Anacharis densa (Planch.) Victorin, Elodea densa (Planch.) Caspary, Philotria densa (Planch.) Small & St. John, leafy elodea, dense waterweed, Brazilian elodea

Egeria densa can inhabit waters with a wide range of temperatures, low CO₂ levels, and low light levels. This species can survive in waters with temperatures of 3-35°C (Yarrow et al. 2009). The plant can overwinter as seeds (although female plants are not found in the U.S.), dormant shoots, or semi-dormant shoots until temperatures rise above 15°C (Parsons and Cuthbertson 2001). Egeria densa exhibits the C4 pathway and utilizes HCO₃^-; thus it is able to photosynthesize in waters with low CO₂ levels (Casati et al. 2000). Egeria densa can tolerate high phosphorous levels, but is susceptible to iron deficiency (Parsons and Cuthbertson 2001).

This species has a low light requirement and can thrive in turbid environments (Parsons and Cuthbertson 2001). Optimal light intensity is about 100 lux. Egeria densa cannot tolerate high light intensities or high levels of ultra-violet and blue light, as it experiences chlorophyll damage to light levels of 1250 lux. Egeria densa cannot tolerate high UV-B radiation, as it can damage the enzymes involved in photosynthesis and can reduce photosynthetic capacity (Casati et al. 2002).

Flowers float above the water surface and are pollinated by insects (Parsons and Cuthbertson 2001). It reproduces asexually in Australia (Parsons and Cuthbertson 2001) and in the U.S. (Hoshovsky and Anderson 2001), where only the male plant has established. Egeria densa is capable of vegetative fragmentation; stems of at least two nodes can break off from the parent colony and disperse by stream flow (Parsons and Cuthberson 2001). Stem fragments that break off can take root in bottom mud or grow as free-floating mats (Hoshovsky and Anderson 2001). Fragmentation can occur as a result of the mechanical shearing of water flows, wave action, waterfowl activity, and boating.

This species may be transported by hitchhiking on recreational gear; E. densa grows in thick mats that can become entangled on boat propellers and trailer wheels, or can be captured in bilge water (Washington State Department of Ecology 2013). Attached fragments can be transported between water bodies. Egeria densa is not known to be taken up in ballast water. As a popular ornamental plant, E. densa is planted in water gardens (Indiana Department of Natural Resources 2013), however, there is not enough information available to determine the frequency of E. densa plantings.

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic

Potential pathway(s) of introduction: Hitchhiking/fouling, unintentional release

Egeria densa is one of the most common species sold in the Montreal aquarium trade (Cohen et al. 2007). The sale and transport of E. densa is prohibited in Illinois, Indiana, Michigan, Minnesota, and Wisconsin (Great Lakes Panel of Aquatic Nuisance Species 2012); but there are no regulations on the sale or transport of E. densa in New York, Ohio, Ontario, Pennsylvania, or Quebec. A survey performed from 2002 to 2003 on aquarium and pet stores near Lakes Erie and Ontario found that 35% of stores surveyed sold E. densa (Rixon et al. 2005). Due to the availability of E. densa in stores near the Great Lakes, and the ability of the species to overwinter, Rixon et al. (2005) predicted that it has the potential to be introduced to the Great Lakes. Based on the number of aquarium stores in Montreal, Quebec that sold E. densa, the number of E. densa sold by each store, and the disposal pathways for aquatic plants, Cohen et al. (2007) estimates that 188 E. densa individuals are released into the St. Lawrence Seaway each year.

Egeria densa is cultured in Florida for the ornamental aquatic plant industry (Fenner); however, there is no indication that E. densa is commercially cultured in the Great Lakes region. Egeria densa was first reported outside of its native range in 1893 in Long Island, New York (Yarrow et al. 2009). Egeria densa is a known nuisance species in California, Oregon, and Washington (California State Parks 2014; Washington State Department of Ecology 2013). Egeria densa has been reported in Powderhorn Lake, Minnesota (City of Minneapolis 2013) and Griffy Lake, Indiana, but has been controlled and eradicated. Egeria densa occurs private ponds in Lake County, Illinois (Illinois Database of Aquatic Non-native Species 2014; Lake County Health Department and Community Health Center 2009; New Invaders Watch Program 2013), which is near Lake Michigan; however, dispersal from these ponds is limited because it does not occur in waters connected to the Great Lakes.

