Water Hyacinth: a threat to Lake Tana

Geospatial Data & Technology Center (GDTC)in collaboration with

V/President for Research & Community Services Office

Bahir Dar University

ContributersDaniel Ayalew(Asso. Prof)

Yetwale Alemayehu Agumasie Genet

Daniel AsfawHelen Asfaw

Andargachew Abeje

@October 2020Bahir Dar, Ethiopia

About the Center

Bahir Dar University (BDU) is one of the first genera-tion Universities in Ethiopia that envisioned to become among the top ten premier research Universities in Af-rica by 2025.To this end, the University has established research centers to achieve its goals and bring changes to the research arena. Geospatial Data and Technolo-gy Center (GDTC) is one of the research centers that established in 2013. GDTC has envisioned to become center of excellence in GIS, Remote Sensing and Sur-veying (Geospatial Technologies) in Ethiopia. GDTC is an official ESRI partner research center, which has the privilege to use licensed ESRI software products and online trainings.In the age of digitalization and automation, geospatial technology plays a vital role in our daily activities and are embedded in most of the systems and applications we use today.

Acquainted of its responsibilities, GDTC has been en-gaged in providing the following services since its es-tablishment in 2013:

• Geospatial data collection maintenance and updating; • Geospatial data provision to academic, research and public institutions; • Conduct demand driven and problem solving re-search; • Capacity Building (basic, intermediate and advanced trainings) to academic and non-academic staff of BDU and other stockholders; •Provide consultancy service in many disciplines; •Deliver research based community services that could contribute to the development of the country; •Contribute for the quality of National Spatial Data In-frastructure (NSDI); •Organize and conduct seminars, national and interna-tional workshop and symposium; •Establish partnership with different Universities, re-search institutions and Governmental and Non-gov-ernmental organizations with geospatial data and tech-nology affiliates;•Build center of excellence in geospatial data and Tech-nologyC

1.INTRODUCTION Lake Tana is one of the highland lakes in Ethiopia, which is located in Amhara Regional State in the north-western part of Ethiopia, between 10°58’–12°47’N and 36°45’-38°14’E. It is the largest freshwater body in the country and contributes about 50% of the freshwater resource of the nation. The Lake covers a total surface area of 3,074 km2 with a total drainage area of 15,096 km2 (Duan and Bastiaanssen, 2013). It is approximately 84 kilometers long from north to south and 66 kilo-meters wide from east to west, with an average depth of 8 meters and a maximum depth of 14 meters, and an elevation of 1,788 meters. The lake is surrounded by ecologically important wetlands and fed by about 40 tributary rivers, of which Gilgel Abay, Ribb, Gumara, and Megech ac-count for 93 % of the total inflow (Setegn et al. 2009).

Lake Tana has emerged as one of the global top 250 lake regions most important for biological diversity conservation in the LakeNet framework (Duker and Borre, 2001). The lake comprises four terrestrial and three freshwater ‘Key Biodiversity Areas’. Hence, in June 2015, the United Nations Educational, Scientific, and Cultural Organization (UNESCO) has registered Lake Tana as a world biosphere reserve site for its being rich in biological and cultural diversity. It also comprises important fish resources and is home for more than 67 different fish species, of which 70% are endemic; this makes internationally known as an Important Bird Area (e.g. the area is one of the most important wintering areas for migratory birds such as the European crane) and is of global impor-tance for agricultural genetic diversity, the region is a gene center for indigenous crops such as nug (Guizotia abyssinica), teff (Eragrostis tef)

dating back to the 13th and 14th Century, with varieties of attrac-tions in the nearby Lake Tana and Blue Nile River, make the Lake Tana region as one of the leading tourist destinations in Ethiopia. Thus, Lake Tana is the largest icon of the region and the nation. However, this multi-pur-pose lake and its surroundings are threatened by multiple-prob-lems. Among these, excessive siltation, recession agriculture, water pollution, overgrazing, and infestation by invasive weed spe-cies are the major threats to the Lake. Annually an average sedi-ment load of 31.02 Mt entered the lake. From this, 96.6% is sus-pended load and 3.4% is bedload (Lemma et al., 2020). Besides, more than 52% of the wetland areas of the lake are severely degraded and converted to other land uses such as cultivation due to siltation (BNWI, 2014). These problems are manifested in the form of sedimentation, clearing of wetland and canalization of tributaries, and increasing trend of eutrophication and toxigenic cyanobacteria.

