technical note g.4 water resources and environment...4. water hyacinth in ooty lake, india 16 5....

Series EditorsRichard Davis

Rafik Hirji

Series EditorsRichard Davis

Rafik Hirji

Environment DepartmentThe World Bank1818 H Street, N.W.Washington, D.C. 20433, U.S.A.www.worldbank.orgFor information on these publications contact theESSD Advisory Service at [email protected] call 202.522.3773

The World Bank

Water Resources and EnvironmentTechnical Note G.4

Management ofAquatic Plants

Water Resources and EnvironmentTechnical Note G.4

Management ofAquatic Plants

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The World BankWashington, D.C.

WATER RESOURCES

AND ENVIRONMENTTECHNICAL NOTE G.4

Managementof Aquatic Plants

SERIES EDITORS

RICHARD DAVIS, RAFIK HIRJI

2

A. Environmental Issues and LessonsNote A.1 Environmental Aspects of Water Resources ManagementNote A.2 Water Resources Management Policy Implementation: Early Lessons

B. Institutional and Regulatory IssuesNote B.1 Strategic Environmental Assessment: A Watershed ApproachNote B.2 Water Resources Management: Regulatory DimensionsNote B.3 Regulations for Private Sector Utilities

C. Environmental Flow AssessmentNote C.1 Environmental Flows: Concepts and MethodsNote C.2 Environmental Flows: Case StudiesNote C.3 Environmental Flows: Flood FlowsNote C.4 Environmental Flows: Social Issues

D. Water Quality ManagementNote D.1 Water Quality: Assessment and ProtectionNote D.2 Water Quality: Wastewater TreatmentNote D.3 Water Quality: Nonpoint-Source Pollution

E. Irrigation and DrainageNote E.1 Irrigation and Drainage: DevelopmentNote E.2 Irrigation and Drainage: Rehabilitation

F. Water Conservation and Demand ManagementNote F.1 Water Conservation: Urban UtilitiesNote F.2 Water Conservation: IrrigationNote F.3 Wastewater Reuse

G. Waterbody ManagementNote G.1 Groundwater ManagementNote G.2 Lake ManagementNote G.3 Wetlands ManagementNote G.4 Management of Aquatic Plants

H. Selected topicsNote H.1 Interbasin TransfersNote H.2 DesalinationNote H.3 Climate Variability and Climate Change

Water Resources and Environment Technical Notes

Copyright © 2003

The International Bank for Reconstruction and Development/THE WORLD BANK

1818 H Street, N.W., Washington, D.C. 20433, U.S.A.

All rights reserved.

Manufactured in the United States of America

First printing March 2003

3

AuthorJan Vermaat

Technical AdviserStephen Lintner

EditorRobert Livernash

Production StaffCover Design: Cathe Fadel

Design and Production:The Word Express, Inc.

NotesUnless otherwise stated,all dollars = U.S. dollars.All tons are metric tons.

Cover photo byAgaba Patrick, Clean Lakes Inc.

UgandaFerry and water hyacinth,

Kisumu, Kenya

This series also is available on theWorld Bank website

(www.worldbank.org).

CONTENTSForeword 5

Acknowledgments 7

Introduction 9

Types of Nuisance Aquatic Plants 10Most nuisance aquatic plants are either flowering plantsor microscopic algae. Some of the most troublesomeare global in distribution.

Scope of the Problem 13Nuisance aquatic plants have impacts on navigation,water quality, human health, the local water balance,and ecosystems. The most serious problems occur inboth the dry and humid tropics.

Methods of Controlling Aquatic Plants 19A variety of manual and mechanical methods areavailable to control aquatic weeds. Other alternativesinclude chemical controls, biological controls, habi-tat alteration, environmental controls, and integratedcontrols.

Beneficial Uses or Disposal of Weeds 24Harvesting aquatic weeds can sometimes provide aproduct for sale.

Advantages and Disadvantagesof Control Methods 25

The selection of a suitable control method dependson direct and capital costs, the skills required, institu-tional needs, and health, safety, and cultural factors.

Monitoring and Prevention 28Early warning of incipient aquatic plant problemsthrough monitoring systems can significantly reducethe costs of control.

Conclusion 29

Further Information 30

4

WATER RESOURCES AND ENVIRONMENT • TECHNICAL NOTE G.4

Boxes1. Some problems caused by aquatic weeds in Southern Africa 102. Myriophyllum or Myriophyllum, which is the weed? 113. Water hyacinth build-up in the Sudd, Sudan 164. Water hyacinth in Ooty Lake, India 165. Deaths from cyanobacterial toxins: Caruaru, Brazil 186. Water plants and lake management: alternative stable states 197. Biological control of weeds in Southern Africa 218. Selection and application of weed control methods in Mexico 239. Integrated weed production and fish farming, Bangladesh 25

10. Historical development of aquatic weed control strategies in Southern Africa 28

Tables1. Growth-form classification of freshwater angiosperm aquatic plants 112. Major weed problems for different growth forms in different types

of water bodies. 143. Comparison of feasibility, complexity and potential side-effects

of different weed control methods 264. Indicative costs of different macrophyte control measures 265. Indicative costs for different nuisance algae control methods 26

Figures1. Major aquatic weed management projects in Southern Africa 29

5

MANAGEMENT OF AQUATIC PLANTS

FOREWORD

The environmentally sustainable development andmanagement of water resources is a critical andcomplex issue for both rich and poor countries. Itis technically challenging and often entails difficulttrade-offs among social, economic, and politicalconsiderations. Typically, the environment is treatedas a marginal issue when it is actually key to sus-tainable water management.

According to the World Bank’s recently approvedWater Resources Sector Strategy, “the environmentis a special ‘water-using sector’ in that most envi-ronmental concerns are a central part of overallwater resources management, and not just a partof a distinct water-using sector” (World Bank 2003:28). Being integral to overall water resources man-agement, the environment is “voiceless” when otherwater using sectors have distinct voices. As a con-sequence, representatives of these other water us-ing sectors need to be fully aware of the importanceof environmental aspects of water resources man-agement for the development of their sectoral in-terests.

For us in the World Bank, water resources man-agement—including the development of surface andgroundwater resources for urban, rural, agriculture,energy, mining, and industrial uses, as well as theprotection of surface and groundwater sources,pollution control, watershed management, controlof water weeds, and restoration of degraded eco-systems such as lakes and wetlands—is an impor-tant element of our lending, supporting one of theessential building blocks for sustaining livelihoodsand for social and economic development in gen-eral. Prior to 1993, environmental considerationsof such investments were addressed reactively andprimarily through the Bank’s safeguard policies. The1993 Water Resources Management Policy Paperbroadened the development focus to include theprotection and management of water resources inan environmentally sustainable, socially acceptable,and economically efficient manner as an emerging

priority in Bank lending. Many lessons have beenlearned, and these have contributed to changingattitudes and practices in World Bank operations.

Water resources management is also a critical de-velopment issue because of its many links to pov-erty reduction, including health, agriculturalproductivity, industrial and energy development,and sustainable growth in downstream communi-ties. But strategies to reduce poverty should not leadto further degradation of water resources␣ or eco-logical services. Finding a balance between theseobjectives is an important aspect of the Bank’s in-terest in sustainable development. The 2001 Envi-ronment Strategy underscores the linkages amongwater resources management, environmentalsustainability, and poverty, and shows how the 2003Water Resources Sector Strategy’s call for usingwater as a vehicle for increasing growth and re-ducing poverty can be carried out in a socially andenvironmentally responsible manner.

Over the past few decades, many nations have beensubjected to the ravages of either droughts or floods.Unsustainable land and water use practices havecontributed to the degradation of the water resourcesbase and are undermining the primary investmentsin water supply, energy and irrigation infrastruc-ture, often also contributing to loss of biodiversity.In response, new policy and institutional reformsare being developed to ensure responsible and sus-tainable practices are put in place, and new predic-tive and forecasting techniques are being developedthat can help to reduce the impacts and managethe consequences of such events. The Environmentand Water Resources Sector Strategies make it clearthat water must be treated as a resource that spansmultiple uses in a river basin, particularly to main-tain sufficient flows of sufficient quality at the ap-propriate times to offset upstream abstraction andpollution and sustain the downstream social, eco-logical, and hydrological functions of watershedsand wetlands.

6


With the support of the Government of the Nether-lands, the Environment Department has preparedan initial series of Water Resources and Environ-ment Technical Notes to improve the knowledgebase about applying environmental managementprinciples to water resources management. TheTechnical Note series supports the implementationof the World Bank 1993 Water Resources Manage-ment Policy, 2001 Environment Strategy, and 2003Water Resources Sector Strategy, as well as theimplementation of the Bank’s safeguard policies.The Notes are also consistent with the MillenniumDevelopment Goal objectives related to environmen-tal sustainability of water resources.

The Notes are intended for use by those withoutspecific training in water resources managementsuch as technical specialists, policymakers andmanagers working on water sector related invest-ments within the Bank; practitioners from bilateral,multilateral, and nongovernmental organizations;and public and private sector specialists interestedin environmentally sustainable water resourcesmanagement. These people may have been trainedas environmental, municipal, water resources, ir-rigation, power, or mining engineers; or as econo-mists, lawyers, sociologists, natural resourcesspecialists, urban planners, environmental planners,or ecologists.

