A southern African invader, a curly-leaved waterweed, is taking over a bay in Lough Corrib, disrupting fisheries and killing off native plants, writes Dr Claire O'Connell
Just when you thought it was safe to go back in the water, an alien invader is discovered lurking in one of Ireland's renowned fishing lakes. But this is no creature from outer space - it is an invasive aquatic plant species from southern Africa called curly-leaved waterweed or Lagarosiphon major.
This mean, green, growing machine is taking over a bay of Lough Corrib, displacing native plants and disrupting the ecology of natural brown trout fishing waters.
Dr Joe Caffrey, a senior scientific officer with the Central Fisheries Board in Dublin, says the curly-leaved waterweed has been in Ireland for several years. Garden centres sell the alien plant, which is often confused with naturalised waterweed species such as Elodea, and biology teachers sometimes use it in photosynthesis experiments.
Caffrey says Lagarosiphon has not caused problems until recently, but now the push is on to find out about its growth habits and to control its spread.
He became aware of the Lough Corrib invasion last summer when local fisheries personnel sent him a sample of a weed they found growing in the upper lake.
"As soon as it arrived on the desk I knew what it was," he says. "I was down to Lough Corrib within three hours and I was confronted with a bay that was almost totally overgrown with this plant."
He says the weed's presence causes serious concern because it can spread rapidly. "Literally a stem on the bottom of a boat brought to another lake is enough to start a new population," he says. And where it grows densely, it can have disastrous consequences for the natural habitat.
"It can take over watercourses, growing over our native plants and very rapidly excluding and killing them off, predominantly by blocking out the light. It really has quite a dramatic impact where it is growing with significant density," he says.
Since the Lough Corrib discovery, Caffrey has found the weed throughout the country, in golf courses and garden ponds, but so far at no other site in the wild. Now researchers are asking why it has taken hold in this one wild location in Ireland and how it can be stopped.
"It hasn't been a problem in Ireland before so there's no research done on it," says Dr Maria Mulholland from Trinity College Dublin's Centre for the Environment. In September, she and undergraduate student Joanne Kohls worked with Caffrey's team to survey the extent of the weed's invasion at Lough Corrib.
The researchers painstakingly sampled plants, water and sediment from different parts of the infested bay to get an idea of the weed's spread and impact. Under difficult conditions, divers used metre-square quadrats to count the density of Lagarosiphon stems, collecting data to estimate the invasive weed population's biomass.
"It was literally a jungle of Lagarosiphon," says Mulholland, describing how the weed occupies the water column's full depth of around four metres. "There was nothing growing underneath it and just at the edges you would have some native vegetation which didn't look particularly healthy - it was being invaded."
In the lab, the TCD researchers compared Lagarosiphon's growth with Elodea canadensis, which is a member of the same botanical family but is naturalised in Ireland. Mulholland says Lagarosiphon grows faster and is more robust.
The problem in Lough Corrib is that Lagarosiphon disrupts the ecological balance by kicking out native plants which provide cover and food for a variety of insects that in turn are food for fish.
Mulholland and Caffrey plan to carry out further investigations on the plant and are in contact with experts in New Zealand where Lagarosiphon clogs electricity turbines in hydrolakes.
A task force, involving the Central and Western Fisheries Boards, Galway County Council and the National Park and Wildlife Service, has been set up to look at the problem.
In the meantime, you can play your part. Lagarosiphon can be difficult to distinguish from naturalised Elodea species, but if the alien weed skulks in your garden pond, Caffrey says to remove the plant carefully and burn, compost or bury it. You can contact him at joe.caffrey@cfb.ie