This topic is designed to introduce the concept of aquatic weeds, their impact and management in Australia. At the end of this topic you will:
(Notes based on: Mitchell, DS and Williams, JE 2000, ‘The management of aquatic weeds’, in Sindel, BM (ed.) Australian Weed Management Systems, RG and FJ Richardson, Melbourne, Victoria, pp. 459-478.)
Water is an essential resource and is used for a variety of purposes. It is first and foremost an essential requirement for human life and is used every day for drinking, cleaning, etc. It is also used for other human activities including:
Aquatic plants are an important part of healthy aquatic ecosystems. However, populations of aquatic plants can become out of balance with the natural environment for a variety of reasons. They can then have negative impacts on the aquatic ecosystem and/or human activities. In some cases the ‘normal’ growth of aquatic plants can also have a negative impact on certain human activities. (These negative impacts will be outlined later).
There are three main types of aquatic ecosystems in Australia:
Most rivers in Australia are slow meandering streams with extensive floodplains which are flooded naturally during times of high rainfall and connect the rivers to nearby billabongs and wetlands.
Human demands for water have meant that many of our rivers have been dammed – this has consequently altered their flow regimes and led to the formation of extensive areas of standing water. The flow of rivers has also been significantly reduced by irrigation practices in some areas.
Extensive levee banks have also been built along some river margins. These levee banks have been built to reduce or prevent flooding in certain areas (particularly around towns and other settlements). Modifications to the flow and structure of rivers generally reduce the amount of water flushing through the system and also reduce natural fluctuations.
This classification also includes some man-made systems (e.g. canals and bore drains).
This classification includes natural lakes and wetlands as well as man-made systems.
Man-made systems include:
Permanent bodies of standing water tend to be more susceptible to water weed problems.
Temporary aquatic ecosystems are water bodies that form episodically, only after periods of high rainfall and run-off. These systems may be extensively flooded for a short period of time, followed by long periods of time where they are continuously dry (from several months to several years or even decades).
They are most common in the semi-arid regions of inland Australia, however they also occur in regions that have a high variability in rainfall patterns (e.g. in the parts of northern Australia where there are very distinct wet and dry seasons).
Examples include the ‘Channel Country’ in south-western Queensland and Lake Eyre in north-western South Australia. The Channel Country region consists of several creeks and rivers that are mostly dry and interspersed by large areas of flat land. However, after large rainfall events that entire system can fill with water and form temporary water bodies tens or even hundreds of kilometres in width. Lake Eyre is a temporary inland lake, which only rarely fills to its full capacity (e.g. there were only four major periods of flooding in this lake in the 100 years to 1986).
Temporary aquatic systems tend to be less susceptible to water weed problems.
Plants that grow in aquatic ecosystems usually possess certain attributes that give them an advantage in such environments. They may have attributes that allow them to:
tolerate low O2 levels in the water or substrate;
Aquatic environments often have low levels of available oxygen in the water or soil (i.e. they are anoxic). Plants that live in such habitats have physiological (e.g. different metabolism pathways in the roots, the ability to release toxic substances from the roots) and physical adaptations (e.g. internal oxygen reservoirs, or a structure that allows the internal movement of oxygen from the aerial parts to the roots) that allow them to tolerate these low oxygen levels.
float on or near the water surface;
Many aquatic plants, and particularly floating plants, have structural adaptations that provide them with extra buoyancy (e.g. they usually have a very highly developed system of air cavities or canals running through their stems and/or leaves, and some species also have water-resistant hairs that prevent water accumulating on their surfaces).
survive in low light intensities;
Submerged aquatic plants often have to survive in relatively low light intensities, especially when growing in water that has a high turbidity. Their chloroplasts are often located in the top cell layer of leaves to ensure that as much light as possible is absorbed. Additionally, they also have a low light compensation point (the point at which the rate of photosynthesis equals the rate of respiration and growth stops) that allows them to grow in conditions where they receive only 1 to 4% of full sunlight.
float on or near the water surface;
Many aquatic plants, and particularly floating plants, have structural adaptations that provide them with extra buoyancy (e.g. they usually have a very highly developed system of air cavities or canals running through their stems and/or leaves, and some species also have water-resistant hairs that prevent water accumulating on their surfaces).
