Chesapeake Bay Introduced Species Database


Trapa natans

Common name(s):
Water Chestnut
Trapa natans (Water Chestnut) was a widespread Eurasian water plant with edible seeds, and was introduced to North America as an ornamental plant in the 19th century. In the 20th century, it formed great blooms, clogging waterways in the Great Lakes, Hudson River, and in the Potomac, and upper Bay. It was cleared by mechanical methods in the 1950s, but has reappeared in 1998 in the Bird and Sassafras River, Maryland, and in 2014 in Pohick Bay, Viriginia, on the Potomac. These blooms have been controlled by state agencies, with the help of volunteers.
Image courtesy of U.S. Army corps of Engineers 1977.

Economic Impacts

Impact:
Boating; Habitat Change; Fisheries; Health
The impacts of Trapa natans (Water Chestnut) have fluctuated drastically with its population in tidal fresh tributaries of Chesapeake Bay. A successful eradication campaign nearly eliminated the plant from the Potomac River and upper Bay by the 1960s, but in 1998, a dramatic resurgence occurred on the Bird and Sassafras Rivers (MD), tributaries of the Upper Bay. Annual eradication campaigns since then have greatly decreased the extent of the infestation (Maryland Department of Natural Resources 2000).

Aesthetic - Trapa natans made swimming unpleasant (United States Army Corps of Engineers 1977). This plant is probably unattractive in large quantities, and likely to cause unpleasant odors when washed ashore. The plant was regarded as ornamental in fish ponds (Mills et al. 1997), and the seeds can be roasted and eaten (Brown and Brown 1984).

Fisheries - Areas of dense growth are avoided by fishes (United States Army Corps of Engineers 1977).

Boating- In the 1920s-1950s, Trapa natans interfered with navigation on the Potomac and upper Bay tributaries and required mechanical harvesting, and later, herbicide control to maintain channels. Overall costs for the 1920s-1950s control program probably exceeded $2.8 million dollars (1950 dollars, converted to 1992 dollars) (Stevenson and Confer 1978; United States Army Corps of Engineers 1977). The cost of the recent 1998-to present upper Bay eradication program, using mechanical harvesters and volunteers with rakes (Maryland Department of Natural Resources 2000), is not known.

Health - The seeds are spiny and can cause injuries if stepped on (Hurley 1990). Dense growths were probably favorable to mosquito breeding, as was the case with Myriophyllum spicatum (Eurasian Watermilfoil) (Stevenson and Confer 1978).

Habitat Change - See under 'Ecological impacts'. Trapa natans killed native submerged aquatic vegetation which were important waterfowl foods (United States Army Corps of Engineers 1977). Control methods-including mechanical harvesting and herbicides probably also had some adverse affects of fishes (Serafy et al. 1993) and on submerged aquatic vegetation beds.

References - Brown and Brown 1984; Mills et al. 1997; Hurley 1990; Serafy et al. 1993; Stevenson and Confer 1978; United States Army Corps of Engineers 1977.


Economic Impacts Outside Chesapeake Bay

Impact:
In areas where Trapa natans (Water Chestnut) has become abundant (Hudson River, Lake Ontario, Potomac, and others), it has required costly mechanical and chemical control programs (Mills et al. 1993; Mills et al. 1997). Effects of fisheries and waterfowl appear to be overwhelmingly negative (United States Army Corps of Engineers 1977). However, it is still frequently cultivated and sold as an ornamental.

References- United States Army Corps of Engineers 1977


Ecological Impacts

Impacts on Natives:
Competition; Habitat Change
The impacts of Trapa natans (Water Chestnut) have fluctuated drastically with its population in tidal fresh tributaries of Chesapeake Bay. A successful eradication campaign nearly eliminated the plant from the Potomac River and upper Bay by the 1960s, but in 1998, a dramatic resurgence occurred on the Bird and Sassafras Rivers (MD), tributaries of the Upper Bay. Annual eradication campaigns since then have greatly decreased the extent of the infestation (Maryland Department of Natural Resources 2000).

Competition - In the Potomac and some Upper Bay tributaries, Trapa natans grew so densely as to make a continuous sheet of vegetation across bodies of water, shutting out light, killing submerged aquatic vegetation (Stevenson and Confer 1978; United States Army Corps of Enginers 1977).

Habitat Change - 'It has also been found that very few fish will frequent these (Trapa natans) areas and that normal biological processes are terminated or severely reduced' (United States Army Corps of Engineers 1977). Decomposition of large biomasses probably depleted oxygen concentrations.

Food/Prey - Plants and seeds are not used as wildlife food (Hurley 1990).

References - Hurley 1990; Stevenson and Confer 1978; United States Army Corps of Engineers 1977
Impacts on Non-natives:
Competition; Habitat Change
Trapa natans (Water Chestnut) probably has had impacts on introduced as well as native fishes and introduced submerged aquatic vegetation similar to those on native species (see 'Impacts on Native Residents'). Plant species especially likely to be affected would be Myriophyllum spicatum (Eurasian Watermilfoil), Najas minor (Eurasian Water-Nymph), Potamogeton crispus (Curly Pondweed), and Hydrilla verticillata (Hydrilla).


This data was last modified on Thursday, September 29th, 2005.
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