The lake supports millions of people in the region depend on its eco-logical and socio-economical values. Lake Tana also consists of 37 is-lands that are scattered about the surface of the lake and these islands shelter fascinating churches and monasteries which contains valuable treasures of the Ethiopian Christian faith, some of which have histories

1.Introduction

Annually an aver-age sediment load of 31.02 Mt entered the lake. From this, 96.6% is suspend-ed load and 3.4% is bedload

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Since 2011, the water hyacinth (Eichhornia crassipes) has invaded Lake Tana, causing significant damage to the biodiversity and provisions of the lake. Water hyacinth is a free-floating perennial (or hydrophyte) plant na-tive to tropical and sub-tropical South America. This dangerous weed was first observed and reported in Ethiopia in 1965 in Lake Koka and River Awash, and from there it started to spread to other nearby water bodies. Water hyacinth has broad, thick, glossy, ovate leaves; it may rise above the surface of the water as much as 1 meter in height and have 80 cm root below the surface of the water. The stems and leaves contain air-filled tis-sue which give the plant its considerable buoyancy. International Union for Conservation of Nature (IUCN’s) lists this species as one of the 100 most dangerous invasive species and the top 10 worst weeds in the world. The vegetation reproduction is sexual and asexual and takes place at a rapid rate under preferential conditions and produces enormous amounts of biomass, thereby covering extensive areas of natural open water. Displace-ment of water by the water hyacinth, which reduced the storage capacity of the reservoirs, is another threat to the Lake. Fishing in Lake Tana is also becoming increasingly time-consuming and more difficult, due to physical interference of the water hyacinth with the nets.

The weed causes serious envi-ronmental and socio-economic problems for millions of ripar-ian communities. The efforts made to control water hyacinth in Lake Tana have costed huge labor and financial resourc-es. According to Enyew et al., (2020), more than 800,000 human labor had been ded-icated to manual removal of the weed from 2012 to 2018, and above one million USD spent for procurement of har-vester machines and bioagent experiments. The socio-eco-nomic activities of the people, whose livelihoods are directly or indirectly contingent on the ecosystem services of Lake Tana, are severely affected by the water hyacinth invasion.

Since its existence in Lake Tana, different intervention mechanisms have been test-ed and implemented. Physical removal by community mobili-zation, mechanical using ma-chines and biological control using weevils at experimen-tal level have been tested so far. Although such efforts are made, the weed is still re-out breaking and invading new ar-eas over time. However, there is no consistent and accurate information about the area coverage, spatiotemporal dy-namics, and density of the water hyacinth on Lake Tana. This makes difficult to under-stand the magnitude of the problem and take appropriate measures to control the water hyacinth on the Lake.

Water hyacinth infestation in Lake Tana

Expansion of Gilgel Abay delta during the period 1973–2010(Top) and gumara river (Down)

1984

1994 2004 2014

2016

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Therefore, in this booklet we attempted to address the spatiotemporal dynamics of water hyacinth on Lake Tana, triggering factors, impacts, and strategies to control water hyacinth.

50.54 369

962 827.2

2627.33

3286.44

5358.68 5685.34

4302

0

1000

2000

3000

4000

5000

6000

2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

Wat

er h

yacin

th co

vera

ge(h

a)

Year

Year Image acquisition date Area covered by water

hyacinth (ha)

Rate of expansion (%)

2012 Dec 2012 50.54 -

2013 Nov 2013 369 630%

2014 Nov 2014 962 160%

2015 Oct 2015 827.2 -14%*

2016 Oct 2016 906.4 9.5%

2017 Oct 2017 2627.33 189%

2018 Oct 2018 3286.44 25%

2019 Nov 2019 5358.68 63%

2020 Jan 2020 5685.34 6%

2020 June 2020 2342.37 -58.7%*

2020 Oct 2020 4302 83.7%

The following tables and figures show the spatiotemporal dynamics of water hyacinth infestation on Lake Tana from 2012 to present (October 2020). As shown in the ta-bles and figures, water hyacinth infested only 50.5ha of the surface of Lake Tana, and limited to one kebele located around the mouth of Megech river. Currently, the weed increases in space and time and infested about nine woredas (Takusa, West Dembia, East Dembia, Gonder Zuria, Libokemkem, Fogera, Dera, Bahir dar zuria) and about 30 kebeles.