respond to rapidly changing water levels;
The physical nature of free-floating plants and anchored plants with free-floating parts means that they are well adapted to rapid changes in water levels.
reproduce rapidly during favourable periods;
Plants that can reproduce rapidly have a great advantage, particularly in temporary aquatic ecosystems. Many of the most invasive aquatic weeds reproduce vegetatively at an astonishing rate, allowing them to cover large water bodies in a short period of time.
survive long periods without water, and
This is also very important in temporary aquatic systems. Some species survive dry periods as seeds, while others form resistant vegetative propagules (e.g. tubers and turions).
tolerate marked changes in the amount of dissolved salts present in the water.
The amount of dissolved salts in water can change significantly in a freshwater environment (i.e. the salt concentration generally increases as water is evaporated and decreases with rainfall). Increased levels of dissolved salts can cause water to flow out of the plant via osmosis, hence less water is absorbed by the plant and this can cause stunted growth, bleaching, and/or defoliation. Saline tolerant species are advantaged in such situations. These species have structural, physiological and/or biochemical adaptations that allow them to tolerate higher salt levels in their cells or minimise the amount of salt that enters their cells. They may also have adaptations that allow them to excrete excess salts or store them internally.
There are three types of aquatic plants; submerged plants, emergent plants and floating plants.
Submerged plants are plants that grow entirely or almost entirely below the water surface. They are usually attached to the substrate, but some parts may occasionally become detached and float near the water surface. The flowers of some aquatic plants are borne on or just above the water surface.
Some examples of submerged plants are:
Emergent plants are plants that are attached to the substrate below the water but some part of the plant emerges above the water surface (but does not float on the water surface).
Examples of emergent plants are:
Floating plants are plants that have parts that float on or near the water surface. There are two main types of floating plants, free-floating plants and attached floating plants.
Free-floating plants are those that float on the water surface and are not attached to the substrate. Hence, they are able to move freely across the water surface depending on factors such as wind and water currents.
Floating attached plants are usually rooted to the substrate, but have at least some leaves floating on the water surface. The stems and flowers of these plants do not necessarily float on the water surface. Some attached floating plants have two separate types of leaves (i.e. their leaves are dimorphic), where one leaf type is only present on submerged stems and the other leaf type floats on the water surface.
Examples of free-floating plants are:
Some examples of attached floating plants are:
Plants that grow in aquatic ecosystems are important because they contribute to the maintenance of good water quality and can help to ameliorate poor water quality. They provide an important habitat for aquatic animals and birds and are a vital food source for these creatures. Aquatic plants also add to the aesthetic appeal of landscapes.
Healthy aquatic ecosystems are often characterised by dense and vigorous stands of native aquatic plants. Aquatic plants should only be regarded as weeds if they are adversely affecting the water body, its use, or the use of the water that is contained in it (or they have the potential to have an impact if left unmanaged).
Both introduced species and native plants can be aquatic weeds.
Many aquatic weeds are introduced to Australia from overseas (e.g. alien invasive species such as salvinia, water lettuce and water hyacinth) – or introduced to one part of the country from another part of the country where it is native (e.g. broad-leaved cumbungi – Typha orientalis – has been introduced from eastern Australia to south-western WA). However, serious problems can also be caused by native plants within their natural range (e.g. starfruit – Damasonium minus – is a native plant that is a problem weed in rice crops in southern NSW and northern Victoria).
ense populations of aquatic plants in waterways and drains can restrict water flows. This can cause flooding, or an increase in the amount of flooding that occurs, along these waterways. It can also prevent or slow the drainage of flooded areas, thereby causing damage to crops, etc. Pumps and irrigation equipment can also become clogged with weeds. This can affect agricultural activities, impact on the provision of vital water supplies, and even hamper the generation of hydroelectricity.
Aquatic weeds can interfere with water transport and navigation on lakes and rivers and inhibit the use of water bodies for recreational boating and fishing, water-skiing, canoeing, and swimming. In some developing countries, where fishing is vital for survival, communities have actually been forced to migrate because dense aquatic weed infestations have made fishing impossible.
