Littoral zone is an
interface region between the land of the drainage basins and the open water of
lakes. This region is characterized by high plant and animal species diversity
and is commonly the site where fish reproduction and development occur. Those
rooted emergent, floating, and submersed vascular plants in the littoral zone
are collectively called macrophytes. Most lakes of the world are relatively
small in area and shallow. In such lakes, the littoral flora contributes
significantly to the productivity and may regulate metabolism of the entire lake
ecosystem.
Aquatic macrophytes
alter sediment, water quality, physical condition, and population dynamics of
the ecosystem by plant growth, metabolism, and decay. Macrophytes also provide
food, shelter, and substrate for a variety of organisms in the aquatic system.
Bacteria, algae, protozoan, rotifers, and invertebrates inhabit underwater
leaves and stems (Engel, 1990). Rotifers and small crustaceans filter bacteria,
algae, and detritus from water flowing by the plants. Snails, certain leeches,
and many insect larvae scrape algae and detritus on the foliate or beneath it.
They in turn are consumed by other aquatic insects, fish, water birds, and some
furbearers. Aquatic macrophytes alter the physical environment by intercepting
water movements and sunlight. Dense vegetation provides relatively quiet area
near shore and blocks turbulence from breaking waves, longshore currents, and
runoffs. High energy shores, exposed to erosion from winds and waves, become
calm depositional plains after plant growth (Engel, 1990). Plants at the water
surface blunt the wind, reducing its potential to stir the bottom. In terms of
casting shade and blocking water movement, plant beds reduce the amount of heat
transfer to the bottom. The foliage acts as a barrier to separate warm water at
the surface and cool water at the bottom. Also, density differences between cold
and warm water masses help maintain this separation. This temperature difference
can reach 10ūC on hot summer days (Dale and Gillespie, 1977). The temperature
gradients create warm and cool microclimates for colonizing organisms (Engel
1985). Animals unable to withstand warm water can approach the shore under
macrophytes; others can be repelled by heat radiated from surface foliage. Cool
bottom water can also reduce photosynthesis in lower leaves and delay hatching
of fish eggs, whereas warm surface water can speed up the development of
invertebrates. Microclimates which is caused by macrophytes growth thus
restructure the ecosystem (Engel, 1990). Sediment supplies nitrogen (N),
phosphorus (P), and micronutrients to aquatic macrophytes. Evidences from field
studies suggest that rooted submersed macrophytes, e. g., Hydrilla verticillata
and Myriophyllum spicatum, are capable of markedly depleting N and P in
sediment. Researchers report that various species of freshwater aquatic
macrophytes not only extract nutrients from the sediment but also release them
into the surrounding water (Carignan and Kalff, 1980). As a result, high
productivity and biomass turnover of rapidly growing macrophytes can result in
high rates of sediment nutrient loss (Barko, 1993). Thus, nutrient uptake by
aquatic macrophytes may significantly reduce sediment nutrient availability.
Aquatic macrophytes
growth also alters the sedimentation rate of the water. Sedimentation rates are
generally much greater in macrophyte beds than in the open water systems due to
the filtering capacity of macrophytes (Barko, 1993). As a result, aquatic
macrophytes not only reduce the turbulence in the water but also stabilize the
sediment. As suspended sediment is removed from the water column, light
penetration increases, so more light becomes available for photosynthesis by
aquatic macrophytes. The value of pH increases in macrophyte beds during periods
of photosynthesis. Elevated values of pH near the sediment surface tend to
increase rates of phosphorus release from sediment, which can stimulate algal
production. Algal production on macrophyte leaf surfaces becomes an excellent
source of food for grazing invertebrate communities. The growth of invertebrate
population can result in an increase in fish population. Besides, the structural
complexity of macrophyte beds can influence fish communities of aquatic systems
directly. These beds provide refugees for smaller fish, and are thus important
in predator-prey relationships. To sum up, aquatic macrophyte communities can
potentially influence bed geometry as well as biomass distribution in aquatic
systems (Barko, 1993).
In the United States,
several species of aquatic macrophytes are considered as nuisance weeds such as
Myriophyllum spicatum and Hydrilla verticillata, and overabundance of aquatic
macrophytes often occurs in most lakes during summer. The excessive growth of
aquatic plants may interfere with fishing, boating, and swimming activities and
be aesthetically displeasing. Besides, respiration by large plant masses in the
littoral zone during hours of darkness may significantly reduce oxygen
concentrations of the lake (Moore, 1994). In addition, nutrient uptake by
aquatic macrophytes may significantly reduce sediment nutrient availability.