Established in North America, but not including the Great Lakes.

Egeria densa has a moderate probability of establishment if introduced to the Great Lakes (Confidence level: High).

Egeria densa has broad physiological tolerances. It can tolerate low light conditions (Lara et al. 2002). Although a tropical plant, it is able to adapt to seasonal changes and overwinter (Indiana Department of Natural Resources 2013; Parsons and Cuthbertson 2001; Rixon et al. 2005; Yarrow et al. 2009). It can tolerate low levels of CO₂ (Casati et al. 2000), nitrogen, phosphorus, and nutrients (Yarrow et al. 2009). This species can survive in waters with salinities up to 8 ppt (Hauenstein and Ramirez 1986).This species is known to have a relatively fast growth rate (Yarrow et al. 2009). Moreover, the introduced ranges of Egeria densa have similar climate and abiotic conditions as the Great Lakes. Egeria densa has established near the Great Lakes previously (e.g. Griffy Lake, Indiana), in areas that have similar water temperature and dissolved oxygen levels (Aquatic Control, Inc. 2008; NOAA CoastWatch 2014). This species can survive in freshwater habitats of varying temperatures, light levels, and CO₂ levels; thus suitable habitats are readily available in the Great Lakes region. Turbidity and nutrient levels of the Great Lakes is likely suitable for E. densa to obtain sufficient light, phosphorus, and nitrogen. It is likely that Egeria densa will benefit from the effects of climate change, including warmer temperatures and shorter duration of ice cover. Increased salinization due to climate change may negatively impact this species’ establishment if salinities exceed 8 ppt (Hauenstein and Ramirez 1986). Grass carp preys on E. densa and occurs in the Great Lakes region (Anderson et al. 2008; Hoshovsky and Anderson 2001; Parsons and Cuthbertson 2001), but it is not likely that it will prevent the establishment of E. densa in the Great Lakes. Each year, it is estimated that 188 E. densa individuals enter the St. Lawrence Seaway through disposal of aquarium plants (Cohen et al. 2007).

Egeria densa reproduces asexually via vegetative fragmentation (Hoshovsky and Anderson 2001), which may aid its establishment in the Great Lakes region. This species spreads rapidly by vegetative fragmentation and recreational activities, resulting in dense mats (Indiana Department of Natural Resources 2013). E. densa has established extensively in 27 countries beyond its native range (Curt et al. 2010). In Australia, E. densa spread quickly; over a period of two years, it doubled its biomass and doubled the area it occupied in the Hawkesbury-Nepean River (Roberts et al. 1999). It is difficult to control once established (Yarrow et al. 2009) and it is recommended to correctly identify the plant due to its similarities with native plants (Indiana Department of Natural Resources 2013).

The model developed by Rixon et al. (2005) predicts that E. densa poses a threat to the Great Lakes.

Egeria densa has the potential for high environmental impact if introduced to the Great Lakes.

Egeria densa acts as an ecosystem engineer by preventing the resuspension of sediments and controlling light and nutrient availability (Yarrow et al. 2009). The dense growth of E. densa can retard water flow and reduce turbidity (Parsons and Cuthbertson 2001). This species can reduce the abundance and diversity of native plant seeds in lake bottoms due to increased sediment accumulation under its weed beds (Hoshovsky and Anderson 2001). Egeria densa removes nutrients from the water column, thereby decreasing the standing stock of phytoplankton (Yarrow et al. 2009). Furthermore, Egeria densa forms mats that can shade out phytoplankton. Egeria densa can outcompete native species. In Duck Lake, Washington, E. densa displaced native stonewart, elodea, and pondweed in a period of 3 years (Washington State Department of Ecology 2013). In Hawkesbury-Negean River, Australia, evidence suggests that E. densa outcompeted native vallisneria (Vallisneria americana) for light (Roberts et al. 1999).