Livestock drowning can occur when animals mistake a water body covered in aquatic weeds for a pasture. People, particularly small children, can also mistake dense infestations of water weeds floating on the water surface for solid ground and can also drown after becoming entangled in dense infestations of submerged aquatic weeds.
Aquatic weeds can compete with ponded crops, such as rice, thereby reducing their potential yield and/or quality. This is a more significant problem in other parts of the world, where rice is the dominant crop species. They can also enhance water loss through increased evapotranspiration. For example, it has been estimated that 3.5 billion cubic metres of water are lost per year in Egypt as a result of evapotranspiration caused by infestations of water hyacinth (Eichhornia crassipes).
In addition to replacing useful plants, dense infestations of some weed species can deplete the concentration of oxygen in the water and cause the death of other aquatic life, leading to the depletion of fish stocks. Aquatic weeds can reduce the biodiversity of aquatic environments and threaten the survival of rare native plant species. Aquatic weeds can do this by simply out-competing and replacing native plant species that would otherwise occupy the same ecological niche. Surface-growing aquatic weeds can also block out sunlight for submerged aquatic plants, preventing photosynthesis and causing their death.
Infestations of aquatic weeds can harbour pests and diseases of humans and animals, as well as pests and diseases of other plants. For example, dense infestations of water hyacinth on Lake Victoria in eastern Africa are known to harbour large populations of the snail hosts of the pathogen that causes schistosomiasis in humans. Infestations of this weed are thought to be a major factor contributing to an increase in the transmission of this disease in this region.
Aquatic plants can become weeds for several reasons: they are alien species that have been introduced to the ecosystem; the natural water regime has been modified, or the nutrient balance of the ecosystem has been modified.
Invasive alien plants are those introduced species which become naturalised and aggressively compete with, and displace, native plant species that are adapted to the local environment. They have a competitive advantage because their natural enemies are not present.
Important examples include:
Highly variable water regimes are usually unreliable for many human enterprises and so considerable effort is often made to improve or stabilise water regimes. For example: the construction of dams in order to create artificial lakes on rivers that have highly variable flows. However, this creates more prolonged or even semi-permanent favourable growing conditions for certain native and alien plant species
Without this stabilisation of the water regime, water levels would naturally fall from time to time - and the associated plant populations would also be significantly reduced or only remain present as resistant propagules.
River catchments are usually seen as ideal locations for human settlements because of the readily accessible source of water for growing crops and supplying human needs. The agricultural practices often involve the application of large quantities of fertilisers, which can result in run-off that contains enriched levels of nutrients. Often significant clearing and land disturbance is involved, which can also increase the amount of run-off that occurs. Wastewater from human settlements, which is usually pumped directly into the river system, also usually contains high levels of nutrients.
The process of enrichment of these water bodies with high levels of nutrients is widely referred to as eutrophication. Eutrophication may allow aquatic plants to grow at faster growth rates than would otherwise be the case, and may also enable the system to sustain a much greater volume of plant matter.
Several of these factors may be acting synergistically to cause an aquatic weed problem. This is usually the case with the most severe infestations of aquatic weeds. You would, for example, expect to have significant weed problems in a situation where one of more alien invasive plant species have been introduced to an artificial lake that has a relatively stable water level and is subject to run-off that contains very high levels of nutrients.
Because Australia is a relatively dry continent, with a relatively variable rainfall pattern, at least some part of most water catchments has been modified in some way or another. Except for the drier inland areas, where there is less intense agriculture and a very sparse population, nutrient enrichment is also a country-wide phenomenon.
Hence, one or more invasive alien aquatic species is usually present in a region or in an aquatic ecosystem. However, water weeds are by no means evenly distributed across the country. Some species are restricted to the tropics (e.g. Mimosa pigra) while others are only a concern in temperate regions (e.g. Elodea canadensis).
Therefore the potential exists for aquatic weed problems to occur in most aquatic ecosystems and they are not isolated or rare events.
For the management of aquatic weeds to be effective, quarantine measures to prevent the introduction and spread of invasive aquatic species must be put in place. There should be a coordinated effort between all levels of government and informed and motivated communities.