Moreover, the decay of plant tissues after the growing season provides nutrients
and stimulates the growth of algae populations. Thus, in the United States and
Canada, aquatic macrophytes control methods such as hand harvesting, drawdown,
dredging, mechanical harvesting, sediment covers and surface shading, dyes,
chemical control, and biological control have been studied intensively for years
. For example, recently in the North America, there is a growing interest in
controlling Myriophyllum spicatum due to overgrowth of this plant population.
Myriophyllum spicatum
(Eurasian watermilfoil), a nuisance aquatic macrophyte, was introduced into the
United States from Eurasia in the late 1800's (Reed, 1977). The plants are often
found in lakes, rivers, drainage and irrigation canals, ponds, and streams
(Jones, 1993). Severe infestations of watermilfoil can restrict boat traffic,
interfere with fisheries, block water flow, and displace native species. A
typical invasion of milfoil is characterized by a pattern of explosive growth,
persisting for 5 to 10 years, followed by declining abundance (Carpenter, 1980).
This type of plant growth is often exhibited when exotic species invade an area
without their natural competitors. The absence of natural enemies in the new
habitat enables the invading species to compete with and often dominate existing
native species (Painter and McCabe, 1988). In this paper, the two most
controversial aquatic macrophyte management methods, chemical control and
biological control, will be discussed in the following paragraphs.
Chemical Control
Herbicide has been used
to control aquatic macrophyte growth for decades. Herbicide
concentration/exposure time relationship is always an issue. Rapid residue
dissipation is generally considered to be desirable from an environmental
standpoint. However, residues that disperse too quickly (via gravity flow,
tides, thermal and wind induced circulation patterns, etc.) can result in a
failure of plant control because of insufficient exposure to the herbicide.
Increased efficiency of weed control and lower dosages of chemicals requirements
are preferred. For example, the concentration of 2,4-D detected in the water
would render it nonpotable for both humans and livestock for a period of about
180 days (Anon, 1980). Its rate of breakdown is directly related to the
temperature and oxygen content of the water and whether sufficient populations
of 2,4-D degrading microorganisms are present. Due to the unique properties of
each compound, for example, rate of application, mode of action, environmental
half-life, and species selectivity, the concentration/exposure time
relationships must be developed for each herbicide and target plant (Netherland,
Getsinger, and Turner, 1993). A variety of chemicals including 2,4-D, Diquat,
Endothal, Simazine, Fenac, Dichlobenil, Acrolein, Fluridone, and copper
compounds are available for aquatic weed control. Combined use of herbicides and
metal ions has advantages such as increased efficiency of weed control and lower
dosages of chemicals required. For example, combined applications of Diquat with
CuSO4 and Endothal with CuSO4 have been shown to be effective at lower
concentrations than those required by single-applications of Diquat and Endothal
(Sutton et al., 1970; Yeo and Dechoretz, 1977). Besides, the use of invert and
bivert emulsions of herbicides has been evidenced to effectively control weeds
with 80 percent less amount of herbicides than the use of ordinary application
methods (Bitting, 1974; Baker et al., 1975). The invert or bivert adheres to the
plants, putting the herbicide in direct contact with the plant and minimizing
the concentration of the herbicide in the water. Another method of herbicide
application that can reduce the herbicide concentration in the environment and
provide season-long control is the use of slow-release formulation of herbicides
(Harris, 1973; Steward, 1981). In addition, the physical conditions of the water
body can also alter the efficiency of herbicide uses. Herbicides are usually
most effective at water temperatures about 15 to 18ūC, in water with low
turbidity, and on young plants. Water hardness and high calcium concentrations
have been shown to increase the efficiency of herbicides (Parker, 1960; Stanley,
1975). To sum up, a good timing, appropriate technique, and suitable physical
conditions to apply herbicide can reduce the amount of herbicide use.
