit a quantitative environmental
assessment of the impact of different lake
management strategies on lake
ecosystems and allow the development of
rational cost-benefit analysis.
(3) Studies in Russia, Poland, and Israel
have demonstrated that ichthyofauna
reconstruction and management have
the potential to alleviate many of the
problems associated with the
eutrophication of water bodies. The
United States currently relies on
engineering techniques, such as nutrient
diversion and nutrient limitation, which
are extremely costly. Other than research
to test the feasibility of manipulating zoo-
plankton populations to control algae,
and the use of grass carp to control
weeds, there have been no major
attempts in the U.S. to use fishery
management techniques to counteract
the eutrophication process. Research is
needed to test concepts of ichthyofauna
reconstruction and plant management
developed by scientists in other parts of
the world. Native and introduced fish
species should be tested as weed control
agents and management evaluations are
made. If new methods prove successful,
considerable monetary savings could be
made as well as enhancement of both
commercial and sportfisheries values.
J. V. Shireman. W. T. Haller, D. E, Colle. C. E. Wat kins, II, D. F. DuRant, andD. E.
Canfield are with University of Florida, Gainesville, FL 32611.
Gerald E. Walsh is the EPA Project Officer (see below).
The complete report, entitled "Ecological Impact of Integrated Chemical and
Biological Aquatic Weed Control," (Order No. PB 83-264 242; Cost: $26.50,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Gulf Breeze, FL 32561
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