Though many aquatic weeds are already present in Australia, every attempt should be made to prevent the introduction of additional species, particularly those that have shown evidence of aggressive weediness in other parts of the world. Troublesome aquatic weeds that are already present should also continue to be prevented entry, as further introductions may add to the gene pool present in the country (and possibly introduce more aggressive forms of a species). AQIS (the Australian Quarantine and Inspection Service) uses a system which prohibits the importation of plants until they have been assessed for weediness and cleared of posing a major risk.
The potential economic benefits of a species should be weighed up against its potential adverse environmental, social and economic impacts. For example, aleman grass (Echinochloa polystachya) and hymenachne (Hymenachne amplexicaulis) were introduced in the 1980’s for use in ponded pastures in northern Queensland. Both species have spread from these pastures and invaded natural tropical wetland areas - where they have become significant aquatic weeds that alter these ecosystems and threaten the survival of native plant and animal species.
In the past, there has not been sufficient cooperation between state and commonwealth governments in the legislation and management of weed species. This has meant a species that is a declared weed is some states can be obtained legitimately in nearby states and carried into an area where it could cause problems (by someone who did not realise that it was an offence to do so).
Problems can also occur when different departments within a state government do not have a coordinated approach. Such a situation is unhealthy as it sends out a mixed message to the community.
Government agencies can establish successful control programs for aquatic weeds infestations and provide leadership, planning and resources. However, if management of these species is to be successful, there must also be a significant contribution by community groups or individuals, including landholders, managers of public and private water bodies, catchment management authorities, environmental associations, and commercial and recreational users of water bodies.
Increasing awareness, by focussing on target audiences and providing general information to the wider community, is the first stage in informing and then motivating the community.
In aquatic ecosystems, this control method usually involves removing the weed material using some type of large mechanical harvesting device. Numerous such devices have been developed and they are widely used in Australia, particularly on large open water bodies such as lakes, dams and larger rivers.
Slightly different methods are required for different types of aquatic weeds. For example, free-floating plants can be collected using a scoop or net near the water surface. Submerged plants can be removed using a cutting device, wire rope or heavy chain that is dragged near or on the bottom of the water body. Emergent plants can be cut below the waterline, but this only retards their growth and does not eliminate the problem.
The impact of this control method is immediate and there is usually also an immediately visible difference. It is safe in water sources that are used for human and animal consumption. Physical removal of the plant material removes nutrients and may slow regrowth and it does not lead to the depletion of oxygen levels in the water.
Mechanical control can be difficult and expensive in aquatic systems. Specialised and very expensive devices are required to undertake such operations, though they may be hired rather than purchased. The chance of re-infestation is high as some of the weed material always remains. Certain species may only be partially removed or may grow faster than they can be removed. This method can be totally ineffective when insufficient resources are used over a large and heavily infested area. In this situation, the re-growth of the species can keep pace or even exceed the rate of removal and so it is mostly a waste of time and money.
Chemical control is generally most suitable for smaller infestations because usually only a small quantity of herbicide is needed in these situations, and therefore water contamination is minimal.
In order to control submerged aquatic weeds, herbicides need to be applied directly to the water. They therefore need to be active against the species at very low rates, or large quantities of chemicals need to be added to the water (this is generally not economically feasible or environmentally acceptable). No chemicals are currently registered for many submerged aquatic weed species in Australia – so this option is not possible for controlling these species.
Herbicide use in aquatic ecosystems is therefore mainly limited to controlling emergent or floating species – with application directly onto the surface of plant parts above or on the water surface.
Most chemical herbicides are relatively quick-acting and there is usually a visible difference in the weed infestation in a relatively short period of time. If the entire area of the weed infestation is sprayed correctly, then control can be very successful. A follow-up application (or hand removal) should be done to remove any plants that were missed in the first application and prevent the re-infestation of the water body.
Because the plants are generally killed, the species is less likely to re-infest the system or recover more slowly
Many chemicals are not permitted to be used near aquatic ecosystems because they are toxic to other plants, fish or wildlife, so the number of chemicals available is significantly reduced. Those that are permitted in aquatic environments can only be used in certain situations under relatively strict conditions.