Environmental impacts
of herbicide use have been observed and intensely studied. Herbicides are toxic
not only to plants but also to animals. Using chemicals to control excessive
aquatic weeds either directly affects the aquatic environment through herbicide
toxicity or causes secondary effects resulting from loss of weeds (Brooker and
Edwards, 1975). There is a possibility of minimizing herbicides' direct toxic
effects on aquatic life by carefully selecting herbicides and applying them
properly (Nichlos and Shaw, 1993). Another example to potentially minimize the
direct harm of herbicide use is escape routes for fish. The secondary effects of
herbicide use due to weed destruction may affect aquatic life more drastically
than the herbicide itself. For example, effects of herbicide treatments on
aquatic fauna are most dramatic on the invertebrates (Brooker and Edwards,
1974). The destruction of aquatic weeds may destroy the habitat of invertebrates
and result in a decline in the invertebrate population. Another secondary effect
is the disturbance of the oxygen-carbon dioxide balance in the aquatic system.
The decline in photosynthesis and the increase in metabolism of dying vegetation
may result in a deficiency of oxygen and an excessive amount of carbon dioxide
in the aquatic system (Brooker and Edwards, 1973). This phenomenon may result in
massive fish and invertebrate deaths where a fast-acting herbicide is used over
a large portion of the lake.
Another effect is that
the decomposition processes of plant tissues release nutrients into the water.
Not only the released nutrients but also carbon dioxide resulting from the
decrease in photosynthesis and improved light penetration resulting from the
decline in aquatic weed populations can be quickly used for growth by
nonsusceptible species. For example, planktonic algal blooms was observed
following chemical control of aquatic weeds (Brooker and Edwards, 1973). In
short, herbicide application of aquatic macrophyte management may result in a
change in water quality, deficiency of oxygen in water, decline in animal
populations, disturbance of habitat, and loss of biodiversity. The loss of
biodiversity is a result of habitat loss and is among the highest-risk
environmental problems in the United States. It is even higher than oil spills,
groundwater pollution, toxic wastes, and pesticides (Environmental Protection
Agency's Scientific Advisory Board, 1991). According to the Center for Plant
Conservation, seven hundred plant species, 30% of native freshwater fish
species, and 30% of duck populations are facing possible extinction in the
coming decade (Studds, 1994). Efforts to protect plants and wildlife on a
species-by-species basis are often ineffective or inefficient. By the time when
a species is on the endangered list, emergency actions are often too slow to be
successful. Habitat preservation could be the first step to prevent the loss of
biodiversity.
Biological Control
Due to the negative
ecological impacts on herbicide use, there is a growing interest in biological
control of aquatic weeds. Biological control uses undomesticated organisms
(usually insects, pathogens, and fish) that feed on a weed species to kill the
weed or reduce its vigor, reproductive capacity, or density. The first
utilization of insect biocontrol agents to manage a noxious plant in the United
States was in 1902. A beetle, Aerenicopsis champoini, was released in Hawaii to
control Lantana (Weber, 1956). To date, biocontrol projects have been initiated
on over 100 species of weeds in more than 70 countries (Julien, 1987). In the
United States, the U.S. Department of Agriculture (USDA), the USACE, the Florida
Department of Natural Resources, and the University of Florida are the leaders
in research programs to identify biological control methods (Cook, 1993).
Sanders (1981) and A. J. Leslie (1983) have described the steps which must be
taken in order to release an insect from quarantine to the environment as a
potential biological control agent. They are (1) seek for, usually in the native
habitat of the nuisance plant, candidate species as potential biocontrol agents,
(2) while in the native country, expurgate candidate insects for host
specificity and for possibility of attack on crop species, (3) transport
candidate insects under quarantine into the United States for further laboratory
studies on host specificity and biocontrol potential, (4) petition a technical
advisory group (TAG) for permission to release the candidate insects from
quarantine to field, and (5) release the candidate species to field sites to
establish a population.
Another biocontrol
method is releasing plant pathogens. The most well-known plant pathogen as a
biological control agent was Mycoleptodiscus terrestris which was isolated to
control Eurasian watermilfoil. The fungal pathogen appeared generally specific
for watermilfoil, and its impact on watermilfoil in laboratory and greenhouse
tests was enormous. The pathogen significantly reduced (80 percent of)
watermilfoil biomass in laboratory and small-scale greenhouse studies (Theriot,
Cofrancesco, and Shearer, 1993). However, the fungus did not become well
established in watermilfoil stem tissues when this pathogen was applied on the
field. The viability and virulence of the fungus could well account for poor
field performance and the failure of the experiment (Shearer, 1993).