The Notes are in eight categories: environmentalissues and lessons; institutional and regulatory is-sues; environmental flow assessment; water qual-ity management; irrigation and drainage; waterconservation (demand management); waterbodymanagement; and selected topics. The series maybe expanded in the future to include other relevantcategories or topics. Not all topics will be of inter-est to all specialists. Some will find the review ofpast environmental practices in the water sectoruseful for learning and improving their perfor-mance; others may find their suggestions for fur-ther, more detailed information to be valuable; whilestill others will find them useful as a reference onemerging topics such as environmental flow assess-ment, environmental regulations for private waterutilities, inter-basin water transfers and climatevariability and climate change. The latter topics arelikely to be of increasing importance as the WorldBank implements its environment and water re-sources sector strategies and supports the next gen-eration of water resources and environmental policyand institutional reforms.

Kristalina GeorgievaDirector

Environment Department

7


ACKNOWLEDGMENTS

The production of this Technical Note has beensupported by a trust fund from the Government ofthe Netherlands managed by the World Bank.

This Note was drafted by Jan Vermaat of IHE Delft,the Netherlands. The Note was reviewed by Hans-Olav Ibrekk of the World Bank.

9


can produce potent toxins that can enter drinkingwater supplies. Consequently, task managers at theWorld Bank confront problems arising from aquaticplants in a wide variety of contexts.

This Technical Note describes the occurrence andmanagement of nuisance aquatic plants in fresh-water. Though not discussed in this Note, estuarine/marine aquatic plants can cause considerable so-cial and economic disruption, particularly in de-veloping countries. The toxins released by somespecies of marine/estuarine cyanobacteria and di-noflagellates are extremely poisonous to humansand stock animals, causing “red tide” paralytic shell-fish poisoning, ciguatera poisoning, and diarrhoealshellfish poisoning. For example, blooms of the di-noflagellate Pyrodinium bahamense caused at least34 deaths in the Philippines in 1983 and 1987, and26 deaths in 1987 along the coastal areas of CostaRica and Guatemala. The Further Information sec-tion includes some references on these estuarine/marine issues.

This Note is organized in six sections. A generaldescription and typology of nuisance aquatic plantsis followed by a description of the problems causedby these plants. The Note then discusses methodsfor controlling these plants, how to obtain benefitsfrom them, and the advantages and disadvantages

of the various controlmethods. It also pro-vides cost estimatesassociated with differ-ent control options.The Note concludeswith suggestions forobtaining further infor-mation from bothwebsites and printedmaterial.

Nuisance aquatic plants come in both macroscopicand microscopic forms. Macroscopic forms includesuch common plants as water hyacinth (Eichhorniacrassipes) and macroalgae such as Cladophorasp., and are generally termed aquatic weeds. Mi-croscopic forms include algae and cyanobacteria,commonly called algal blooms when their concen-trations are great enough to be visible and discolorthe water. Aquatic plants can seriously interfere withmany water uses, including drinking water qual-ity, accessibility, navigability, irrigation and hydro-power production, water withdrawals, andecological functioning. In addition, aquatic weedscan harbor vectors of disease and cyanobacteria canrelease potent toxins.

Water managers typically underestimate the eco-nomic loss from invasions of weeds and algae.Box 1, which is based on a 1997 study by the WorldBank, provides examples of the severity of the prob-lems caused by water hyacinth in many subsectors.Since that study was completed, the World Bank hassupported a number of aquatic weed managementprograms, including GEF-funded projects to controlwater hyacinth and algal blooms in Lake Victoriaand the control of a range of aquatic weeds in South-ern Africa. The Bank has also been active in helpingset up a secretariat for the Global Invasive SpeciesProgramme (GISP) in South Africa with Bank-Neth-erlands-Water-Partner-ship-Program fundingsupport. The potentialhealth consequences ofnuisance aquatic plantscan also be very signifi-cant. Weeds can harborvectors for disease andprovide habitat forsnakes and other dan-gerous animals, andsome species of algae

Water weed, Senegal

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10


TYPES OF NUISANCE AQUATIC PLANTS

DEFINITIONS AND CAUSES

Aquatic plants are considered nuisances when ex-cessive growth interferes with desired water uses,including human health.

Aquatic plant problems have increased in the lasttwo centuries, in line with increases in industrial-ization, travel and communications, agriculturalproductivity (including the agricultural “green”revolution), the growth of the human population,and changes in consumption patterns. Increasedtravel has expanded the opportunities for transmis-sion of aquatic plants from their home ranges tonew environments. Factors such as population

growth and changing consumption have led tochanges in land and water use, which have in-creased pressure on endemic aquatic plants andprovided suitable habitats for introduced species.Aquatic plant problems are usually a symptom ofbroad land and water use changes and poor watermanagement. Aquatic plant management shouldtherefore be based on preventing or minimizing theside effects of developments that disturb aquaticenvironments. In practice, this means counteract-ing the causes of excessive plant growth, includingvectors that spread weeds from their native habitatinto new areas, and retaining ecological restraintssuch as herbivores, competitors, and nutrient scar-city (see Box 2).

BOX 1.SOME PROBLEMS CAUSED BY AQUATIC WEEDS IN SOUTHERN AFRICA

Weeds that form dense mats, such as water hyacinth and Kariba weed (Salvinia molesta), can obstruct the flow ofrivers, which in turn can obstruct floodwaters and increase the extent of flooding. For example, mats of water hyacinthcaused extensive flood damage in the Swartkops River in South Africa in 1977. Aquatic weeds also cause direct loss ofwater via transpiration; for example, water hyacinth is reported to increase evapotranspiration by a factor of 3.5 overthat from open water. This water loss represents a significant cost in water-scarce areas.

These mats also make fishing and water gathering difficult and impede navigation. The dense mats that chokedKisumu harbor during the 1996-8 outbreak of water hyacinth in Lake Victoria are estimated to have cost the Kenyaneconomy at least Ksh 2 million ($25,000) in extra fuel costs for ferries and Ksh 79 million ($1 million) in lost catches fromartisinal fishing. Drifting mats of Kariba weed caused local fish catches to decline in the former Lake Liambezi on theborder between Namibia and Botswana. It is believed that the mats reduced oxygen concentrations in the watersufficiently to affect zooplankton and fish numbers.

Mats of water weeds also block water intake points and canals. For example, dense mats blocked the intake screensand filters of the Owen Falls power station in Uganda; shutting down the turbines and removing the weeds cost theUgandan Electricity Board $1 million per day. Rooted weeds, such as Parrot’s Feather (Myriophyllum aquaticum), havecaused problems in irrigation canals in South Africa by impeding flow and causing siltation.

Aquatic weeds also affect the quality of drinking and irrigation water. They cause odors and tastes and affect the pH.Thus, irrigators who draw water from the Mokolo River in South Africa have found that Parrot’s Feather infestationsdiscolor their tobacco crop, halving its market value.

Weed mats provide habitat for the vectors that spread tropical diseases. They provide shelter for mosquitoes that carrymalaria as well as habitat for Bilharzia snails.

Finally, aquatic weeds affect native plants and animals by altering local ecosystems. The reduced sunlight penetrationwill suppress the growth of some plants and cause some animals to move to new locations; the reduced oxygencontent of the water will favor some fish and zooplankton species over others; and the dense mats provide physicalhabitat that suits some species and not others.

Sources: Bethune S. and K. Roberts. (2002); Mogaka, H., S. Gichere, J.R. Davis, and R. Hirji. 2003. Impacts and Costs of Climate Variabilityand Water Resources Degradation in Kenya. Washington: World Bank.

11


BOX 2.MYRIOPHYLLUM OR MYRIOPHYLLUM, WHICH IS THE WEED?

In North America, considerable effort has been spent to control the introduced Eurasian milfoil (Myriophyllum spicatum)as it is considered a weed, while the native Myriophyllum species are protected. However, “it requires expert knowl-edge to distinguish M. spicatum from a native species (M. sibiricum) and it is an absurd situation when the watermanagement authorities have to call in a taxonomic botanist to know if they have a weed problem or not. I myselfhave seen M. laxum being sprayed in the southeast United States, where it is a rare and endangered species. Thereason for the increased growth of Myriophyllum, native or introduced, in North America is usually the result of changesin land and water usage.”

In short, caution is warranted, particularly when officials may have difficulty in properly identifying the species andconfuse desired native species with the perceived weed.

Source: adapted from Cook, C.D.K. (1990).

PLANT FORMS

In freshwater, most aquatic weeds are either mac-roscopic angiosperms (flowering plants), alsotermed macrophytes, or microscopic algae. It isuseful to categorize macrophytes by growth form,

partly because particular growth forms are morecommon in particular types of water bodies, andpartly because management options depend on thegrowth form (Table 1). The major aquatic weedspecies are found in the categories “submerged” and“free-floating.”

TABLE 1.GROWTH-FORM CLASSIFICATION OF FRESHWATER ANGIOSPERM AQUATIC PLANTS

Type Description

Emergent Roots occur from approximately 1.5 m underwater to waterlogged soils on land. Stemsand leaves can be partially submerged, but some part emerges above the watersurface. Most species have well-developed roots and long-lived rhizomes that anchorthe plant in the sediment and may provide considerable aeration to the underlyingsediment. Flowering occurs above water. Examples are cattail (Typha) and commonreed (Phragmites).

