Water lettuce (Pistia stratiotes)

Present distribution

Scientific name:	Pistia stratiotes L.
Common name(s):	water lettuce

This weed is not known to be naturalised in Victoria

Habitat:

Forms dense floating mats on rivers, dams, irrigation canals, channels, (CSIRO 2007), slow-moving streams, drainage ditches, ponds, lakes, springs (FNA 2008), surface flow wetlands (Mitsch et al. 2008), reservoirs, polluted water (Parsons & Cuthbertson 2001), nearly dried-out lagoons, mud at water’s edge (Blake 1954), rice paddies (Rao et al. 2007). Grows in nutrient rich, muddy water, low light intensities (Tewari et al. 2008), “disturbed aquatic habitats, particularly those that are man-made” (CSIRO 2007), lower altitudes (Muniappan et al. 2008). “Can survive for long periods when stranded on mud banks or in other damp situations such as a roadside culvert where it may even take root” (Land Protection 2006). Occurs in a seasonally artificially flooded floodplain, whilst rapid seed production enables it to survive drought and high salinity (den Hollander et al. 1999).

Potential distribution

Potential distribution produced from CLIMATE modelling refined by applying suitable landuse and vegetation type overlays with CMA boundaries

Map Overlays Used

Land Use:
Broadacre cropping; water

Ecological Vegetation Divisions
Freshwater wetland (permanent); freshwater wetland (ephermeral)

Colours indicate possibility of Pistia stratiotes infesting these areas.

In the non-coloured areas the plant is unlikely to establish as the climate, soil or landuse is not presently suitable.