2. Spatio temporal Dynmaics of water hyacinth on Lake Tana

* The reduction was due to human mobilization

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Map of Spatio-temporal dynamics ofwater hyacinth on Lake Tana (2012-2020)

a) b)December 2012 November 2013

November 2014 October 2015c) d)

4

SPATIO -TEMPORAL COVERAGE MAP

October 2016 October 2017

October 2018 November 2019

e) f)

g) h)

5

SPATIO

-TEM

PORAL COVERAGE M

AP January 2020

June 2020

i)

j) k)

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Triggering factors for the prevalence of water hyacinth on Lake Tana

Water hyacinth spreads prolifically and reproduces quickly. According to Chopra et al. (2001) and Keller & Lodge (2009), water hyacinth doubles its area coverage every 6 to 15 days. The most favorable environmental conditions for op-timum growth of water hyacinth are quiescent- nutrient-rich wa-ter, shallow water depth (< 6m) covered with sediment deposit (Makhanu, 1997). Nitrogen, potassium, phosphorous, and sulfates are favorite nutrients. Besides, temperature (28°C to 30°C), pH (6.5 to 8.5), and sa-linity (< 2%) of the water are suitable conditions for its prev-alence (Dersseh et al., 2019). Even though, the exact triggering factors and sources of the water hyacinth prevalence in Lake Tana are not well known yet; sedimen-tation, extensive fertilizer applica-tion and runoff from agricultural fields, and pollutants (nutrients) from the surrounding industrial zones, and residential areas of cities mainly Bahir Dar and Gondar probably create favorable condi-tion for its prevalence in the Lake.

Crop production

Water hyacinth has dichotomous socioeco-nomic impacts. One of the impacts of wa-ter hyacinth is its negative effect on paddy crop production. Water hyacinth affected paddy crop production by destroying the plant, inhibiting germination, and interfer-ing with harvesting. During flooding and wave time, a mat of water hyacinth makes rice production frustrating by totally cov-ering the crop field. Water hyacinth makes the farmland more compacted due to its long root and makes the farmland diffi-cult to plough. The collected water hya-cinth (heap) has also a noticeable impact on farm management because they took large places and make the farmland frag-ile and spent much amount of time and money on managing the weed when they prepare their farmland for recessional agriculture (Tewabe et al., 2016).

Fishing

Water hyacinth provides a highly complex habitat structure by restricting the growth of other submerged macrophytes. This modification and habitat complexity at the surface of the water are likely to affect fishes and another invertebrate’s habitat (Meerhoff et al., 2003). Water hyacinth can greatly affect fish catch rates because mats of water hyacinth can block access to fishing grounds clogging and damag-ing the eye of the net, and increasing costs (effort and materials) of fishing.

4.Impacts of Water HyacinthFurthermore, water hya-cinth tears gill nets and damage the boat’s motor which increases to the cost of fishing (Tewabe et al., 2016).

The invasion of lakeshore, river mouths, and wetlands by water hyacinth has ex-acerbated the deteriora-tion of the fish population. Oreochromis niloticus and Labeobarbus species are the most seriously affect-ed species because of the obliteration of their spawning areas and en-tangled by water hyacinth. The average weight of fish caught after the infestation of the weed was decreased by 46.4% (13 kilograms/ day). The elimination of fish feed and obstruction of fish migration to the spawning habitat by the weed were accounted for the reduc-tion of the amount of fish caught per fisherman.

on livestock feeds

The weed invades smother grazing lands at an alarm-ing rate which directly or indirectly harms livestock. Due to the expansion of water hyacinth and its com-petition with the native spe-cies the submerging grass-es and other native species becomes devastated.

3.Triggering factors for the prevalence of water hyacinth on Lake Tana

4.1.Crop production

4.2.Fishing 4.3.On livestock

feeds

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These results farmers purchase supplementary feeds for their cattle be-cause the grass on the grazing lands around the lake has been destroyed by the invasive water hyacinth.