Rooted with floating leaves Similar to emergent plants in most characteristics, but the leaves rest on the watersurface. The depth range is 0.5-3 m. In some species, submerged and aerial leaves alsoexist. Flowering is aerial. Examples are water lilies (Nymphaea, Lotus, Nuphar).

Submerged, rooted These rooted plants are fully submerged, and flowering may occur underwater. Theymay occur at any depth, depending on light availability. Examples are pondweeds(Potamogeton) and hydrilla (Hydrilla). Roots are useful as anchorage, but also for nutrientacquisition, since nutrient availability in the sediment is often greater than in the overlyingwater.

Submerged, non-rooted Several species are not firmly rooted in the sediment but float freely underwater. Ex-amples are hornwort (Ceratophyllum) and bladderwort (Utricularia). Fragments and matsof rooted submerged plants may become detached and effectively act as if non-rooted.

Free-floating These are freely floating on the water surface and suspend their roots into the water.Examples are duckweed (Lemna), water hyacinth, and water lettuce (Pistia stratiotes).

Source: Wetzel, R.G. 1983. Limnology. New York: CBS College Publishing.

12


Emergent plants are not often considered weeds,although in irrigation schemes emergents can beproblematic when channel maintenance has beenneglected. For example, Glyceria maxima is con-sidered troublesome along the margins ofeutrophic waters in New Zealand, typha is a nui-sance plant in Senegal, and water lilies sometimesimpede flow in drains, particularly in areas whereit is not native.

Microscopic algae can be either attached (periph-ytic) or freely suspended in the water column(planktonic). Algal concentrations grow to bloomproportions as a consequence of excessive nutri-ent loading (eutrophication), particularly whenphysical water-body conditions such as tempera-ture and flow are suitable and there are favorableecological conditions, such as absence of grazers.Cyanobacteria (commonly called “blue-green al-gae,” but taxonomically not algae at all) are a par-ticular threat to freshwaters because somegenera—for example, Anabaena and Microcystis—contain species that can release toxins that arehighly poisonous to domestic animals and hu-mans.

Freshwater macrophytes and algae are readily iden-tified. National keys to the aquatic flora are avail-able in many developing and all developedcountries, and most botanically trained staff mem-bers in government agencies are familiar with theappropriate keys. Several aquatic plant species oc-cur in many parts of the world. Among these cos-mopolitan macrophytes are water hyacinth andhydrilla, which both cause serious problems. Thereare references to the identification of these wide-spread weeds in the Further Information sectionof this Note. Although it is usually sufficient to iden-tify aquatic weeds to genus level, in some cases itcan be critical to identify them to species level (Box2). Similarly, some of the most common nuisancefreshwater algae and cyanobacteria occur in manyparts of the world—for example, Microcystis—andcan be identified from readily available keys andidentification guides.

FACTORS INFLUENCING GROWTHAND SPREAD OF AQUATIC PLANTS

Nuisance aquatic plants, both weeds and algae, candevelop and spread with remarkable speed whenthe conditions favor them, often taking managersby surprise. There are a number of factors behindthese outbreaks.

Both weeds and algae can grow quickly because oftheir rapid doubling times (the time taken to doubleits biomass) and (sometimes) lack of predators.Water hyacinth has a reported maximum doublingtime of just 6–7 days, while Kariba weed and redwater fern (Azolla filiculoides) have been reportedto double in 3–5 days. Where the plants have beenintroduced from another environment, it is com-mon to find that they have no local herbivores orparasites and so these rapid growth rates are un-checked. Thus, one method of controlling theirgrowth is to introduce biological controls such asweevils for water hyacinth or invertebrates in thecase of cyanobacteria (bio-manipulation).

Some nuisance plants have more than one repro-ductive strategy. For example, water hyacinth andred water fern can reproduce either vegetativelyfrom small plant fragments or from spores that areresistant to desiccation. Similarly, some species ofcyanobacteria, such as Anabaena circinalis, canform spores that can remain viable through droughtperiods.

The growth of most nuisance aquatic plants is nu-trient limited; that is, they will respond to an in-crease in the availability of nutrients that are in shortsupply. Thus, waterbodies that have experienced in-creases in nutrient concentrations, often becauseof increased human activities, are susceptible toaquatic weeds and algae. Because of their rapiddoubling and, often, freedom from grazing, they arebetter able to utilize these nutrients than otherplants.

Physical conditions can also play an important rolein the growth and spread of these plants. Floatingforms, such as cyanobacterial scums and floating

13


macrophytes, can be concentrated by wind condi-tions on a small length of shoreline, causing animmediate problem, sometimes even overnight.Impoundments (lakes and reservoirs) provide anideal habitat for buoyant species of cyanobacteria.They can take advantage of the non-turbulent con-ditions and congregate near the surface, where theyhave maximum access to sunlight for growth. Theseproblems are likely to increase as more waters areimpounded for development.

Finally, aquatic weeds benefit from a lack of un-derstanding by the general population about theproblems that can be caused by their introductionsto new waterbodies. Plants are transported in sev-eral ways: attached to boats and vehicles, introducedas ornamental garden plants, and for shelter pur-poses in hatcheries. The public needs to be edu-cated about the economic and environmental lossesthat can occur because of these inadvertent intro-ductions.

The major problems caused by nuisance aquaticplants are:n Physical impediment to navigation, fisheries,

and the intakes of water treatment plants, irri-gation pumps, and hydroelectric turbines. Theseproblems occur mainly in the tropics and sub-tropics, and are often caused by water hyacinthin larger lakes, reservoirs, and rivers. Only inthe very shallowest waters do submerged spe-cies cause weed problems.

n Water quality problems (such as anoxia) occurmost often as a consequence of either high res-piration rates or the die-off and decompositionof plant biomass. This can arise from excessivegrowth of both macrophytes and algae.

n Health problems from harboring vectors of dis-ease or the release of toxins from some speciesof cyanobacteria.

n In arid regions, the local water balance may beaffected where emergent stands and dense bedsof the larger floating species increase evapo-transpiration.

n Ecological problems, such as threats to en-dangered species, can arise from changes inphysical habitat, water chemistry, and lightregime.

n Significant aesthetic or recreational problemscan occur because of large mats of nuisancemacrophytes or blooms of algae, although theseare largely limited to the industrialized worldor areas in the developing world that rely ontourism.

Table 2 provides a summary of the problems com-monly caused by different aquatic weeds.

Algal blooms occur in both temperate and tropicalconditions, although the biomass produced is oftengreater in tropical conditions because of the highertemperatures and greater light availability. Toxiccyanobacterial species are prevalent in both tropi-cal and temperate conditions.

The same aquatic plant species may be considereddesirable in some environments and weeds in oth-ers. Considerable sums have been spent to main-tain or restore beds of water plants in lakes of Europeand North America because they are a desired andcrucial component of a healthy littoral ecosystem,whereas the same species can be a serious prob-lem in irrigation channels in other countries. Evenwhen they are classified as weeds, aquatic plantscan still provide some useful environmental servicessuch as stripping nutrients from the water columnin irrigation drains.

DIRECT PHYSICAL PROBLEMS

Impediments to flow. Emergent aquatic weeds canbuild up enough biomass to fill the entire watercolumn and so cause serious resistance to waterflow, particularly in comparatively small (< 5 mwidth) and shallow (< 1.5 m depth) channels,streams, and drains. As a result, flooding can in-crease, agricultural productivity can be reduced as

SCOPE OF THE PROBLEM

14


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intakes to water treatment plants and other rela-tively restricted areas. However, they are never denseenough to impede water flow significantly in chan-nels and open water bodies.

Water losses. When emergent aquatic plants, whetherrooted or not, form dense mats they can consider-ably increase the water lost through transpiration.A recent report1 states that water hyacinth mats haveincreased evapotranspiration rates by a factor of 3.5over those of free water surfaces. These losses canbe very costly in water-scarce areas. Smaller float-ing plants like duckweed probably have less effecton evapotranspiration, simply because the watertransport system within these small plants is of littleimportance to the physiological functioning of theplant. There are even reports, in an arid climate, ofa reduction in total water loss by a duckweed matbecause the extra transpiration is more than offsetby the reduced evaporation from the water surface.

Impediments to navigation, fishing, or recreation.Very dense and extensive accumulations of aquaticweeds, especially water hyacinth, may smothermooring sites and harbors as well as restrict ac-cess to both traditional and commercial fishing ar-eas (Box 3). In some cases, the density of the weedmats impedes navigation to the point where signifi-cant financial losses occur.

In North America and Europe, dense weed beds andalgal blooms have impeded recreational boating,fishing, and swimming. They are also unattractive.However, recreational losses are of secondary im-portance in developing countries, except in areaswhere tourism provides a major source of income(Box 4).

CHEMICAL CONSEQUENCES

Daytime photosynthesis and nighttime respirationof dense submerged weed beds may strongly influ-ence the chemistry of the surrounding waters. Dur-ing daytime, oxygen concentrations will be elevated;at nighttime, the levels may be seriously reduced.

a result of poor water delivery, and environmentalproblems can arise from poor drainage of tailwaters.In larger water bodies, which are normally too deepfor emergent plants, floating weed mats may be-come colonized by emergent species and conse-quently obstruct access to shore or open water.

Rooted species with floating leaves are rarely prob-lematic, and emergent species generally only be-come so when littoral areas are poorly maintained.Submerged rooted plants are probably the most im-portant nuisance group, but in the tropics and sub-tropics, the larger floating forms such as waterhyacinth may also play a significant role.

