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Natural Heritage -  Lagarosiphon major (African curly leaved waterweed)

The weed

Lagarosiphon major is an aggressive, invasive, alien plant species that originated in Southern Africa, where it is regarded as a nuisance weed. The plant can grow in water up to 6 m deep. In addition to occupying the full water column, plant stands produce dense vegetation on the water surface. The leaves are strongly recurved and are borne in whorls of 3 or in a spiral arrangement. The long stem is brittle and easily broken (aiding dispersal). Only female plants are present and all reproduction is by fragmentation or vegetative reproduction. Detached stems, when they sink, root from the nodes and establish new populations.

It dramatically alters the ecology for native plants, insects and fish. It creates a poorer ecosystem for native plants, insects and fish. It oses a serious threat for tourism, angling, boating and other recreational pursuits. It can cause fish kills through oxygen depletion. It is potentially a more major threat than the Zebra mussel. The devastation caused is clearly evident in the photographs.

Reproduction is vegetative (only female plants are present in Ireland). Studies have revealed two dispersal strategies:

- Fragmentation

- Self-layering (in summer)

Studies have also revealed two distinct morphological phases: (1) Tall, canopy-forming growth – maximum biomass (winter) (2) Collapsed condition – reduced biomass (summer)

How did Lagarosiphon major arrive in the Corrib?

It is generally accepted that it arrived in the Corrib as a fragement or a small plant mass in the late 1990s. It was probably dumped from an aquarium or a garden pond and was trasnported to the lake. It spread rapidly throughout the upper lake.

How was the problem of controlling it addressed?

A number of meetings took place which involved the various stakeholders around the Corrib. Arising from the deliberations, a number of potential control measures were proposed:

Experiments were carried out using hand removal in recenlty colonised sites, using the herbicide dichlobenzil on submerged weeks in localised areas in the upper lake in consultation with teh NPWS, mechanical cutting using V-blades and trailing knives with containment of the fragments. Twelve infected sites were treated using mechanical cutting in 2008/2009 with removal of ca. 6,000 tonnes of Lagarosiphon and about 35 hectares of lake were cleared. This resulted in a dramatic improvement in angling in Rinneroon post-cut in 2009.

Light  Exclusion due using biogradable geotextile

By far the most successive method of dealing with Lagarosiphon involved the use of biogradable geotextile, a technique invented by Dr. Joe Caffrey of the Central Fisheries Board (now Inland Fisheries Ireland). Despite it not being in this area of responsibility, Joe took on the role of dealing with Lagarosiphon - he as also an ardent supporter of the WRZNCI. The method involves the use of a natural jute- or hessian-based open-weave fabric which saturates and sinks rapidly. This is environmentally safe as it is 100% biodegradable. Initial trails began in Autumn 2008. However they had to be suspended due to a Government embargo which did not allow the recruitment of the temporary personnel required to implement the process. The WRFB did not have the required permanent staff.

The method could be used for relatively low lying Lagarosiphon less than 2 metres high. It required a modified boat, a team of at least two divers and calm weather. Approximately 10,000 sq metres of Lagarosiphon infected lake bed were covered and it was found that the plant dies rapidly beneath the geotextile and within 8 months there was no longer any evidence of its existence. Only an organic lake bed was observed.

Surprising and unexpected outcome

Within 8 weeks of the disappearance of Lagarosiphon, charophytes were observed to be growing through the geotextile and dense meadows were established withing 8 monthos of the laying of the geotextile. Other indigenous species also grew through the geotextile but thankfully, no Lagarosiphon grew through it. This avoided the need for costly transplantation programmes.

2014 Update from 2014 report of Inland Fisheries Ireland

In 2014 approximately 24 ha of Lough Corrib was treated for Labarosiphon major. At the beginning of 2014 there were 31.5 ha of weed in the lake which by year end had been reduced to 18.9 ha. The details of the technology used are shown in the following table.








There is a detailed map of the sites treated in the actual report.

The furthest down the lake Lagarosiphon has penetrated is still the single site in Annaghdown Bay.

Method

Sites treated

Area, m2

Jute matting

8

32,180

Hand picking

6

350

Mechanical cutting

9

206,820

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