This control method usually results in the breakdown of large amounts of plant material in the water body in a relatively short period of time. This can significantly deplete oxygen levels which may threaten the survival of fish and other aquatic species. As the weed material is not removed from the water body, and the nutrients are eventually returned to the water, this can provide good conditions for the regrowth of the weed. Because weed infestations in aquatic environments can sometimes be difficult to access, this can make chemical control more expensive and time-consuming.
Many problems with aquatic weeds occur when invasive alien species are introduced into an aquatic ecosystem without their natural enemies. By introducing the natural enemies of the weed, or ‘biological control agents’, into the environment this can help to bring the ecosystem back closer towards equilibrium.
Though it can be extremely useful in some circumstances, it is rarely the complete solution for a weed problem. Some weed species are not amenable to biological control, as there are no suitable agents available for their control (or potential agents cannot be introduced because they may attack native species or crop plants). There is also great variability in the effectiveness of biological control agents. Some are extremely effective while others may only have a limited impact and only provide partial control. They may also only provide control in certain geographic regions, at certain times of the year, or even in certain habitats within an aquatic ecosystem.
Because of these factors, biological control often has to be supported by other control measures.
Biological control is safe in all water sources, including those that are used for human and animal consumption, because no toxic chemicals are released into the water.
Biological control methods are easy to implement in aquatic ecosystems, as the agents will naturally spread throughout the water body over time. However, it is usually sensible to release the agents at several strategic points, as this will increase the chances of establishment and/or quicken the rate of spread of the agent throughout the entire water body.
Though successful biological control will not totally eradicate a species, it can reduce the population of the weed to acceptable levels and provide very long term control. As the weed numbers decrease the population of the agent will also decrease, and vice versa (unless the weed is totally eradicated using other measures, which may also lead to the loss of the agent).
This method is relatively slow and it can take several years for adequate control to be achieved. Even when successful and damaging agents are available, they take time to build up sufficient numbers to have a noticeable effect on the target weed species. This control method can also result in the breakdown of large amounts of plant material in the water body over a short period of time. Though it is not likely to be as significant as is the case with chemical control, this can still lead to depletion of oxygen levels in the water. As the weed material is not removed from the water body, the maintenance of high nutrient levels can provide good conditions for the regrowth of the weed.
Temporarily draining or removing all water from the system is a relatively drastic measure, but it may be possible in certain situations (e.g. in relatively small man-made water bodies). Where possible, all the water can be drained from the system and it can be left to dry out for several weeks in order to kill the plants.
The influx of nutrients can be diverted to other areas (e.g. by stopping the flow of effluent into the system, preventing livestock direct access to the area, revegetating catchment areas, and limiting soil erosion and run-off from nearby agricultural areas).
Integrated management involves combining two or more of the available control techniques into a unified program
An example of integrated management in a small dam would be to:
Water hyacinth is an attractive free-floating weed of tropical and sub-tropical water bodies, which is native to South and Central America and was introduced as a garden ornamental because of its attractive flowers. Its free-floating nature means that it can quickly and easily spread throughout an aquatic ecosystem. Water hyacinth is one of the most successful invasive plants in the world. It can produce large numbers of seed, but its ability to reproduce vegetatively at a rapid rate is vital to its success as a weed (it is capable of doubling its population in less than ten days).
It is widespread in Australia and is still spreading to previously uninfested waterways, probably as a result of intentional and unintentional human introductions. Water hyacinth has been recorded in every state and territory except Tasmania. Dense carpets of this species can alter the ecology of the infested area (as can the control measures that are used against it). These impacts are lessened if infestations are tackled at the earliest possible opportunity.
The growth rate of this species is so great that mechanical harvesting may not even keep up with the increase in population. Hence, this method is not particularly useful for managing this species.
Biological control is most successful when multiple agents are used against an infestation. Results can also be quite variable depending on the climatic conditions prevailing in the infested area, as the agents are not all equally well adapted over the entire range of the species in Australia (for example, some agents tend to be less effective in cooler regions within the range of water hyacinth).