Closed pipes rarely have weed problems, sinceplants need light for photosynthesis. Pipes, pumps,filters, and water intakes may become blocked,however, if upstream plant biomass becomes mo-bile, such as after mowing or manual uprootingwithout planned removal of the plant material.These problems are particularly acute with high-value water uses such as hydropower generation.

Algal blooms, too, can cause physical problems.Dense mats of algae and cyanobacteria can block

1 Bethune, S. and J. Roberts (2002).

Water hyacinth

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The die-off of nuisance aquatic plants, whether weedbeds, floating plants, or algal blooms, will lead tohigh carbon loads being deposited onto the sedi-ments. Benthic oxygen concentrations will be re-duced significantly as the biomass decomposes. Thisis particularly noticeable when the aquatic plantsdie abruptly, such as after herbicide treatment. Thelow oxygen levels will, in turn, cause phosphorusto be released from sediments via a reducing reac-

BOX 4.WATER HYACINTH IN OOTY LAKE, INDIA

At Ooty Lake in Tamil Nadu province in India, tourism is being threatened by water hyacinth, which is slowly spreadingalong the surface of the lake and threatening to choke it.

The lake was artificially formed by John Sullivan in 1823–1825 by damming the mountain streams flowing down theOoty valley. Over the decades, the lake’s area has shrunk due to lack of management, and the lake has becomeseriously polluted from sewage. The recent infestation of water hyacinth is believed to have resulted from the highnutrient levels now present in the lake. In spite of the importance of the tourism industry to the local economy, little hasbeen done to remove the causes of the weed problem.

BOX 3.WATER HYACINTH BUILDUP IN THE SUDD, SUDAN

The Sudd is a large, highly productive papyrus wetland area (10,000 km2 in the dry season and 92,000 km2 in the wetseason) in the south of Sudan. The Sudd also provides a good habitat for water hyacinth. Considerable quantities offloating vegetation are brought downstream from the Sudd to the Jebel Awila Dam above Khartoum. These floatingrafts may build up to large concentrations in side arms and dead ends of the Sudd. Along the Sudanese White Nile,these accumulations seriously impede navigation, particularly when seasonal winds further pile up the plant mass. TheSudanese government declared water hyacinth control a national priority in the early 1970s. With technical supportfrom Germany, a large-scale study was launched to investigate control of the weed, as well as to develop locallyfeasible and economically viable ways of using the large quantities of biomass.

The study concluded that:n chemical control was feasible, though at considerable cost, particularly since the main source of drifting water

hyacinth lies upstream in the inaccessible marshes of the Suddn mechanical control by locally available labor was not very effective without supervisionn biological control had not been very successfuln most of the alternative use options were not very promising.

Water hyacinth was tested for biogas generation, fodder for cattle and sheep, bricketing for fuel, as well as compostingand mulching. Only the latter proved promising, but not on all crops. The high water content of the biomass alwaysnecessitated drying as a pre-treatment. Still, the report concluded that “it is necessary to stop regarding the waterhyacinth as a weed and to regard it as a beneficial or economic plant.”

The weevils that were released for biological control around 1978 continued to multiply. By 1982, in spite of the report’sconclusion, no more problems were reported and all observed plants showed distinct scars of weevil feeding. Thewater hyacinth is no longer regarded as a national priority, but it is unclear whether navigation is still substantiallyhampered in the White Nile.

Oxygen concentrations below about 4 mg l–1 maylead to fish kills; most fish are unable to live in con-centrations below 2 mg l–1. Active photosynthesiswill cause the pH to rise rapidly to alkaline levels(around pH 10), particularly in poorly bufferedwaters, with a consequent increase in NH3 concen-trations. The combination of low oxygen and el-evated ammonia concentrations may affect fisheriesand aquaculture.

17


tion. If this phosphorus reaches surface waterswhere light levels are high, it can fuel an algal bloom.Thus the die-off of water hyacinth following treat-ment resulted in heavy cyanobacterial blooms intwo of the bays on the Ugandan shoreline of LakeVictoria. Even if the nutrients are restricted to thebottom waters and algal blooms are avoided, thelow oxygen levels can lead to mortality amongaquatic invertebrates and vertebrates, such as fish.

HEALTH CONSEQUENCES

Several diseases can be promoted by beds of waterweeds. The association of bilharzia-bearing snailswith water plants is well known. Cholera may alsobe associated with weeds; cholera bacteria (vibrio)are reported to survive well in organisms attachedto the weeds.

Serious health effects can also arise from toxinsproduced by some species of cyanobacteria.2 Thereasons why the cells produce toxins are not known.Different cyanobacteria produce different types oftoxins:n Hepatotoxins, which affect the liver and are

produced by many common species of cyano-bacteria, including Microcystis and Anabaena.

n Neurotoxins, which affect the nervous system,are produced by some strains of Aphanizomenonand Oscillatoria.

n Toxic alkaloids, which cause gastrointestinalproblems and affect the kidneys.

n Dermatotoxins, which cause skin rashes.

Toxins from Microcystis are also reported to act astumor promoters, even in low doses.

These toxins can be removed or inactivated by chlo-rination, filtration through activated carbon, andozonation. These treatments may not be sufficientduring bloom situations or when a high organic loadis present in the water supply. Removing these tox-ins is difficult and costly in developed countries; indeveloping countries, the best solution is to avoidsituations that promote cyanobacterial blooms.

There have been few documented cases of humandeath from freshwater cyanobacteria (for an excep-tion, see Box 5). However, there is believed to be arelationship between cyanobacterial contaminationof surface water and cancer mortality, particularlygastric, esophageal, and liver cancers. There havebeen a number of studies in China using differenttechniques that show a link between algal toxinsand liver cancer.

ECOLOGICAL CONSEQUENCES

Ecosystem changes. Water plants form a key com-ponent of littoral, shallow-water ecosystems, andmost weed species also have important roles as foodand as habitat. There are mixed opinions about thedirect impact of aquatic weeds, but the indirect rolesare highly significant: weed beds are refuge to manyinvertebrates and juvenile fish and are home tospecies-rich guilds of invertebrates that feed on at-tached or suspended algae and bacteria in a vari-ety of ways.

Water plant beds generally harbor considerablymore biodiversity than unvegetated sediments. Com-mercial fisheries may incur losses after the influxof aquatic plants, when they previously relied onspecialized fish species that inhabit bare sediments.However, this is not always so; one study in Ugandafound there was little change as a result of an in-creasing cover of water hyacinth on both benthicinvertebrates and fish. In most instances, a scien-tific investigation will be needed to assess theecological impacts.

In bloom concentrations, algae and cyanobacteriacan also affect food chains. Larger species of inver-tebrates such as some Daphnia (water fleas) cangraze on small-celled algae. Their numbers havebeen observed to increase with persistent algalblooms. Increases in such invertebrates will leadto changes in higher levels of the food chain, in-cluding fish. Dense algal blooms also reduce lightlevels within the water column, adversely affectingmacrophytes. In fact, there is a direct competitionbetween free-floating algae and rooted aquaticplants, such that there appear to be two stable states2 Bartram, J. and I. Chorus. (1999).

18


for many water bodies—dominance by phytoplank-ton or dominance by macrophytes (Box 6).Cyanobacterial toxins can affect fish as well asmammals. Salmon, striped bass, and shrimp havebeen found to harbor toxins from Microcystis, withthe fish usually dying from acute liver failure. It isnot clear if the toxins affect the shrimp.

SPREAD OF WEEDS

Some species of aquatic weed can rapidly form largenumbers of viable fragments that are easily dis-persed over large distances. Nonnative species canspread extremely rapidly after inadvertent, careless,or even deliberate introduction. Examples are avail-able from most continents: two Elodea species intoEurope, hydrilla into North America, and water hya-cinth into Africa and Asia. Means of dispersal arehighly varied and include wildlife (migrant ducks),cattle transportation, ships’ ballast water, smallboats, and careless dumping of plants by horticul-turists or salesmen.

Some control methods can actually promote dis-persal. For example, several species have adaptivesurvival organs (tubers, turions, seeds) that remainin the sediment after the aboveground vegetationis removed. They can regenerate within the sameor following season once the water body has beencleared. Also, fragments detached during mechani-cal removal will easily form adventive roots andsuccessfully reestablish.

Rapid growth in transportation and tourism hasincreased the opportunities for weed dispersal. Inresponse, several countries—including the UnitedStates, Australia, and New Zealand—have drawn uptight preventive regulations and border controls tolimit these anthropogenic introductions. Natural dis-persal is much more difficult to control, particu-larly when water bodies are connected in a riverinedrainage network or through wildlife or cattle mi-gration routes.

BOX 5:DEATHS FROM CYANOBACTERIAL TOXINS: CARUARU, BRAZIL

A dialysis center in the town of Caruaru, Brazil obtained its water supply by truck from a reservoir about 40km away. Inmid-February 1996, patients began experiencing nausea, vomiting, and visual disturbances after dialysis. During thefollowing two weeks, 12 patients died of seizures or acute liver failure. Eventually, 50 deaths were attributed to liverfailure following dialysis. Experts from the U.S. Centers for Disease Control were asked to assist local authorities with theirinvestigation.

The water was not chlorinated or filtered, although the center passed the water through its own treatment / filtrationequipment before use in dialysis. Maintenance of the equipment was poor, and neither the sand filter nor themicropore filter had been changed for at least three months.