Water supply

Lake Tana, river mouths, and swamps were the major sources of water for livestock watering, washing clothes and baths. How-ever, the infestation of water hya-cinth on the shores of Lake Tana affected the water supply for vari-ous purposes. According to Enyew et al. (2020), the weed blocked the digging of boreholes and fetching of water for drinking and cooking from the lake and river mouths. Hence, Livestock watering has been difficult for herders due to the blockage of the weed and poor odor of water. Swimming in the lake and river mouths is not also possible especially during the peak infestation seasons of water hyacinth. It also affects the hydro-logic balance of water bodies by increasing the evapotranspiration rate because of its large surface area.

Water quality

The weed can form a floating thick mat on the surface water which deterio-rates the water quality used for irriga-tion. Death and decay of water hyacinth vegetation in large masses deteriorate the quality and the quantity of potable water and increase treatment costs for drinking water. It alters the clarity of the water, reduce dissolved oxygen, nitrogen, phosphorous, heavy metals, and other contaminants. Water bodies infested by water hyacinth are char-acterized by higher turbidity, a higher level of chlorophyll, a higher level of chemical oxygen demand (COD), lower dissolved oxygen (DO), lower pH, and lower nitrates than non-infested areas (Brendonck et al., 2003).

Livestock health

Feeding of water hyacinth caused gut bloating and contin-uous diarrhea on livestock. The consumption of stalk tissues of water hyacinth which con-tain intercellular spaces filled with air as the principal factor for the bloating of ruminant guts and continuous diarrhea (Enyew et al., 2020). However, water hyacinth can be used for a healthy livestock feed if the weed chopped to eliminate in-tercellular spaces filled with air. The weed also increases the intensity of breeding of leech and other internal parasites in the lake and associated marsh-lands which contributed to the loss of body weight and death of livestock.

Human health

The water hyacinth may re-duce water quality in various ways and become breading habitat for mosquitoes, snails, and other organisms asso-ciated with human illnesses, including malaria, schistosomi-asis, encephalitis, filariasis, and cholera. The stagnant water resulting from the obstructing effect of water hyacinth creates a microhabitat suitable for the breeding of many vectors of human diseases and for host-ing poisonous snakes (Dega-ga, 2019).

4.4.Water supply 4.5.Water Quality

4.6.Livestock health

4.7.Human health

Impacts of Water Hyacinth

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BiodiversityWater hyacinth invasions change the natural diversity and balance of eco-logical communities. The weed out-competes all other species growing in the vicinity and posing a threat to aquatic biodiversity. Besides suppress-ing the growth of native plants and negatively affecting microbes, water hyacinth inhibits the growth and abundance of phytoplankton under large mats through lowering dissolved oxygen concentrations, ultimately affect-ing fishes. This is because fish feed on phytoplankton. Reduced phyto-plankton productivity can decrease zooplankton abundance by decreas-ing food availability (Richards et al., 1985). Water hyacinth also affects diversity, distribution, and abundance of life in aquatic environments and enhances evapotranspiration, thus affecting all aquatic organisms. The death and decay of water hyacinth vegetation in large masses create an-aerobic conditions and production of lethal gases (Tewabe et al., 2017). Coverage of water hyacinth causes de-oxygenation of water, and at times anoxia below the dense mats.

Hindrance to water transport

Access to harbours and docking areas can be seriously hindered by mats of water hyacinth. Hence water hyacinth is becoming a serious hazard to water transport on Lake Tana as large floating and densely intertwined carpets of the water hyacinth forms.This has an advese effect on the tourism sector

Water hyacinth Controlling methods

The most appropriate management strategy for water hyacinth is not al-ways obvious and will depend on the site. Factors such as the nature and use of the waterway, climate, size and age of the infestation, presence or absence of an upstream infestation, as well as current and ongoing resources available, all need to be considered in the control and man-agement of water hyacinth. Several controlling mechanisms are available which include chemical, physical (mechanical and manual removal), bi-ological control, and a combination of these. Each has its benefits and drawbacks.

Chemical Controlling method

This technique involves the application of herbicides to get rid of the weed. Paraquat, 2, 4-D acid, and Glyphosate are chemicals widely used for this purpose especially in Africa (Martínez Jiménez, 2003). This meth-od of control is quick, and not costly when compared to mechanical and manual control techniques but it requires a select skill to be efficient. Experts and stakeholders in issues concerning the environment, advised that caution be taken when chemical control is being considered because of the effects it may have on non-target organisms. Therefore, the chemi cal option is the least desirable as the chemicals used to kill the weed

The chemicals could also enter humans via the food chain, and precipitate health problems.