Numbers of fish have
been suggested as biological control agents of aquatic weeds. These include the
common carp (Cyprinus carpio) and the Israeli strain of the common carp, roach (Rutilus
rutilus), rudd (Scardinus erythopthalmus), various species of tilapia including
Tilapia zillii and Tilapia mossambica, silver dollar fish (Metynnis roosevelti
and Mylossoma argenteum), white amur (Ctenopharyngodon idella), and a variety of
hybrids of the white amur. The most promising fish appears to be the triploid
hybrid between the white amur and the bighead carp (Hypopthalmichys nobillis).
All of these species are not native to the United States (Cook, 1993). Among
those fish, the most extensively studied herbivorous fish is the white amur, or
Chinese grass carp. At least two States, Arkansas and Iowa, use white amurs as a
standard management tool (Nichols and Shaw, 1993). Under good conditions, a
white amur eats the same amount of weeds as its body weight every day (Stott,
1972). However, the environmental concerns about using white amur center on four
areas. They are white amur's (1) potential to become a pest, (2) potential for
recycling nutrients or causing other water quality problems, (3) selectivity of
feeding habits, and (4) ability to destroy aquatic habitat (Nichols and Shaw,
1993). Grass carp provide long periods of plant control (8 to 10 years or more
for diploids, 3 to 4 years for triploids). However, the disadvantage of using
fish as a biological control agent is that fish are difficult to capture and
remove from the lake once they are stocked. As a result, when grass carp are
adopted as a plant management technique, lake users are committed to a long term
change in their lakes (Cook, 1993).
The target weeds for
biocontrol are typically introduced plant species that are not consumed by
indigenous organisms. They thus have an advantage over native plants, which are
subject to consumer pressure from pathogens, insects, and vertebrate herbivores.
Consequently, the introduced weed dominates and excludes native plants and
animals from their habitat. Thus, potential biological control agents were
tested and introduced to balance the system. However, fear has been expressed at
most public meetings on weed biological control. When they have depleted the
supply of the target weed, the biological control agents may attack rare,
desirable native plants related to the target weed as well as various crop and
desirable plants (Harris, 1988). For example, Hypericum perforatum L. is an
herbaceous European plant and introduced into the U.S. as a garden flower in
1793. By 1900, it had spread westward into California, and a total of 2 million
ha in the western United States and Canada were infested by 1940 (Goeden, 1978).
H. perforatum is toxic and formed dense stands that excluded other herbaceous
plants. Thus, three foliage feeding beetles, Chrysolina hyperici, Chrysolina
quadrigemina, and Chrysolina varians, which had been investigated and applied
successfully by the Australians in 1939, were released in the United States
between 1945-1950. The result of this application was successful, and H.
perforatum was reduced to less than one percent of its former density in all but
a few habitat (Harris, 1988). However, the problems did not start to surface
until 1975 with reports from California that C. quadrigemina was damaging the
introduced ornamental plant called rose of sharon, Hypericum calycinum L. Also,
the beetles were found attacking the native herbaceous gold wire, Hypericum
concinnum Benth (Andres, 1985). Before the beetles were released into the field,
scientists in Australia had done feeding tests on 75 plant species in 35
families to show that these plants were safe from attack (Currie and Garthside,
1932). However, the Australian tests were designed to show that the test plants
were not at risk. There are too many desirable plants to test them all, thus the
tests have little predictive values. There is always fear of the possible
problems in the future. Another example is Lantana, Lantana camara L., an
ornamental shrub created in Britain and Germany by hybridizing wild South and
Central American species (Harris, 1988). L. camara occupied much of the
previously cultivated land at serere, Uganda, and the remaining land was
surrounded by L. camara thickets. The tingid Teleonemia scrupulosa Stal was
introduced and released to control the L. camara population in 1963. Teleonemia
scrupulosa Stal was first introduced into Australia in 1936 after extensive
feeding tests, and it has since been used as a biological control agent in 22
countries. The prevalence of T. scrupulosa successfully kept L. camara in a more
or less defoliated condition and killed many stems (Greathead, 1968). However,
the problem was not discovered until several years later. T. scrupulosa attacked
one variety of sesame crop (Sesamun indicum L.). The insect caused loss and
distortion of the sesame by its feeding and laying of eggs in the tissue.