It became evident that during dialysis the patients had been exposed to microcystins—the group of toxins producedby the cyanobacterium Microcystis aeruginosa. Microcystins were found to be present in the reservoir water, thedelivery truck, the water storage tank, and treatment equipment at the center. Microcystins were also found in liversamples from deceased patients.

It appears likely that an algal bloom in the reservoir led to small amounts of toxin passing through the municipal watertreatment plant. When the decision was made to bypass part of the municipal treatment process and truck the waterto the center, the patients were exposed to high levels of toxin. Some of the histological observations of liver tissuewere not typical of microcystin, and it is believed that cylindrospermopsin was also present in the water. There were noalgal counts performed at the time of the incident, but low numbers of Microcystis and Cylindrospermopsis werepresent in the reservoir during the investigation.

Source: Jochimsen, E.M. et al. “Liver Failure And Death after Exposure to Microcystins at a Hemodialysis Center in Brazil.” New EnglandJournal of Medicine 338 (13): 873–78.

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MANUAL CONTROL

Nuisance macrophytes can be controlled withmanual methods when cheap labor is available.Hand tools are used to cut plants and/or roots andto pull plant biomass out of the water. Manual meth-ods are mainly applicable in small water bodies,such as irrigation and drainage canals or small riv-ers and lakes. Complete removal of weeds is usu-ally not achieved, and in many cases a quick

regrowth occurs. Consequently, more than one cutper season is usually necessary.

Manual control has advantages when access forequipment is difficult and certain plant species orplant parts need to be harvested selectively. Animportant drawback is the potential health haz-ard from waterborne diseases, for example bilhar-zia, that workers encounter when entering thewater.

BOX 6.WATER PLANTS AND LAKE MANAGEMENT: ALTERNATIVE STABLE STATES

As a consequence of increased nutrient loading, many shallow lakes in industrialized Europe, North America, andAustralia have undergone a transition from a period with clear water, when water plants were present but not inexcessive numbers, to a phase with turbid water, no water plants, and cyanobacterial blooms. This transition processinvolves a complex interaction among aquatic plants, light penetration and turbidity, anoxia and nutrient release fromsediments, and fish and invertebrates.

Both the macrophyte and phytoplankton states are stabilized by feedbacks. Stabilizing mechanisms for the clear-water,macrophyte-dominated phase are:n The dense submerged foliage reduces flow, thereby enhancing sedimentation, leading to clearer water.n Submerged leaf surface (often leaf area indices >2 m2 m–2) is colonized by attached algae and bacteria that

reduce nutrient and BOD concentrations in the water.n Photosynthesis by the plants affects inorganic carbon equilibria, pH, and oxygen concentrations in the water, which all

retard planktonic growth potential.n Sediment oxygenation by roots creates an oxidized upper sediment zone, which retains nutrients in the sediments.n Weed beds provide refuges for juvenile zooplankton and habitat for adult zooplankton as well as fish.n Several macrophyte species reportedly produce compounds that reduce algal growth.

The algae-dominated, turbid phase also has stabilizing mechanisms:n The sediment is no longer protected by plants and therefore easily resuspended, creating a harsh environment for

reestablishing seedlings of water plants, as well as low light levels.n Increased turbidity enhances survival rates of fish that eat zooplankton, which in turn graze on phytoplankton and stir

up the sediment. Top predator fish, in contrast, are severely hindered by increased turbidity.n High turbidity causes heat absorption in the upper layers and stratification of the water body. The lower water layer is

cut off from the atmosphere and will turn anoxic, leading to increased nutrient losses from the sediments. Thesenutrients can feed algal blooms in the surface water layers.

Both states are stable. The transition is normally triggered by increases in nutrient concentrations and the crucial stagebefore algal “takeover” is often the development of attached algae that bloom when the water is still clear. Reducingnutrient loads to these lakes once algal domination has been established has often had disappointingly little effect onalgal blooms because of the stability afforded by the feedback mechanisms. The interactions are usually complexenough to require an integrated management effort that tackles foodwebs, water clarity, nutrient inputs, and sedimentstabilization simultaneously. A scientifically based understanding is needed if management intervention is to besuccessful in these cases.

Sources: Blindow, I. 1992. “Long- and short-term dynamics of submerged macrophytes in two shallow lakes.” Freshwat Biol. 28: 15–27; VanVierssen, W., M.J.M. Hootsmans, and J.E. Vermaat. 1994. “Lake Veluwe, A macrophyte-dominated system under eutrophication stress.”Geobotany 21. The Netherlands: Kluwer.

METHODS OF CONTROLLING AQUATIC PLANTS

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portant that personnel are trained in the safe stor-age, handling, and application of herbicides.

Special herbicides are available to control algalgrowth (algicides), although they are expensive.Copper sulfate is commonly used as a cheap, highlyeffective algicide. In developed countries, it is sub-ject to stringent controls because of residual effectson fish and other aquatic life.

In general, herbicides and algicides are expensive,so chemical control is usually confined to relativelysmall bodies of higher-value water such as irriga-tion canals. It can be applied by hand, by helicop-ter, or plane. However, the latter two methods, whileefficient, will often result in dispersal of the herbi-cide outside the target area.

Both physical and chemical control methods usu-ally need to be repeated at least every growing sea-son. Physical and chemical methods rarely providea permanent solution to weed problems; as a re-sult, a long-term management program will needto be implemented.

MECHANICAL CONTROL

A wide variety of harvesting machinery is availableon the market. The equipment is usually mountedon a boat or on a tractor that operates from the bankor the shore. Some mechanical equipment has onlycutting bars, whereas others also include means forharvesting the plant material.

Dredging is a mechanical method that not onlyharvests the plant material but also removes the un-derground material of rooted plants and seeds. Thisprovides an important advantage over other me-chanical methods, because removal of roots, rhi-zomes, and other plant propagules causes a delayin regrowth of the weed. Unfortunately, even dredg-ing can leave enough plant propagules behind forfairly quick regrowth. More critically, it is devas-tating to benthic organisms, as well as affectingsuspended organisms through increased turbidity.Physical damage to the habitats of macro-inverte-brates will affect fish feeding and growth. Fisheriesmay therefore encounter reduced fish yields afterdredging or even other physical methods of weedcontrol. Consequently, dredging is not a recom-mended management technique except in extremecases.

CHEMICAL CONTROL

Herbicides are powerful tools in aquatic plant man-agement. Herbicides are potentially more thoroughand sometimes longer lasting than mechanical con-trol methods. However, they can only be appliedeffectively and safely with a solid understanding ofthe chemistry and biology of the ecosystem.

Broad-spectrum herbicides affect all aquatic plants,while selective types only damage particular spe-cies. Application of herbicides should not surpassthe recommended dose, in order to minimize thechance of side effects on other organisms. The ef-fectiveness of herbicides can be improved by con-trolled dose applications using granular pellets orpolymer carriers that slowly release the herbicideclose to the plant or its roots. Given the potentiallytoxic nature of many agrochemicals, it is also im- Algal bloom, Darling River, Burke, Australia

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selective control. For example, the weevil Neochetinaeichhorniae has brought infestations of water hya-cinth in Lake Victoria under control (a fortuitousEl Nino-induced drought probably also contributed).There have been no side effects reported from us-ing arthropods for biological control, probably be-cause these organisms are highly selective.Microbial herbicides (pathogenic fungi) that can besprayed on emergent or floating plants offer poten-tial for short-term management following suddengrowth spurts of these weeds. However, pathogenicfungi need to be screened for their effect on otherplants and fish before application.

The introduction of herbivorous fish such as grasscarp into the affected water body provides an ex-ample of nonselective control of weeds. Not onlydo the fish reduce weed densities, but they may alsoprovide additional food production.

An interesting biological approach to the control offlow resistance is the planting or stimulation ofparticular species of Nymphaeids that have limitedsubmerged biomass, hence a low resistance to flow,and that shade out fully submerged, nuisance wa-ter plants.

Biological control can also be used with nuisancealgae. Biomanipulation through the introduction of

It is important that aquatic plants are removed fromthe water body if they have been killed in largenumbers by either physical or chemical methods.Unfortunately, this is usually difficult with algaebecause of their small size. If the dead biomass fallsto the bottom of the water body, it can often resultin anoxic conditions, and the release of nutrientsand hydrogen sulfide from the sediments. These con-ditions are conducive to cyanobacterial or algalgrowth and even fish kills.

BIOLOGICAL CONTROL

Biological control methods potentially provide bet-ter long-term success than mechanical or chemicalmethods. To be effective, biological methods requiremore insight into ecosystem functioning than is re-quired for chemical or physical methods. For aquaticweeds, biological control methods are based on theintroduction of a herbivorous organism, fungus, orvirus into the affected ecosystem (Box 7).

Biological control of aquatic weeds is regarded assuccessful if the control agent multiplies, spreadsover the infected area, and reduces the weed popu-lation to acceptable levels. These agents may beselective, only affecting the weed, or nonselective,affecting all vegetation. Several reports exist of thesuccessful introduction of arthropods and fungi for

BOX 7.BIOLOGICAL CONTROL OF WEEDS IN SOUTHERN AFRICA

In the 1970s, biological control still met with substantial skepticism in Southern Africa. For instance, the National Institutefor Water Research of South Africa and the Science Council of Zimbabwe advised against using biological methods tocontrol water hyacinth. However, the use of biological control methods was stimulated by the concern of both thepublic and scientists about other control methods. Although the first biological control method was initiated in SouthAfrica in 1962, biological methods became accepted only in the 1980s.