Physical/Mechanical Controlling method

This method involves harvesting, cutting, rot ovation, weed raking, hand pulling, dredging, channel cleaning, and excavation using mechanical methods such as ma-chinery, containment booms or fences. Water hyacinth removal by this method is a practical con-trol method often used for small areas or when numbers are low. Physical removal is most effective for small infestation and should be made ahead of flowering and seed set. Mechanical control of water hyacinth can help take advantage of flooding or water flush that deposit water hyacinth in dams and calm water areas of rivers. When using this approach, it is essential that water hyacinth should be removed before its rapid growth commences. Me-chanical harvesting has limitation when it is applied to controlling of nuisance species. It affects water quality, survival of aquat-ic animals and plants, available nutrients, and turbidity. The cut material remains in the water and can decompose and increase nutrients and turbidity (James et al., 2002). Physical removal of water hyacinth is quite an expen-sive and laborious task. However, if systematically implemented it may be of great value to reduce a moderate stand of the weed.

4.8.Biodiversity

4.9.Hindrance to water trans-

5.2.Physical/Mechanical Controlling method

5.Water hyacinth Controlling methods

5.1.Chemical Controlling method

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Biological Controlling method

Biological control of water hyacinth is a practice by which the weed is reduced by introducing biological agents such as weevils and pathogens. In this method, the removal of the invasive plant can be either by directly feeding on the plant, causing diseases for the nuisance plant species or by resources competition. The application of biological natural enemies for the control of water hyacinth is the most effective, economical and sustainable control methods as the con-trol persists for long time with little ongoing cost. However, if biological control is not implemented in an adequate way, its reputation will be damaged. This normally happens when release of the weevils is not carried out systematically and the level of water hyacinth infestation is very high. From the biological point of view, there should be a population threshold of the weevils needed to reduce the weed stand and, based on this, advice on the number of the insects and frequency of release should be given, but most of the time this does not happen. Most popular weevils, Neochetina eichhorniae and Neochetina bruchi species are common biological water hyacinth control methods (Center and Dray, 2010). In Ethiopia, although the control of water hyacinth by using bio-logical control agent still is not beyond experimental stage, the use of bio-con-trol agent at the national level has received attention and researchers have been engaged in surveys and evaluate classical and native bio-control agents. The main challenge for this method is the difficulty of production of sufficient number of bio-agents. However, in countries like India, they used the biological control method successfully. For instance, in Banglor, India from 20,000 ha water hyacinth affected water surface, more than 95% was successfully cleared within 32 months using the biological method (Neochetina eichhorniae ). In Africa, countries like South Africa in Clairwood Quarry, Kenya in the Lake Victoria successfully controlled the water hyacinth with biological control (weevils).

Fig. Water hyacinth infected by planthopper

Water hyacinth control using harvest machine (left) hand removal (right)

5.3. Biological control

Neochetina Spp.

The main chal-lenge for the Biological wa-ter hyacinth control method is the difficul-ty in the pro-duction of suf-ficient number of bio-agents.

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Integrated Management Approach

Integrated control of water hyacinth is a sensible approach that includes the combination of mechanical, biological, and chemical methods that complement each other. Because of the rapid dispersal habit of water hyacinth and among practical constraints and financial costs associat-ed with physical, mechanical, biological, and chemical control measures alone are not effective for the control of water hyacinth. Thus, integrated managements that stress the weeds over a longer period are usually re-quired for effective control particularly for the established infestation. To sustainably manage the abundance of water hyacinth weed; an advance integrated weed management approach is very essential, where manual, mechanical, biological and herbicides are being jointly implemented (Afe-work, 2008). For instance, in Malaysia water hyacinth has been success-fully controlled through the combination of manual removal with biologi-cal control (Nai Kin, 1995). Mexico has been also successful in removing water hyacinth from Trigomil Dam by using mechanical method integrated with herbicide. Similarly, USA was also successfully control the weed from the water bodies by integrating mechanical and chemical methods.