map of the potential distribution of pistia stratiotes

Red= Very high	Orange = Medium
Yellow = High	Green = Likely

Impact

QUESTION	COMMENTS	RATING	CONFIDENCE
Social
1. Restrict human access?	“Once established it spreads quickly over the entire surface of freshwater lakes, rivers and canals, impeding traffic and interfering with water flow and recreational activities” (Parsons & Cuthbertson 2001). Major impediment to access waterways	H	MH
2. Reduce tourism?	“Recreation and aesthetics: Large infestations of water lettuce stop the passage of boats by clogging the inlets of boat engine water-cooling systems. The mats of weed also interfere with swimming and make fishing impossible. The natural beauty of an open water body can be spoilt and further degraded as native aquatic plants, birds and animals are displaced.” (Land Protection 2006). Major impact on recreation. Weeds obvious to most visitors, with visitor response complaints and a major reduction in visitors	H	M
3. Injurious to people?	Poses a “safety and health risk: Children and livestock may be in danger of drowning if they become entangled in the roots of a heavy infestation. Water Lettuce mats also create a haven for mosquitoes that are vectors of Ross River Fever and Encephalitis.” (Land Protection 2006). Spines, burrs or toxic properties at most times of the year, or may be a major component in allergies, hayfever and/or asthma; in aquatic situations: probability of the plant causing death through drowning.	H	M
4. Damage to cultural sites?	“Damage to structures: Under flood conditions, rafts of plant material build up at fences and bridges that in turn collect other floating debris. The combined weight may cause these structures to collapse.” (Land Protection 2006). Major structural damage to site.	H	M
Abiotic
5. Impact flow?	“Growth is rapid forming dense mats, often covering rivers, dams and irrigation canals. These mats, with their long roots can restrict the flow of water, increase water loss through transpiration and reduce light infiltration that is necessary for photosynthesis in submerged aquatic plant life.” (Land Protection 2006). “Once established it spreads quickly over the entire surface of freshwater lakes, rivers and canals, impeding traffic and interfering with water flow” (Parsons & Cuthbertson 2001). Serious impacts both to surface and subsurface water flow	H	M
6. Impact water quality?	“Floating aquatics, including… water cabbage (Pistia stratiotes)… assist in removing dissolved nutrients from the water, control mosquitoes, and provide a nutrient-rich food source for the pigs” (Lansing et al. 2008). Found to reduce BOD, COD, TSS, Turbidity and ammonium in sewage water (Mishra & Tripathi 2008). At sites with floating aquatic vegetation, including P. stratiotes, it was found to cause thermal stratification, poor oxygenation with water at the bottom approaching “anoxic conditions” and light extinction coefficients were “substantially greater” than coefficients for open water sites (Chimney et al. 2006). “Degradation of water quality: Heavy water lettuce infestations reduces the infiltration of sunlight necessary for native plant growth in creek and riverbeds. Heavy weed cover also prevents the exchange of air, which normally occurs on an open water surface. As the plant dies and decomposes, oxygen is removed from the water causing water pollution and stagnation. This stagnation affects water quality and may result in the death of aquatic animals” (Land Protection 2006). High effects in dissolved oxygen and lights; causing eutrophication	H	M
7. Increase soil erosion?	As it is a free-floating macrophyte (Mitsch et al. 2008), hence does not occur on land, it is not likely to affect the probability of soil erosion	L	H
8. Reduce biomass?	“Once established it spreads quickly over the entire surface of freshwater lakes, rivers and canals, impeding traffic and interfering with water flow” and can have an “annual biomass production of about 72 tonnes dry matter per hectare” (Parsons & Cuthbertson 2001). Biomass may increase.	L	MH
9. Change fire regime?	As it is a free-floating macrophyte (Mitsch et al. 2008), hence does not occur on land, fire would not occur on those areas at all, thereby having no effect on fire regime	L	MH
Community Habitat
10. Impact on composition (a) high value EVC	EVC = Estuarine Wetland (V); CMA = East Gippsland; Bioregion = East Gippsland Lowlands; H CLIMATE potential. Monoculture within a specific layer; displaces all spp. within a strata/layer.	H	H
(b) medium value EVC	Aquatic weed. All water bodies in Victoria are given a high value EVC.	L	H
(c) low value EVC	Aquatic weed. All water bodies in Victoria are given a high value EVC.	L	H
11. Impact on structure?	“Perennial aquatic weed spread all over the world, considered noxious and extremely invasive for freshwater environments” (Mishra & Tripathi 2008). A “species that produce[s] acute impacts on ecosystem structure and function” (García-Llorente et al. 2008). “Once established it spreads quickly over the entire surface of freshwater lakes, rivers and canals, impeding traffic and interfering with water flow and recreational activities. The dense mass reduces light penetration, oxygen concentration and pH levels in the water, thus destroying the habitat of fish and waterfowl. It can also be a serious competitor in rice paddies, taking root in the soil and competing for nutrients and space” (Parsons & Cuthbertson 2001). Major effects on all layers. Forms monoculture; no other strata/layers present	H	MH