A good example is the biological control of Kariba weed in the rivers and floodplains of the Easter Caprivi in Namibia.Initially, research was conducted into the causes of the weed problem in order to evaluate the effectiveness of severalcontrol strategies. It was found that biological control was both the most promising method and the most ecologicallysound option. Today, Kariba weed is kept under control by C. salviniae weevils.

International cooperation between organizations such as the Plant Protection Research Institutes in South Africa andZimbabwe, the International Institute of Biological Control in Kenya, the Entomology Division of the CSIRO in Australia,and the Aquatic Plant Control Research Laboratory in Florida played a key role in the development of biologicalcontrol programs in Southern Africa. Most biological control programs started with the import of a biological agentfrom the Australian or American partner institute. After quarantine, careful screening, and breeding, the agent wasreleased in the affected area, generally with success.

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top-level predatory fish, such as bass, has been usedto successfully reduce algal biomass in a numberof shallow European and North American lakes.These large fish are often palatable and, in somecases, provide good recreational opportunities. Thusthey can have economic side benefits, as well asproviding algal control. Other biomanipulations,such as removal of all fish species, have been usedwith mixed success.

In a number of cases, the decline in algal biomassafter biomanipulation was followed a few years laterby a resurgence of algal numbers. Food-chain in-teractions are complex, and it is essential thatattempts at biomanipulation are preceded by thor-ough ecological studies of the lakes, so that the likelyeffects of the interventions can be understood. Giventhe cost and expertise required for these studies,biomanipulation is more likely to be used in devel-oped rather than developing countries.

HABITAT ALTERATIONS

The physical habitat of weeds and algae can bemodified by, for example, lowering the water levelin smaller, well-controlled water bodies such asirrigation channels and reservoirs. This method willexpose weeds and algae to dehydration and directsunlight. Weeds were managed successfully in anirrigation network in India at very low costs byimplementing an annual five-day draw-down pe-riod. While this method can remove nuisance mac-rophytes, it can also promote phytoplankton suchas cyanobacteria, which are quicker to colonizenutrient-rich waters if competition from macro-phytes has been removed.

Reducing the penetration of light into the water bodyis another way of changing the habitat of submergedplants. This can be achieved by increasing the wa-ter turbidity with nonharmful dyes, by regulatingflows, or by introducing an appropriate density ofbottom feeding fish that stir up sediment. For smallerdrains and streams, trees planted on the banks willshade the water and significantly reduce weeddevelopment.

Freshwater cyanobacteria are buoyant and are par-ticularly favored by stratified water bodies wherethey can move to the surface layer to obtain lightfor photosynthesis. Destratification of thesewaterbodies removes this habitat advantage and al-lows other less harmful phytoplankton to dominate(see Note G.2). However, it takes considerable en-ergy to destratify a mass of water, and this tech-nique is only feasible for high-value water bodiessuch as drinking water supplies for major cities.

ENVIRONMENTAL CONTROL

Environmental control methods are aimed at re-storing the key environmental conditions of theecosystem. These prior conditions have often beendisturbed by anthropogenic sources, such as waste-water discharge or runoff of fertilizers from agri-cultural land. Introducing proper wastewatertreatment and limiting runoff prevents an increasein nutrient availability and hence will reduce thechances of excessive growth of weeds or algae.

INTEGRATED CONTROL

Integrated control refers to the application of arange of physical, chemical, biological, and envi-ronmental control methods when single methodsare unlikely to succeed or when longer-lastingcontrol of weeds or algae is required at reducedcosts and with fewer undesired side effects (Box8). However, it must be based on an ecologicalanalysis of the aquatic ecosystem, including inter-actions with the surrounding terrestrial or aquaticecosystems and socioeconomic considerations.Consequently, integrated control takes longer toimplement and calls for a major investment in sci-entific investigation and monitoring. The examplealso shows the importance of involving stakehold-ers when several methods are being applied andlong-term monitoring is required.

For example, the herbicide 2,4-D can be applied ata lower-than-normal dose if it is combined with in-troduction of the water hyacinth weevil Neochetinaeichhorniae. In other cases, mechanical control ofaquatic weeds can be combined with the introduc-

23


tion of grass carp, or catchment management thatreduces nutrient inflows can be combined withdestratification of lakes to disrupt the habitat thatis conducive to cyanobacteria. When the integratedapproach includes a biological control measure, itnaturally combines a short- and long-term perspec-tive. For the long-term control of weeds to be effec-tive, the ecological interventions need to be analyzedfrom an overall catchment perspective, rather thanat the scale of the infected aquatic ecosystem. Thisapproach will prevent quick reinfection of a clearedarea with plants from remaining populations in thesame catchment area.

Integrated approaches should also consider the tim-ing of the control action in relation to the life-cyclestages of aquatic plants. For instance, the develop-ment of emergent as well as submerged weeds dur-ing one season depends on the number of

propagules that were produced in the previous sea-son. The number of propagules may be influencedby manipulating certain environmental factors. Forexample, reducing light availability by raising thewater level or increasing turbidity causes one spe-cies of pondweed (P. pectinatus) to form relativelysmall tubers. This effectively hampers sprouting inthe next season. Harvesting aquatic plants just be-fore the formation of tubers and turions, or reduc-ing light availability for young seedlings, are similarexamples of approaches that make prudent use ofbottlenecks in the weed’s annual life cycle and in-teractions in the ecosystem.

Management of integrated control methods. Aquaticweed control programs are preferably executed aspart of an integrated water management program.Integrated water management per se is not discussedhere, but some specific requirements that are re-

BOX 8.SELECTION AND APPLICATION OF WEED CONTROL METHODS IN MEXICO

Starting in 1993, an Aquatic Weed Control Program was implemented in three phases for three man-made lakes—Tacotán, Trigomil, and Miraplanes—in the Ayutla River watershed of Mexico. The first two of these lakes were heavilyinfested with water hyacinth, while the third lake was infested by both water hyacinth and cattail. The control programoccurred in a number of phases.

Phase 1: Initial evaluation, participation, and communicationsDuring this phase, water uses, agricultural activities, and weather conditions were inventoried and the extent of weedcoverage was estimated from satellite images. Possible control methods were identified. Simultaneously, water usersthat were willing to work with government authorities were identified and informed about various aspects of theproposed control program.

Phase 2: Selection and application of control methodsThe following methods were considered: chemical treatment, trituration (a mechanical method using large harvestingmachines), biological treatment, water-level management, and manual removal. Control methods were selected foreach lake based mainly on the characteristics of the lake, available budget, and domestic availability of the controlmethod. In Tacotán Lake, water-level reduction resulted in the dessication of about 100 hectares of water hyacinth.These plants were burned. The remaining area of water hyacinth was sprayed with 3.3 kg ha–1 of 2,4-D, followed by anapplication of 1.7 kg ha–1 of diquat. For Trigomil Dam, a combined chemical-mechanical method was selected.Spraying with 3.35 kg ha–1 of glyphosate weakened the plants, and mechanical control with triturators resulted in 100percent clearance. In Lake Miraplanes, 3.3 kg ha–1 of glyphosate was selected as the preferred treatment, becausethis chemical is reported to be very effective at killing cattail (Typha sp.). Two applications of this chemical (with a 200day interval) resulted in 100 percent clearance.

Phase 3: Environmental monitoring and maintenanceAn ongoing water quality monitoring program was established. Coverage due to regrowth was monitored and com-pared to established criteria for initiating new control actions. A users’ committee was involved in the monitoring andpreventive maintenance.

Source: Gutierrez, E., R. Huerto, P. Saldana, and F. Arreguin. 1996. “Strategies for water hyacinth (Eichhornia crassipes) control in Mexico.”Hydrobiologia 340: 181–185.

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may constitute an early stage of an integratedapproach.

A catchment perspective is usually necessary forintegrated control. A waterbody that spans admin-istrative or national boundaries cannot be effectivelydealt with by an institution with jurisdiction overonly part of the waterbody. Without coordination,upstream pollution in another administrative areaprohibits an integrated approach downstream. Thisproblem occurs, not only with transboundary wa-ter bodies, but also if the administrative body re-sponsible for aquatic plant control cannot influenceactivities in other sectors. Thus, manual and chemi-cal control of algal blooms may be futile in the longrun, if high loads of nutrients continue to enter thewater body from other jurisdictions.

lated to integrated control of weeds are presented.Design and implementation of integrated controlmethods require a multidisciplinary managementstaff that includes specialists in the various meth-ods, as well as ecologists to provide the ecologicalanalysis on which the integrated weed managementis based.

Sustainable solutions require a long-term perspec-tive with an ongoing commitment of funds andskilled personnel. This can be difficult to sustainwhen there is a strong urge for broad-scale appli-cation of herbicides, so that immediate results areobservable. However, long-term integrated controlmethods do not necessarily exclude quick solu-tions to urgent weed problems. Thus, the tempo-rary application of an effective chemical method

BENEFICIAL USES OR DISPOSAL OF WEEDS

Harvesting aquatic weeds can sometimes provide aproduct for sale. Many people in low-income coun-tries harvest aquatic plants on a small scale and usethe material as a resource, without being interestedin weed control. Also, algae are sold as a health foodin many developed countries. However, this is not arecommended practice because of its potential con-tamination with toxins. The following uses of har-vested biomass refer to aquatic weeds only. Thereare few cases of large-scale weed control programsthat include economically viable use of the harvestedbiomass. The financial return is usually not suffi-cient to contribute significantly to weed control costs.