Interventions to control water hyacinth in Lake Tana

Water hyacinth removal campaigns have been done annually since 2012. The local farmers are playing a great role in these campaigns. In 2017, the invasive weed became an issue of the community, especially for the youth. As a result, the youth from different corners of the country partic-ipated in a water hyacinth removal campaign, in what came to be known as the “Save Lake Tana” movement. Local and international charities have been organized and got legal recognition to participate in aware-ness creation and community mobilization to remove the invasive weed from the Lake. Even though campaigns are continuously undertaken, the expansion of the invasive weed in Lake Tana is far from control. The removal of the invasive weed by hand is becoming hard due to the ex-pansion rate of the weed is extremely rapid and there are insects and snakebites of people involved in manual removal. Hence, institutions and the Diaspora community bought harvest machines to remove the water hyacinth from the deep water part of the lake, which is difficult to use the labor force. So far, about seven harvesting machines are operating in the Lake. However, factors like road accessibility, depth of the lake, and maintenance, operational cost, dumping site, and other problems made the effort ineffective. With regard to the biological control, the rearing of the weevils is done however releasing to the weevils to the lake requires approval from Lake Tana Agency.

5.4.Integrated Management Approach

6. Water hyacinth control measures on Lake Tana

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The way forward

Lake Tana is the largest freshwater body in the country. Besides, it is registered by UNESCO as a world biosphere reserve site for its being rich in biological and cultural di-versity. However, the lake and its surroundings have been threatened by siltation, recession agriculture, water pollu-tion, overgrazing, and infestation by invasive weed species. Water hyacinth is one of the invasive species that severe-ly threatened the Lake. This invasive species has not only threatened the aquatic biodiversity but also hindered eco-nomic and social development among affected communi-ties. As a results of this, several attempts have been made to control the invasion of water hyacinth on Lake Tana. However, previous attempts were not successful due to various reasons mainly due to lack of coordination among the stakeholders. Therefore, based on the experience from other countries which succeeded in controlling the water hyacinth, we have forwarded the following actions. •Coordinated effort among the stakeholders,•Strong organizational structure with adequate financial resource,•Use of integrated water hyacinth controlling mechanisms instead of using a single method,•Delineation of buffer zone around the Lake,•Integrated watershed management,•Proper waste management specially the domestic and in-dustrial wastes from Bahir Dar and Gondar City,•Design and implement monitoring and evaluation system,•To use the removed water hyacinth for fertilizer, animal fodder and other purposes•To take a lesson from the mistakes we made on the water hyacinth control and give proper attention and take pre-ventive measures to the newly emerging weeds such as አዞላ (Duckweed)፤ ሶኬ፤ የአምባዛ ሳር እና ችግኙ

References Afework D: Efficacy of integrated water hyacinth (Eichhornia crassipes [mart] solms) management strategies at won-ji-shoa sugar factory. Ambo university college, 2008.Brendonck, L., Maes, J., Rommens, W., Dekeza, N., Nhiwatiwa, T., Barson, M., Callebaut, V., Phiri, C., Moreau, K., Gratwicke, B., Stevens, M., 2003. The impact of water hyacinth (Eichhornia crassipes) in a eutrophic subtropical im-poundment (Lake Chivero, Zimbabwe). II. Species diversity. Arch. Hydrobiol. 158 (3), 389–405.Center, T.D.,Dray, F.A., 2010.Bottom-up-controlof water hyacinthweevil popula-tions: do the plants regulate the insects? J. Appl. Ecol. 47 (2), 329–337.Chebud, Y.A.; Melesse, A.M. Modelling lake stage and water balance of Lake Tana, Ethiopia. Hydrol. Process. 2009, 23, 3534–3544Chopra, R.; Verma, V.; Sharma, P.K. Map-ping, monitoring and conservation of Harike wetland ecosystem, Punjab, India, through remote sensing. Int. J. Re-mote Sens. 2001, 22, 89–98Makhanu, K.S. Impact of water hyacinth on Lake Victoria. Water and Sanitation for All Partnerships and I n n o v a -tions. In Proceedings of the 23rd WEDC International Conference, Durban, South Africa, 1–5 September 1997; pp. 165–166Martínez, M. (2003). Progress on water hyacinth (Eichhornia crassipes) manage-ment. FAO Plant Production and Protec-tion Paper (FAO).Richards DI, Small J and Osborne J. 1985. Response of zooplankton to the reduction and elimination of submerged vegetation by grass carp and herbicides in four Florida lakes. Hydrobiologia; 123: 97-108.James, W.F., Barko, J.W., Eakin, H.L., 2002. Water quality impacts of me-chanical shreddingof aquatic macrophytes. J. Aquat. Plant Manage. 40, 36–42.

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