12. Effect on threatened flora?	“Once established it spreads quickly over the entire surface of freshwater lakes, rivers and canals, impeding traffic and interfering with water flow and recreational activities. The dense mass reduces light penetration, oxygen concentration and pH levels in the water, thus destroying the habitat of fish and waterfowl. It can also be a serious competitor in rice paddies, taking root in the soil and competing for nutrients and space.” (Parsons & Cuthbertson 2001). “These invasives [including Pistia stratiotes] are altering the aquatic ecosystem for fish and animal species. These invasives are choking waterways, changing nutrient cycles, and reducing the recreational use of rivers and lakes” (Pimentel 2009) – although it is highly likely to have a negative impact on threatened flora, as it is not yet naturalised in Victoria, it is not know to effect Bioregional Priority 1A spp or VROT spp.	MH	L
Fauna
13. Effect on threatened fauna?	“Once established it spreads quickly over the entire surface of freshwater lakes, rivers and canals, impeding traffic and interfering with water flow and recreational activities. The dense mass reduces light penetration, oxygen concentration and pH levels in the water, thus destroying the habitat of fish and waterfowl. It can also be a serious competitor in rice paddies, taking root in the soil and competing for nutrients and space.” (Parsons & Cuthbertson 2001). “These invasives [including Pistia stratiotes] are altering the aquatic ecosystem for fish and animal species. These invasives are choking waterways, changing nutrient cycles, and reducing the recreational use of rivers and lakes” (Pimentel 2009), although it is highly likely to have a negative impact on threatened fauna, as it is not yet naturalised in Victoria, it is not know to effect Bioregional Priority 1A spp or VROT spp.	MH	L
14. Effect on non-threatened fauna?	“Destruction of wildlife habitats: A large infestation of water lettuce is a physical barrier for aquatic and semi-aquatic animals, restricting their territorial movements and breeding activities.” (Land Protection 2006) “Destroying the habitat of fish and waterfowl (Parsons & Cuthbertson 2001). Habitat changed dramatically, leading to the possible extinction of non-threatened fauna	H	M
15. Benefits fauna?	“Once established it spreads quickly over the entire surface of freshwater lakes, rivers and canals, impeding traffic and interfering with water flow and recreational activities. The dense mass reduces light penetration, oxygen concentration and pH levels in the water, thus destroying the habitat of fish and waterfowl” (Parsons, Cuthbertson 2001) – provides very little support to desirable species	H	MH
16. Injurious to fauna?	Poses a “safety and health risk: Children and livestock may be in danger of drowning if they become entangled in the roots of a heavy infestation” (Land Protection 2006)	H	M
Pest Animal
17. Food source to pests?	“Rapidly forms dense mats covering rivers, dams and irrigation canals. It can restrict water flow, increase water loss by transpiration and serve as a breeding ground for mosquitoes” (Land Protection 2006). “Fresh plant material is accepted by weaner and adult pigs” (Parsons & Cuthbertson 2001). Supplies food to serious pest, but at low levels.	MH	MH
18. Provides harbour?	It may provide “shelter for, and a breeding site of, disease-carrying mosquitoes, especially the malaria-carrying Anopheles spp., and the filiariasis-carrying Mansonia spp.” (Parsons & Cuthbertson 2001). Provides harbour for minor pest species.	ML	MH
Agriculture
19. Impact yield?	“Water lettuce can invade the rice fields, seriously limiting the growth of this crop” (Alliota et al. 1991), as it takes “root in the soil and [competes] for nutrients and space” (Parsons & Cuthbertson 2001). Poses a “safety and health risk… livestock may be in danger of drowning if they become entangled in the roots of a heavy infestation.” (Land Protection 2006). “These species [including P. stratiotes] impede boat transport and fishing activities and also cause eutrophication and loss of productivity” (MacKinnon 2002). Serious impacts on quantity (e.g. >20% reduction). Unviable to harvest crop/ stock.	H	MH
20. Impact quality?	“Once established it spreads quickly over the entire surface of freshwater lakes, rivers and canals, impeding traffic and interfering with water flow and recreational activities. The dense mass reduces light penetration, oxygen concentration and pH levels in the water, thus destroying the habitat of fish and waterfowl.” (Parsons & Cuthbertson 2001). May impact quality of stock drinking affected water and stock in infested aquaculture ponds – major impact on quality of produce (e.g. 5-20%).	MH	ML
21. Affect land value?	“Rapidly forms dense mats covering rivers, dams and irrigation canals. It can restrict water flow, increase water loss by transpiration and serve as a breeding ground for mosquitoes” (Land Protection 2006). The degradation of water ways may result in a decrease in land value <10%.	M	M
22. Change land use?	“Interference with irrigation and stock watering: Stock may have difficulty gaining access to drinking water if the surface is completely covered by weeds. Water flow to irrigation equipment is reduced due to the restrictive action of the roots, which in turn increases pumping times and costs” (Land Protection 2006) . Downgrading the priority land use, to one with less agricultural return.	MH	M
23. Increase harvest costs?	“Interference with irrigation and stock watering: Stock may have difficulty gaining access to drinking water if the surface is completely covered by weeds. Water flow to irrigation equipment is reduced due to the restrictive action of the roots, which in turn increases pumping times and costs.” (Land Protection 2006). “Clearing these weeds [including P. stratiotes] annually involves great labour costs” (MacKinnon 2002). Major increase in time or labour, or machinery in harvesting.	H	M
24. Disease host/vector?	Although it provides breeding ground for mosquitoes (Land Protection 2006), it does not appear to be a host or vector for diseases of agriculture.	L	M