Another problem caused
by releasing biological control agent is that when the host plant continues to
be abundant but is unavailable to the insect at a certain time of year, annual
occurrence of a large number of hungry insects will be the result. The abundant
hungry insects may attack nontarget plants and cause damage. A more recent
example is the weevil Neochetina eichhorniae, which has been released against
water hyacinth in the United States (Center, 1982). After the collapse of the
host plant population in the early fall, massive populations of adults cause
minor damage to ornamental canna, Canna spp., as well as to the water plant,
pickeral weed, Pontederia spp. A better approach for this situation recommended
by scientists is to introduce a competing insect into the ecosystem (Harris,
1988). For example, the seed-head gall fly Urophora solstitialis L. was
established to compete with R. conicus which was introduced as a biological
control agent. The presence of the fly reduced the host weed population, and the
two insect species were both at low densities (Zwolfer, 1979). However, there is
still uncertainty. The competing insect may also attack nontarget plants in the
future. Like the above examples, the attack may not be predicted in advance. The
problems may come to the surface after decades of the application. Even though
there is always fear in the future, some scientists still support biological
control. Bell (1983) reported that although biocontrol agents may attack rare
native plants related to the host, not releasing them may endanger other native
species. For example, in Australia, over 50 plant species are considered
endangered because of competition from introduced plants (Bell, 1983).
Another example is the
Brazilian red fire ant, Solenopsis invicta, which was out of control in the
southeastern United States and Puerto Rico (Grisham, 1994). The fire ant has
killed off as much as 40% of all native insect species in some areas. It was
also threatening to other insects, wildlife, and people. Besides, up to 90% of
total native ant populations, and 70% of all native species of ant have been
wiped out in some areas. It also damaged vertebrate, bird, livestock, and pets.
Researchers at the University of Texas were looking for biocontrol agents to
stop the fire ant's spread. Another evidence which supports biocontrol is that
the biocontrol of a dominant plant increases plant diversity (Harris, 1988). For
example, at Loftus, California, after the control of H. perfortum by C.
quadrigemina, there was at least a 35% increase in the number of plant species.
However, instead of increasing plant diversity, biocontrol may simply allow
another introduced weed to occupy the habitat (Harris, 1988). For example, H.
perforatum was previously dominated in some sites of British Columbia. After the
biocontrol application, the habitats were recently occupied by another
introduced weed, spotted knapweed (C. maculosa).
Obviously, a solution
to one problem does not guarantee that other problems will not occur. Ecologists
are currently debating whether biological control should be used against native
plant species. Some scientists believe that biocontrol can reduce individual
plant species without affecting the functional ecological structure and
processes of a region (Johnson, 1985). In contrast, other scientists pointed out
that as most of the native target plants are ecological dominants, their control
was likely to render the habitat unsuitable for many native organisms
(Pemberton, 1985). It seems likely that biological control will be used against
native plant species in the near future in the United States, if the beneficial
ecological impacts are greater than the negative ones (Harris, 1988).
Conclusion
"A new code of
ethics, environmental ethics, is required to provide a basis for the protection
and preservation of nature, rather than its exploitation for what humans
consider to be their needs and interests," says Bankowski. Improving the
quality of human life and maintaining a healthy balance of nature are two
principles to be considered while managing aquatic macrophyte populations. Most
problem plants in the United States, particularly in aquatic and wetland
environments, are exotic species. The plants usually have been introduced into
favorable environments without their natural enemies. Outcompeting the native
vegetation for habitat and resources, these exotic plants have the ability to
increase rapidly and result in overgrowth of plant populations. Many aquatic
macrophytes control methods have been applied to control the plant populations,
and evidences of beneficial and negative ecological impacts have been found. It
is extremely important to carefully choose suitable control methods to adapt
each individual aquatic ecosystem. Sound understanding of the ecosystem of a
treatment site is indispensable, thus a detail physical, chemical, and
biological pretreatment investigation is strongly recommended. Many scientists
stated that historical records of biocontrol attempts show that there needs to
be more thorough reviews of prerelease data and better postrelease monitoring
than there have been in the past (Gillis, 1992). Gillis also pointed out "a
major problem underlying the use of biocontrol is an attitude on the part of
many government agencies that, if a new arrival becomes a problem, biological
agents can simply be introduced to control them. Instead, the real focus of
control strategies should be on cutting the influx. Once we close the door on
new species introductions, then we can figure out how to control what we already
have."
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