Invasive Alien Species in Northern Ireland

Elodea nuttallii, Nuttall's pondweed

Elodea nuttallii (Planchon) H.St.John
 
Introduction
Elodea nuttallii is a submerged aquatic, rooted in mud or free-floating, similar in appearance to E. canadensis, found in still or slow-moving nutrient-rich waters.
 
Description
A totally submerged aquatic plant similar to E. canadensis and easily confused with it but differing by having narrower leaves (less than 1.75mm wide) and the leaves are also folded down the centre line (flat in E. canadensis) and have pointed tips (blunt in E. canadensis). They are also a paler green.
The two sexes are on separate male and female plants, but only females occur in Ireland and Great Britain. The tiny female flowers float on the surface at the end of long stalks.
 
Country of origin
North America from Quebec south to North Carolina.
 
Current distribution
This species has now spread to parts of Europe and Asia.
 
Location in Ireland
First found in the wild in Ireland in 1984 in western Lough Neagh. In Northern Ireland it is now recorded from many sites in Lough Neagh, Lough Beg, the Rivers Lagan and Lower Bann, a few lakes in County Down and near Belcoo in County Fermanagh.
Prior to c.2000, in the Republic of Ireland it was known only from a few sites around Dublin, and one in County Cork but is now scattered across the country in a number of lakes and reservoirs, The Royal and Grand Canals and rivers including the Shannon and Barrow.
 
Life cycle
Reproduction and spread in Ireland is entirely by breakage of the shoots. The resulting fragments can readily form new plants. Seeds are not produced in Ireland because there are no male plants, but even in its native North America seed set is rare and dispersal is mainly by shoot fragmentation. It overwinters as prostrate shoots which start to regenerate new lateral shoots as the temperature reaches 6-8°C. Unlike E. canadensis it does not produce turions (overwintering buds).
 
Wildlife and habitat impacts
Nuttall's waterweed can produce dense monocultures in some sites which impacts on the general species diversity, but in many other sites it seems to have had only minor impact on habitats and existing wildlife, and may have been present in Ireland for some years prior to its discovery in the 1980s. It occupies similar habitats to those of E. canadensis, but reportedly is increasing across Britain while E. canadensis has apparently declined. This has been linked with generally increasing eutrophication of waters. It is a food source for wildfowl.
 
Human impacts
None evident.
 
Key vectors
Fragments of the plant can be dispersed by becoming attached to machinery, footwear, fishing equipment, dredgers and possibly the feet of wildfowl.
 
What you can do as an individual
The plant is sold as an ‘oxygenator’ for ornamental ponds. Excess growth of this should not be dumped into watercourses or lakes. Dispose of the excess material by drying and burning or composting.
 
Advice to specific groups relevant to this species
Water management authorities, boat users and fishermen should be aware of the ease with which this species can be spread by fragments of shoots.
 
Management measures
The plant can be controlled in similar ways to Elodea canadensis – by mechanical removal, application of suitable herbicides and a biological method of control using grass carp (Ctenopharyngodon idella) which graze the plant. Seek expert advice before using any of these methods. The avoidance of eutrophication will inhibit growth of this species.
 
Management groups
In Northern Ireland a number of bodies are responsible for various aspects of watercourse and water body management, water quality, recreational use and aquatic habitat management. These include the Rivers Agency, the Water Service, Environment and Heritage Service and Waterways Ireland.
 
Further information
Links
Flora, Fauna, Earth, and Sky — The Natural History of the North Woods (USA)
http://www.rook.org/earl/bwca/nature/aquatics/elodeanut.html
Elodea spp. on Flora of North America on line
http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=111486
Methods of control – see: Centre for Aquatic Plant Management – Information Sheet 25
http://www.nerc-wallingford.ac.uk/research/capm/pdf%20files/25%20Elodea%20nuttallii.pdf
 
Literature
Simpson, D.A. (1986). Taxonomy of Elodea Michx. in the British Isles. Watsonia 16: 1-14.
(available on line at http://www.bsbi.org.uk/Vol16p1.pdf).
Paul Hackney 18 April 2007.
 
See the Invasive Species Ireland web site for further information -
http://www.invasivespeciesireland.com/mostunwanted/
 
Text written by:
Paul Hackney, Keeper of Botany, Ulster Museum