Invasive

QUESTION	COMMENTS	RATING	CONFIDENCE
Establishment
1. Germination requirements?	Seeds are photosensitive. “The light requirement for germination does not restrict germination to any particular season. In fact, nondormant seeds will germinate at any time during the growing season, if light and soil moisture are not limiting” (Tang et al. 2008). Germinate in temperatures above 20 C and “in tropical areas, seeds require a short after-ripening period of about 20 days before germinating but, in the Netherlands, the after-ripening period extends to more than six weeks” (Parsons & Cuthbertson 2001). Requires natural seasonal disturbances	MH	MH
2. Establishment requirements?	“Free floating plants of Pistia stratiotes have widespread habitat and potential to grow in nutrient rich environment having muddy water with low light intensities” (Tewari et al. 2008). Requires more specific requirements to establish.	ML	H
3. How much disturbance is required?	“Free floating plants of Pistia stratiotes have widespread habitat and potential to grow in nutrient rich environment having muddy water with low light intensities” (Tewari et al. 2008) and “grows vigorously in polluted water” (Parsons & Cuthbertson 2001). “Water lettuce thrives in ecologically-disturbed aquatic habitats, particularly those that are man-made” (CSIRO 2007). It can also occur on rivers, lakes, wetlands and rice paddies (CSIRO 2007; Parsons & Cuthbertson 2001; Mitsch et al. 2008). Establishes in relatively intact or only minor disturbed natural ecosystems (e.g. wetlands, riparian, riverine, grasslands, open woodlands); in vigorously growing crops or in well-established pastures.	MH	MH
Growth/Competitive
4. Life form?	Free-floating macrophyte (Mitsch et al. 2008). Aquatic.	H	H
5. Allelopathic properties?	Although P. stratiotes is affected by allelopathic properties of water hyacinth (Kathiresan 2000), it is thought to have some allelopathic properties of its own, inhibiting “the growth of some microalgae in solid medium” (Aliotta et al. 1991). Allelopathic properties seriously affecting some plants.	MH	H
6. Tolerates herb pressure?	Eaten by the Amazon freshwater turtle, Podocnemis expansa (Oliveira-Júnior et al. 2009). And has “some potential as stockfeed... fresh plant material is accepted by weaner and adult pigs but cattle reject the ration when 5% of the plant is present.” However, “in Australia, N. pulchellus, released in 1982, cleared water lettuce from a dam near Bundaberg, Queensland, within seven months of release, and significantly reduced infestations in other dams near Brisbane.” (Parsons & Cuthbertson 2001). Consumed and recovers slowly. Reproduction strongly inhibited by herbivory but still capable of vegetative propagule production; weed may still persist.	ML	MH
7. Normal growth rate?	“growth is rapid” (Land Protection 2006)	H	M
8. Stress tolerance to frost, drought, w/logg, sal. etc?	“Water lettuce can survive for long periods when stranded on mud banks or in other damp situations such as a roadside culvert where it may even take root. “(Land Protection 2006). Has been found in the Northern Territory (1953) “covering surface of nearly dried-out lagoons and on mud at water’s edge” and as “free-floating tufts on a lagoon” (Blake 1954). Frost susceptible (Parsons & Cuthbertson 2001). Seeds bank persists through drought and freezing temperatures (Šajna et al. 2007). P. stratiotes died at very low levels of salinity (Haller et al. 1974). However, den Hollander et al. (1999) states that “rapid seed production by Pistia permits it to survive these unfavourable conditions, i.e. high salinity coupled with drought during the second half of the dry season”. Tolerant to waterlogging. Somewhat tolerant to drought, frost and salinity.	ML	M
Reproduction
9. Reproductive system	Sexual and vegetative (Parsons & Cuthbertson 2001).	H	MH
10. Number of propagules produced?	Not given in the literature (den Hollander et al. 1999; Parsons & Cuthbertson 2001). Unknown.	M	L
11. Propagule longevity?	Seeds survive the dry season in Senegal (den Hollander et al. 1999) – extent of longevity not given in the literature (den Hollander et al. 1999; Parsons & Cuthbertson 2001). Unknown.	M	L
12. Reproductive period?	Perennial (Parsons & Cuthbertson 2001). Mature plant produces viable propagules for 3 – 10 years	MH	M
13. Time to reproductive maturity?	“Flowering and reproduction commences early in the plant’s life – about the 4th or 5th leaf stage, when plant densities are high” (Land Protection 2006). Reaches maturity and produces viable propagules or vegetative propagules become separate individuals in under a year.	H	M
Dispersal
14. Number of mechanisms?	Both seeds and vegetative propagules “may be carried some distance by water flow... spreading the infestation over a wide area... also spread through the aquarium trade” (Parsons & Cuthbertson 2001). “New infestations may become established when plants are brought into new areas by flooding” (Land Protection 2006). Seeds may be spread by animals, such as pelicans (den Hollander et al. 1999). Bird dispersed or readily eaten by mobile animals.	H	MH
15. How far do they disperse?	Both seeds and vegetative propagules “may be carried some distance by water flow... spreading the infestation over a wide area... also spread through the aquarium trade” (Parsons & Cuthbertson 2001). Seeds may be spread by animals, such as pelicans (den Hollander et al. 1999). Very likely that at least one propagule will disperse greater than one kilometre.	H	MH

References

Aliotta G, Monaco P, Pinto G, Pollio A, Previtera L (1991) Potential allelochemicals from Pistia stratiotes L. Journal of Chemical Ecology 17(11), 2223-2234.

Biosecurity New Zealand (Undated) Biosecurity New Zealand Pest Plant Assessment list. Available at: http://www.biosecurity.govt.nz/files/pests/plants/nppa/nppa-tagassessments.pdf (verified 21 January 2009)

Blake ST (1954) Botanical contributions of the Northern Australian Regional Survey. II. Studies on miscellaneous northern Australian plants. Australian Journal of Botany 2, 99-140.