FOOD FOR HERBIVORES

A number of aquatic plants, such as duckweed, havehigh protein content, favorable amino acid compo-sition, low fiber content, and good digestibility. Thismakes them desirable for animal feed. Others—forexample, emergent plants—have a high fiber con-tent. The high moisture content of aquatic plant bio-mass usually necessitates extensive pre-processing,such as by mechanical pressing or solar drying. Thisadds to the cost of this food source. However, this

pre-processing cost can be avoided by feeding aquaticplant material to herbivorous fish (Box 9).

SOIL AMENDMENTS

Aquatic plant material may be used to increase theorganic and nutrient content, the microbial activ-ity, and the texture of soils. The plant material canbe applied after composting, directly spread on thesurface, or mulched into the top layer of the soil.Mulching results in reduced evaporation, weed sup-pression, increased soil moisture and organic con-tent, and reduced erosion. These applications maybe helpful where economic conditions do not per-mit the purchase of artificial fertilizers.

FIBER AND PULP PRODUCTS

Several fiber products may be produced from aquaticplants. Paper, for instance, has been produced frompapyrus plants for millennia. In Romania, a programhas been developed in which common reed(Phragmites australis) is harvested from a 434,000-hectare delta for fiber board, alcohol, insulationmaterial, roof cover, and fertilizer. Handicraft prod-

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BOX 9.INTEGRATED WEED PRODUCTION AND FISH FARMING, BANGLADESH

The 10-hectare Mirzapur Farm Complex in Bangladesh contains about 7 hectares of fish ponds. In part of this complex,the fish are fed on duckweed, which receives its nutrients from sewage. Between 125 and 270 m3 of sewage isreceived per day from 2,000–3,000 persons. The sewage is passed through a sedimentation pond before entering aplug-flow lagoon where the duckweed is grown. Duckweed production averages 17 tons dry weight per year, and fishproduction averages about 11 tons/ha/yr. The fish are ultimately used as fertilizer in banana production. More than 100tons of bananas are produced per year with the aid of this fertilizer.

BOD5 of the influent sewage is 125 mg/l, while the effluent at the end of the plug-flow lagoon is 5 mg/l. Fecal coliformsin influent average 45,700 cfu/ml, while in the effluent it is <100 cfu/ml. Since the WHO standard for discharge ofeffluent water into a natural body is <100 cfu faecal coliform per ml, the effluent is considered safe. The levels ofheavy metals and pesticides are monitored and are acceptable under international standards.

ucts such as woven mats, baskets, fish traps, hand-bags, and hats are also produced from numerousfibrous emergent species, as well as submerged ones.

ENERGY PRODUCTION

Chopped plant material can be fermented to pro-duce methane or ethanol. These fermentation tech-nologies are widespread in many countries. Theseprocesses are practiced in several places inBangladesh, Brazil, India, and the United States.However, the economic feasibility of fermentationis very sensitive to international energy prices.

3 Microbial contamination is measured in ‘colony-form-ing units (cfu)’ per ml where a cfu is a bacterial cell orclump of cells capable of developing into a colony whengrown in laboratory conditions.

SANITARY LANDFILL

Disposal of aquatic plant material to sanitary land-fills is subject to environmental concerns, as is truewith any other waste. However, the leachate fromlandfills containing plant material should containonly natural organic compounds and nutrients, andshould not be contaminated with hazardous com-pounds. Odors from decomposing biomass, insectbreeding, and nutrient pollution of shallow aqui-fers are potential problems that need to be prevented.Biogas production in the landfill offers the possibil-ity for energy generation and partial cost recovery.

ADVANTAGES AND DISADVANTAGES OF CONTROL METHODS

As the preceding discussion makes clear, there area large number of options for managing nuisanceaquatic plants. The selection of a suitable methodwill depend on costs, the skills needed, and institu-tional needs, as well as health, safety, and culturalaspects (Table 3).

COSTS

The costs of aquatic plant management quoted inthe literature are often qualitative or poorly defined.This, along with the site-specific nature of many ofthe costs, makes a cost comparison difficult. Nev-ertheless, an attempt is made here to give a rule-of-thumb estimate of costs for managing macrophytes

(Table 4) and algae (Table 5). Note G.2 providesfurther information, including costs, on the man-agement of aquatic weeds and algae in lakes.

Although the reported costs vary widely, they clearlysuggest that the use of heavy equipment increasesboth capital investment costs and running cost.Thus, the “mechanical removal” category in Table4 is notably expensive. This is partly because dif-ferent techniques are grouped together, including

26


dredging, which is particularly expensive. The fewreports available on the costs of integrated controlmethods suggest that they are cost-effective. How-ever, biological treatment costs were not assessedbecause of the difficulty of converting them to anareal basis.

FEASIBILITY

The feasibility of a control method is highly depen-dent on the spatial scale. In smaller drains orstreams, most methods would be fairly easy to ap-ply. In large lakes, reservoirs, and rivers, chemicalcontrol is usually more feasible than mechanical

control because the dredging and cutting machin-ery is slow compared to aerial applications. Fur-thermore, biological control may not be feasible invery large systems when the biological agent is tobe strictly confined to the waterbody under treat-

TABLE 3.COMPARISON OF FEASIBILITY, COMPLEXITY, AND POTENTIAL SIDE-EFFECTS OF DIFFERENT WEED CONTROL METHODS.

Control method Feasibility /Applicability Complexity Side Effects

Small systems Large systems Technological Institutional(drains, streams) (lakes, reservoirs, rivers)

Manual removal ++ – o o oMechanical removal ++ + ooo oo o/ooChemical control ++ ++ oo ooo oooBiological control ++ ++ oo ooo oHabitat alterations (shading, draw-down) ++ – oo ooo oo

Notes: A qualitative scale of judgment is used to measure feasibility (– = unfeasible, + = reasonable, ++ = good option) and complexityand side effects (o = low, ooo=high). Since the integrated approach involves combinations of these methods, it is not evaluatedseparately.

TABLE 5.INDICATIVE COSTS FOR DIFFERENT NUISANCE ALGAE

CONTROL METHODS

Technique Costs

Phosphorus inactivation $640–$3,900 per ha

Algicides (Note these $30 to $700 per ha perchemicals can have application.detrimental environmentaleffects and should beused only for crises)

Biomanipulation Typically $300-600 perha capital cost

Aeration of bottom waters. Highly variable; TegelerSee, Germanyinstallation cost was$2.7 million with$6,500/ha/yroperational cost.

Artificial destratification Typical installation costsare $720/ha;correspondingoperating costs are$320/ha/yr

Sediment removal Lake Trummen, Swedenrestoration cost about$5,700/ha (1970 costs)

TABLE 4.INDICATIVE COSTS OF DIFFERENT MACROPHYTE CONTROL

MEASURES.*

Method Mean Range Number($ ha–1yr–1) ($ ha–1yr–1) of reports

Manual cutting 54 3-150 4Mechanical removal 189 20-530 7Herbicide application 110 46-145 3Combinations 45 23-82 3

* Based on published literature from irrigation canals and lakesfrom various parts of the world. Data were converted to $/ ha–1/ yr–1,assuming a typical mean width of 5 m for irrigation canals andneglecting fluctuations in US dollar rates between 1980 and 1999.

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ment. The spatial scale thus places serious con-straints on the methods that are applicable.

Shading using trees or fences may be feasible onlyin smaller systems, such as along secondary andtertiary irrigation channels. Draw-down of the wa-ter level is also only a feasible alternative in sys-tems where the water level can be manipulated, suchas irrigation systems that have a fallow period.

COMPLEXITY

The feasibility of a method is directly linked to itscomplexity. It has to be capable of being managedby locally relevant institutions and stakeholders. Me-chanical and chemical controls require skilled la-bor before, during, and after treatment. Equipmentneeds to be maintained and used properly, andproper chemical dosing requires training. Con-versely, field application of biological control meth-ods is comparatively simple, although high qualityscientific expertise will be needed during the plan-ning phase. With all methods, the health of fieldworkers must be protected, which may add a de-gree of complexity.

The design and conduct of eradication campaignsrequires a level of institutional sophistication. Au-thorities responsible for the design of complex treat-ment programs should include proper training fortheir field staff, particularly for chemical and bio-logical control. Managers also need to make surethere is a good understanding of the ecology of theweed as part of the control program. These man-agement requirements demand skilled people andfacilities, either in regional water management au-thorities or in irrigation departments.

SIDE EFFECTS VERSUS SPECIFICITY

In general, the more specific a method is, the fewerthe unwanted side effects.

Potentially hazardous side effects are well-knownwith chemical control. Herbicides may be toxic tohumans, fish, and other organisms, and so herbi-cide treatment will often make water unsafe for

drinking by humans and animals. Furthermore,vegetation die-off after herbicide application is of-ten rapid and depletes the dissolved oxygen, lead-ing to fish kills. Most herbicides undergo a rigorousscreening program before being allowed on themarket, but legal restrictions and administrativeoversight vary substantially among countries. Evenwhen there are legal restrictions, banned or re-stricted herbicides continue to be available in manydeveloping countries.