The use of chemical herbicides is often necessary to adequately control large and rapidly spreading populations of this species, which is particularly sensitive to diquat and the acid and amine salts of 2,4-D. Follow-up spraying of surviving plants is necessary to prevent the problem quickly returning.
Meticulous surveillance, along with the manual removal and destruction of rogue plants, can lead to successful eradication of this species from a water body or an ecosystem. This surveillance should also be maintained after the weed has been eradicated, in case the weed is re-introduced into the system.
Salvinia is a free-floating fern of tropical and sub-tropical water bodies, which is also native to South America. It was first reported in Australia from a farm dam in 1952 and is thought to have originally been introduced and spread as an aquarium plant. Like water hyacinth, salvinia can double the size of an infestation in less than 10 days when growing in tropical environments. It has been recorded from every state in Australia except Tasmania.
Salvinia is sterile and therefore relies totally on vegetative reproduction. It breaks apart easily at the nodes and new branches grow readily from buds at each stem node. It is also capable of growing from very small stem segments (i.e. a single node with two leaves). Infestations are known to decrease light penetration into the water, change water quality, and affect the composition of associated plant and animal communities.
Biological control has been spectacularly successful in some situations. The weevil Cyrtobagous salviniae has been used to great effect on some occasions (e.g. in northern Queensland and in Papua New Guinea). However, there are climatic constraints to the effectiveness of this organism and it has been nowhere near as effective when used in more temperate situations.
Chemical herbicides can be used but the upper leaf surface of this species is covered in waxy egg-beater shaped hairs that prevent water accumulating and therefore aid floatation. A mixture of surfactant, kerosene and diuron has been particularly effective in controlling this species in the past. Formulations involving glyphosate are the favoured option nowadays, because of its relative safety in aquatic ecosystems.
Mechanical barriers, such as booms and nets, can prevent the spread of the weed to other areas of the water body in the short-term and make control programs easier. Mechanical removal is an option for relatively small and dense infestations, but care needs to be taken to remove all plants to prevent rapid re-growth.
Successful management relies on early detection and control may require a combination of biological, chemical and mechanical techniques, depending on the size of the infestation.
Alligator weed is a creeping perennial herbaceous weed of tropical, sub-tropical and warmer temperate regions and is also native to South America. It was first reported in Australia in the 1940’s and is seen as a threat to natural waterways as well as ponded crops and pastures.
It has a variable growth habit, ranging from a terrestrial plant to a free-floating species. This plant is probably most commonly encountered growing around the edges of water bodies as an emergent species that is rooted to the substrate. However, it will also grow on dry land and occasionally as a free-floating aquatic plant (usually as a result of emergent plants being dislodged from the substrate).
Alligator weed relies mainly on vegetative reproduction and has also been grown as a vegetable, especially by the Sri Lankan community, and it appears that this has played a major role in its spread. Public awareness and engagement with these communities have helped to minimise further spread.
Mechanical removal, which breaks the plant into small segments, is not recommended, because regrowth from roots and underground stems will quickly recolonise the areas previously occupied, and there is the added risk of spreading the plant segments to new areas.
Several biological control agents have been tested and one, the alligator weed flea-beetle (Agasicles hygrophila), has been moderately effective. However, this agent tends to have a wide range in its effectiveness depending on the form of the plant and the habitat in which it is growing. It is effective against the weed when it is growing as an emergent in aquatic situations, but it is ineffective in controlling terrestrial populations along the banks of waterways.
Chemical control is often required to manage the terrestrial form of this species. Three treatments during the growing season, each consisting of two applications separated by one week, are recommended to control this species as it can recover from a single application via reserves in its underground stems. However, there are problems in this approach, including concerns with using chemicals near waterways.
There are a variety of different aquatic ecosystems in Australia, with varying levels of susceptibility to aquatic weed problems. There are also several different types of aquatic weeds, with different characteristics and impacts, each requiring an emphasis on different control methods.
Aquatic weeds are often difficult to manage because they are not easily accessed and there are restrictions on using chemical herbicides near waterways. Early detection and action are important for successful management, and so is the implementation of an integrated control program. Ongoing surveillance is also crucial to prevent the problem from recurring in the future.
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