Chimney MJ, Wenkert L, Pietro KC (2006) Patterns of vertical stratification in a subtropical constructed wetland in south Florida (USA). Ecological Engineering 27, 322-330.

CSIRO (2007) Biological control of water lettuce, Pistia stratiotes. Australian Government. Available at http://www.csiro.au/resources/BiologicalControlWaterLettuce.html (verified 21 January 2009)

den Hollander NG, Schenk IW, Diouf S, Kropff S, Pieterse AH (1999) Survival strategy of Pistia stratiotes L. in the Djoudj National Park in Senegal. Hydrobiologia 415, 12-27.

FNA (2008) Flora of North America. Vol. 22. eFloras. Available at http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200027300 (verified 4 September 2008)

Garía-Llorente M, Martín-López B, González JA, Alcorlo P, Montes C (2008) Social perceptions of the impacts and benefits of invasive alien species: implications for management. Biological Conservation 141, 2969-2983.

Kathiresan RM (2000) Allelopathic potential of native plants against water hyacinth. Crop Protection 19, 705-708.

Land Protection (2006) Facts: Pest series – Water lettuce. Queensland Government, Natural Resources and Water. Available at
http://www.nqccs.com.au/library/weeds/water_lettuce.pdf (verified 22 January 2009).

Lansing S, Víquez J, Martínez H, Botero R, Martin J (2008) Quantifying electricity generation and waste transformations in a low-cost, plug-flow anaerobic digestion system. Ecological engineering 34, 332-348.

MacKinnon JR (2002) Invasive alien species in Southeast Asia. Asian Biodiversity, October – December, 9-11. Available at http://www.arcbc.org.ph/arcbcweb/pdf/vol2no4/09-11_sr_invasive_alien_species.pdf (verified 24 February 2009).

Mishra VK, Tripathi BD (2008) Concurrent removal and accumulation of heavy metals by the three aquatic macrophytes. Bioresource Technology 99, 7091-7097.

Mitsch WJ, Tejada J, Nahlik A, Kohlmann B, Bernal B, Hernández CE (2008) Tropical wetlands for climate change research, water quality management and conservation education on a university campus in Costa Rica. Ecological engineering 34, 276-288.

Muniappan R, Reddy GVP, Raman A (2008) Biological control of weeds in the tropics and sustainability. Western Pacific Tropical Research Centre, University of Guam. Available at http://www.wptrc.org/userfiles/file/Reddy/pages%201-16.pdf (verified 3 February 2009).

Oliviera-Júnior AA, Tavares-Dias M, Marcon JL (2009) Biochemical and hematological reference ranges for Amazon freshwater turtle, Podocnemis expansa (Reptilia:Pelomedusidae), with morphologic assessment of blood cells. Research in Veterinary Science 86, 146-151.

Parsons WT, Cuthbertson EG (2001) Noxious Weeds of Australia; 2nd edition. CSIRO Publishing.

Pimentel D (2009) Chapter 1. Invasive plants: Their role in species extinction's and economic losses to agriculture in the USA. In: Inderjit (ed.) Management of Invasive Weeds. Invading Nature: Springer series in invasion ecology. Springer.

Rao AN, Johnson DE, Sivaprasad B, Ladha JK, Mortimer AM (2007) Weed management in direct-seeded rice. Advances in Agronomy 93, 153-255.

Sahoo SK, Giri SS, Chandra S (2008) Rearing performance of Claris batrachus larvae: effect of age at stocking on growth and survival during fingerling production. Aquaculture 280, 158-160

Šajna N, Haler M, Škornik S, Kaligarič (2007) Survival and expansion of Pistia stratiotes L. in a thermal stream in Slovenia. Aquatic Botany 87, 75-79.

Tang AJ, Tian MH, Long CL (2008) Environmental control of seed dormancy and germination in the short-lived Olimarabidopsis pumila (Brassicaceae). Journal of Arid Environments xxx, 1-4.

Tewari A, Singh R, Singh KS, Rai UN (2008) Amelioration of municipal sludge by Pistia stratiotes L.: role of antioxidant enzymes in detoxification of metals. Bioresource Technology 99, 8715-8721

Global present distribution data references

Australian National Herbarium (ANH) (2009) Australia’s Virtual Herbarium, Australian National Herbarium, Centre for Plant Diversity and Research, Available at http://www.anbg.gov.au/avh/ (verified 22 January 2009).

Global Biodiversity Information Facility (GBIF) (2009) Global biodiversity information facility, Available at http://www.gbif.org/ (verified 22 January 2009).

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