22nd September

Threat to native aquatic species from ornamental plants

Ireland needs to avoid repeating the mistakes made by countries that tried to eradicate freshwater invasive aquatic species, if it is to have an effective programme of control, an international expert, has indicated.

Dr John Clayton, a senior scientist and aquatic plants specialist with NIWA - the National Institute of Water & Atmospheric Research of New Zealand – told a public meeting in Galway that a national programme to tackle invasive plants like the Africa weed, Lagarosiphon major, were likely to be very expensive and would succeed only if they were well planned from the start.

The carpet of Lagarosisphon is so dense in places that swans have been seen walking on it.

The carpet of Lagarosisphon is so dense in places that swans have been seen walking on it.

“Unfortunately, it’s not as simple as going out and just cutting the weed,” he warned.
“For a control or eradication programme to be really effective, and to get the ‘best bang for your buck’, you have to put a lot of thought into it from the start.  You must know exactly what you are doing and where you are doing it.”


Dr Clayton revealed that reluctance by the New Zealand authorities to provide adequate funding at the start was regarded now as a serious mistake, giving rise to “a stop-go approach to controlling Lagarosiphon early on. I’d hate to see you in Ireland making the same mistakes and having the same experiences that we had in New Zealand in the beginning. The main problem we had to face was a complete lack of continuity and commitment to funding which in turn gave rise to a very chequered history of success.”

He added that whenever funding was significant they always managed to cover a lot of ground.

“But then, as soon the problem appeared to go away, no one wanted to go on putting money into it anymore.”

Threat to flora and fauna
A native of South Africa, Lagarosiphon is a submerged freshwater plant introduced here to oxygenate ornamental garden ponds and aquaria.

In the past few years it has become a serious problem in some Irish waterways and is being seen increasingly as a major threat to the country’s indigenous aquatic flora and fauna.

Given optimum condiditons, Lagarosiphon grows quickly and often produces a dense vegetation on the water surface. It grows to depths of six meters and shades any native plants growing beneath it.

Deprived of sunlight, these quickly die off - as do the various invertebrate communities dependent on them for food and shelter.

Next, the trout that feed on the invertebrates starve, and eventually they too die off to be replaced by a variety of coarse fish such as roach, bream and pike. The result – in a relatively short time - is a complete change in the predominent fish community of the water body in question.


Lagarosiphon was first confirmed in Lough Corrib when it was spotted in Rinneroon Bay in the upper part of the lake in May 2005, about six kilometers north-west of Oughterard.

At the end of 2005 it had been noted at eight other locations. By 2006 however, the number of confirmed sightings had tripled, indicating just how rapidly it can spread when conditions are right.

According to Dr Clayton the cost of trying to control Lagarosiphon in New Zealand “have been significant.”
“Over the past thirty years we have spent about NZ$4 milllion trying to control it. And currently we spend over NZ$ 300,000 a year on this invasive weed alone.”

He argues however that expenditure like this should be seen in context, and reckons that it is money well spent if it manages to protect and preserve a priceless natural resource like the country’s lakes:

“In Ireland you will have to ask yourselves the same questions we had to ask: Just how much are these high-value water bodies like the Great Western lakes and rivers worth to us?

“They cannot be left to look after themselves, so you are going to have to do it. And once an invasive species like Lagarosiphon gets in, it very quickly changes the whole value of any water body in terms of the way people see it.

Therefore, you have to act quickly to preserve what you have. You may not be able to eradicate this weed but you should be able to control and retain it where it is now.”

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