Mechanical removal has comparatively few sideeffects. However, plant material should be removedto minimize the effects of oxygen depletion on fishand other organisms. Cutting without removal ofdebris may also cause reestablishment of the weedsat other undesired locations, since many aquaticweed species are efficient at vegetative propaga-tion. Dredging sediments, as already discussed,has severe side effects and should be avoided ifpossible.

Draw-down of water in channels and reservoirs mayhave serious side effects on desirable biota. Whenno refuges are present in the system, major lossesare observed. Also, some aquatic plant species pro-duce organs (tubers, rhizomes, turions, seeds) thatremain in the deeper layers of the sediment. Thiscontrol method, therefore, will select for particularweed species. Finally, during the period of draw-down, emergent weeds may encroach onto the driedsediments from the banks and can be difficult toremove later.

The possible dispersal of an introduced bio-controlagent or an unexpected change in its diet are themajor inherent risks with biological control. Al-though herbivorous insects are often very host-spe-cific, a new race that attacks a local food crop mayevolve. Consequently, biological introductions mustbe screened extensively for their possible side-ef-fects before the decision is taken to introduce them.

IMPEDIMENTS

Some of the more common impediments to the suc-cess of a weed management program are:

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n Strong political pressure for immediate actionwhen a weed problem emerges at a late stage.Funding for these programs tends to come fromnonsustainable budget sources and may favorquick and sometimes environmentally harm-ful solutions.

n Lack of an integrated ecological and catchmentperspective.

n Lack of confidence in the effectiveness of bio-logical methods.

n Impossibility of full control over the problemor problem area.

n Lack of consultation with and support from lo-cal stakeholders.

n Lack of sufficient skilled personnel and a stronginstitutional framework with political and le-gal status.

MONITORING AND PREVENTION

Early warning requires the identification of anaquatic plant at an early stage of its development.This is generally long before problems become ap-parent to the general public or immediate stake-holders (Box 10). Since the life cycles of mostspecies have a clear seasonality, the timing of sucha monitoring program should be linked to seasonalgrowth periods. Long-term monitoring schemesgenerally involve the selection of a few represen-tative plots or stations, which are subsequentlymonitored after a broad system-wide survey has

been conducted. The program should also includewater quality monitoring (see Note D.1), as wellas algal species and cell concentrations. The col-lection of water samples is important for algalmonitoring; results can be skewed by the collec-tion of wind-borne blooms or surface scums. Sinceplant growth and seasonality are governed byphysical factors—light, water stratification, andtemperature—as well as nutrient availability, thelong-term data collection from such a monitoringprogram can be used to check for correlations

BOX 10.HISTORICAL DEVELOPMENT OF AQUATIC WEED CONTROL STRATEGIES IN SOUTHERN AFRICA

The introduction of water hyacinth occurred at the end of the 19th century in South Africa. Other weeds appeared inthe first half of the 20th century. Parrot’s feather was introduced from South America around 1918. Water lettuce firstoccurred in African waters in 1937. Kariba weed was first reported in southern Africa from the Zambezi River in 1948,and red water fern was first reported from South Africa in the same year.

For most of these species it took decades before serious problems were reported. These weeds have now spreadthroughout the southern Africa region, especially since the 1970s, and have caused many problems. Water hyacinthcaused problems on Lake Chivero in Zimbabwe in the 1950s. The control method applied at that time was a combi-nation of manual control and spraying with the herbicide 2,4 D. Initially this operation was successful, and the lakeremained clear until 1967. However, a drop in water level at that time caused germination of seeds in the lakesediments. After this reinfestation, large-scale manual control was initiated. This was unsuccessful, so the problem wassolved by herbicides. A more recent example of the shortcomings of manual control occurred in Zambia in 1994. TheZambian Army attempted to remove water hyacinth from the Kafue Gorge Water Reservoir manually. Initially thereservoir was cleared, but after some months the weeds recolonized the waterbody.

Because of the shortcomings of manual and mechanical methods, herbicide spraying became the most popularcontrol method in the 1970s (Figure 1). Effective control was achieved in many cases with chemicals—for example,using the Terbutryn herbicide “Clorosan 500” for water hyacinth in 1978 on the Hartbeespoort Dam in South Africa. Thisproject included a study on the health effects of the herbicides; local health authorities could not detect any danger.However, over the years public resistance toward herbicides increased. For this reason, herbicide control of aquaticweeds has now declined in favor of biological control.

Source: Bethune, S., and K. Roberts (2002).

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between plant development and these environ-mental factors.

There are a number of early warning indicators thata weed problem may be developing. The presenceof the plants themselves offers the most reliablepredictor. Even the past occurrence of submergedvegetation is an indicator, since the sediment oftenstill harbors seeds or other propagules. Sedimentcores (with processing and enumeration) are a fea-sible means to assess propagule presence. Algal andcyanobacterial cells are almost always present inthe waterbody and can multiply extremely rapidlywhen the physical and chemical conditions are right.Some species of algae can deposit seeds in sedi-ments. Unfortunately, there is little information onthe conditions that cause germination to providemanagers with early warning indicators of thissource of algal growth.

Environmental parameters such as high nutrientconcentrations and high light availability (low tur-bidity), as well as moderate flow velocities, allowthe rapid growth of submerged and/or floating spe-cies of weeds as well as algae and cyanobacteria.Buoyant species of cyanobacteria, such as Anabaenaand Microcystis, are also strongly favored in strati-

Source: adapted from Bethune & Roberts, 2002.

FIGURE 1.MAJOR AQUATIC WEED MANAGEMENT PROJECTS INSOUTHERN AFRICA, GROUPED BY CONTROL METHOD.

7

6

5

4

3

2

1

0

Manual/mechanical

Chemical

Biological

Nu

mb

er o

f pro

ject

s

‘70-’79 ‘80-’89 ‘90-’99

fied water bodies because they can float to the sur-face to make use of the good light conditions there.Together these factors create a “habitat probabilitywindow” of nuisance aquatic plants. Some of theseenvironmental conditions, such as nutrient concen-trations and stratification, can be manipulated bymanagers.

CONCLUSION

Nuisance aquatic plants can lead to serious eco-nomic losses in most water-using sectors. They canblock water off-takes for hydropower production,irrigation, and drinking water supply; interfere withtransportation; affect fisheries by blocking land-ing sites, alter habitat and lower oxygen levels; in-crease health problems by providing habitat forvectors such as mosquitoes; reduce water qualityby changing water chemistry and releasing tox-ins; and reduce the overall availability of water forconsumptive uses through enhanced transpiration.They also lead to ecological losses by smotheringother plants, competing for nutrients and alteringhabitat and, in the case of phytoplankton, modify-ing foodchains.

Once established, these plants are very difficult tocontrol and usually impossible to eliminate. Chemi-cal control methods are not recommended exceptin emergencies because of their side effects. How-ever, physical methods and biological methods haveproven successful in a proportion of cases withminimal side effects when undertaken carefully. Inthe long term, the conditions that provide theseplants with relative advantages need to be controlled.In combination with physical/biological controltechniques, successful strategies may include inte-grated control methods that combine long-termnutrient reduction activities; education for the com-munity; and consistent legislation and policies, es-pecially in transboundary waterbodies.

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FURTHER INFORMATION

Two scientific journals are dedicated to aquatic weedmanagement:

Aquatic Botany. Elsevier Science BV, Amsterdam, TheNetherlands (http://www.elsevier.com/locate/aquabot)

Journal of Aquatic Plant Management. University ofFlorida, Fort Lauderdale, FL 33314 (http://www.apms.org/japm.htm)

Information on weed identification is provided bythe Centre for Aquatic and Invasive Plants of theUniversity of Florida (http://aquat1.ifas.ufl.edu/),which also has a large on-line data base availableon aquatic weeds.

The following document provides definitive adviceon management of cyanobacteria in freshwaters:

Bartram, J. and I. Chorus, eds. 1999. Toxic Cyanobacteriain Water: A Guide to their Public Health Conse-quences, Monitoring and Management. Geneva:World Health Organization.

The following website provides reliable informa-tion on cyanobacterial toxins:

http://www.worldwaterday.org/2001/disease/cyano.html

Information on marine and estuarine algal bloomscan be obtained from:

http://state-of-coast.noaa.gov/bulletins/html/hab_14/references.html

General references for occurrence and managementof freshwater weeds include:

Cook, C. D. K. 1990. Aquatic Plant Book. The Hague: SPBPublishing.

Joffe, S. and S. Cooke. 1997. Management of the waterhyacinth and other invasive aquatic weeds: issuesfor the World Bank. Report for the World BankRural Development Department. Washington:World Bank.

Pieterse, A. H. and K. J. Murphy, eds.1990. Aquatic weeds,the ecology and management of nuisance aquaticvegetation. Oxford: Oxford University Press.

Scheffer, M. 1997. Ecology of Shallow Lakes. Populationand Community Biology Series 22. London:Chapman & Hall.

Bethune, S. and K. Roberts, 2002. Aquatic weeds and theircontrol. Chapter 8 in Defining and MainstreamingEnvironmental Sustainability in Water ResourcesManagement in Southern Africa. R. Hirji, P.Johnson, P. Maro and T. Matiza Chiuta, eds.Maseru/Harare/Washington: SADC, IUCN,SARDC, World Bank.

technical note g.4 water resources and environment...4. water hyacinth in ooty lake, india 16 5....

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