Egeria densa is not known to pose a threat to the health of native species. This species is not known to alter predator-prey relationships or genetically affect native populations.

Egeria densa has the potential for high socio-economic impact if introduced to the Great Lakes.

The dense growth of E. densa can interfere with irrigation projects, hydroelectric dams, and urban water supply (Hoshovsky and Anderson 2001; Parsons and Cuthbertson 2001). In New Zealand, there was an infestation of E. densa in the Wikato River that clogged the water intake pipes resulting in the shut-down of an electrical plant (Washington State Department of Ecology 2013).  Egeria densa can inhibit recreational activities as a nuisance for navigation, fishing, swimming, and water skiing (Washington State Department of Ecology 2013). The removal of E. densa is costly; Washington local and state governments spend thousands of dollars each year to control the species. Egeria densa may pose a risk to human safety. In 2006, police reports indicate that E. densa may have contributed to the death of a physician in San Joaquin County, who drowned after becoming entangled in the “tentacle-like Delta weeds trap” in attempts to save his nephew (Breitler 2006; Victorian DPI 2013).

Egeria densa has gained widespread recognition by parks departments and local and state governments as a nuisance species (e.g. Great Lakes Panel on Aquatic Nuisance Species 2012; Lake County Health Department and Community Health Center 2009; Mcglynn 2013). Resources have been devoted in various cities and states to remove infestations of E. densa due to its costly impacts on water supply, infrastructure, and recreation (e.g. California State Parks 2014; Indiana Department of Natural Resources 2013).

Egeria densa has the potential for moderate beneficial impact if introduced to the Great Lakes.

It is an ornamental plant for aquariums and small ponds. Egeria densa has been recommended as a submerged oxygenator plant for water gardens (Creative Homeowner 2010). Egeria densa is also utilized in plant biology classes for students to study photosynthesis (Berkely 2014).

The sale and transport of E. densa is prohibited in Illinois, Indiana, Michigan, Minnesota, and Wisconsin (Great Lakes Panel of Aquatic Nuisance Species 2012); but there are no regulations on the sale or transport of E. densa in New York, Ohio, Ontario, Pennsylvania, or Quebec.

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control

Biological

White amur or Chinese grass carp, and Congo tilapia (Tilapia meanopleura) can be a biological control agent for Egeria densa (Hoshovsky and Anderson 2001, Parsons and Cuthbertson 2001). Sterile triplorid grass carp usage to control E. densa is permitted with California Department of Fish and Game authorization in several counties in California (Hoshovsky and Anderson 2001). In Argentina, the leafminer fly Hydrellia sp. is an herbivore that specializes on E. densa (Walsh et al. 2013). Peking ducks (Anas platyrhynchos) were experimentally used to remove E. densa, but they were not effective (Curt et al. 2010).

Physical

Hand-pulling, cutting, and mechanical removal provide temporary control, but can encourage dispersal by vegetative fragmentation (Curt et al. 2010). Flowing water treatments are used in New South Wales and Victoria, Australia which can help control other aquatic nuisance plant species (Parsons and Cuthbertson 2001).

Chemical

In California, herbicides are used with the consultation of a specialist (Hoshovsky and Anderson 2001). Contact-type diquat, copper, acrolein, and fluridone are used in California (Hoshovsky and Anderson 2001). In Sacramento-San Joaquin Delta, fluridone treatment started in June 2014 and will continue through September 2014 (California State Parks 2014). In an attempt to control E. densa in Griffy Lake, Bloomington, Indiana, fluridone treatments started in April 2006 and continued until September of that year (Jones 2006). The dose needed to control E. densa was high enough to kill all aquatic plants in the lake. Post-treatment surveys found only fragments of native species.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

Nonindigenous E. densa populations in Río Cruces, Chile have similar genotypes as populations in Western Oregon, suggesting that the two populations experienced similar bottlenecking events at introduction, or there is a lack of genetic diversity in the native population (Carter and Sytsma 2001).