What is lagarosiphon?Lagarosiphon major (common name: Oxygen weed) originates from Southern Africa and is a submerged perennial (long-lasting) aquatic plant that can grow in both the shallows of a river and in water up to 6.5 metres deep. It has dark green shoots with leaves (6-20 mm long) that are typically curved downwards in a spiral arrangement.
Lagarosiphon was first recorded in the Hutt Valley in 1950 but can now be found all over NZ. It is believed to have found its way into the waterways from domestic aquariums where it is a common ornamental plant. Read more from ORC |
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Lagarosiphon issuesIn ideal conditions, Lagarosiphon can grow up to 2cm a day to a maximum height of 5 metres meaning it can quickly dominate waterways in a matter of years. The lack of natural predators and disease has led to Lagarosiphon being incredibly successful and native flora and fauna species cannot compete. Large weed beds can reduce water flow and light penetration leading to changes in the water’s oxygen levels and pH. These conditions are unfavourable for native species resulting in local extinction and ecosystem collapse.
500ha of the lake bed (1/5th of the total lake) is suitable for Lagarosiphon and is already saturated; 139ha of that is used for recreation. In recent years we have seen a decline in water-orientated activities due to the increased safety risk of weeds entangling around feet, propellers and possessions. Recreation is incredibly important to the mental and physical well-being of the Cromwell community which continues to suffer as weeds saturate the lake. Furthermore, as clumps of weed wash ashore, they reduce the appeal of our shoreline by creating unpleasant odours and flies. |
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Who manages lagarosiphon
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Toitū Te Whenua Land Information New Zealand (LINZ) has responsibility for the management of Crown owned lands and waterways including the bed of Lake Dunstan and associated weed/pest control programmes. Contact Energy and Otago Regional Council also financially support management activities in the area.
In 2016 LINZ created the Lake Dunstan Weed Management Committee to implement a 10 year plan to control Lagarosiphon in Lake Dunstan. Local governing bodies, stakeholders and trusts are on the committee including LDCT. The committee works together to create annual plans for weed management that take all aspects of the lake and community into account. View Lake Dunstan Weed management Plan 2016 |
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Options to manage lagarosiphon
A no-management approach is not an option for Lagarosiphon in Lake Dunstan. Although it has already established itself in all suitable areas of the lake, it will continue to threaten public safety, water quality and biodiversity as beds become denser. There are a variety of control methods available with factors such as depth, turbidity and density deciding which is used.
Methods that have and are used in Lake Dunstan are:
Methods that have and are used in Lake Dunstan are:
Hand cutting is where divers remove all or part of the weed. Partial removal is short-lived as Lagarosiphon grows incredibly fast, but, is beneficial to high-use areas. Although very labour-intensive and limited by depth and visibility, it can have a high success rate when paired with suction dredging.
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Suction dredging removes the weed down to the roots and is used to clear dense weed beds in small areas. This method is costly but can slow regrowth by up to three years if all fragments are collected to prevent re-establishment. There is however a risk to native aquatic ecosystems and it can increase suspended sediment and pollutants in the water.
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Mechanical harvesting is where the top of the weed is cut down to a certain depth, collected and then disposed of off-site. Boat ramps and swimming areas are regularly chosen for this method as it is quick and precise. The equipment does however create fragments that can wash ashore, create floating rafts of weed and potentially wash downstream through the dam.
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Diquat is a contact herbicide that removes moisture and leaves from plants causing them to die. Diquat effectively kills dense weed beds but only when there is little or no suspended sediment in the water. When diquat comes into contact with sediment, it quickly deactivates meaning calm conditions are needed for the application. This does also mean that it does not bioaccumulate in the water or soil, quickly becoming untraceable a few hours after application.
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What is being done
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A combination of methods for Lagarosiphon removal is currently used to increase the success of weed management. NIWA (National Institute of Water and Atmospheric Research) and LINZ create annual plans where each location is individually assessed to ensure the correct method is applied.
Eradication is very costly and labour-intensive so there is a need for a systematic approach to the problem. Lake Wanaka is upstream of Lake Dunstan and therefore has priority on weed removal before clearing can begin here. This is to ensure fragments of Lagarosiphon won't be washed downstream into cleared areas and undo eradication efforts. Containment of Lagarosiphon is incredibly important to ensure the problem does not continue to spread. NZ spends millions of dollars every year to control aquatic weeds and we have already lost native ecosystems as a result. One way to reduce the risk of transfer to non-infected water bodies by boats and gear is by enforcing the ‘Check, Clean, Dry’ initiative. |
What will be done in the futureLagarosiphon is found on most waterbodies in Otago and is very expensive and time-consuming to control. LINZ, NIWA, local stakeholders and the council are continually improving practices and researching new methods to help combat this issue. As technology and scientific studies advance, we can create new approaches (e.g. biological control and Endothall).
Once cleared from an area, the bottom lining with materials such as jute hessian can exclude light and reduce re-establishment in the river or lake bed. Studies have found it can prevent regrowth for up to five months but need replacing every 10 months. Once Lagarosiphon is removed upstream and from Lake Wanaka, this method can be used to systematically remove and prevent regrowth in Lake Dunstan. |
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References
[1] NIWA (2020) Freshwater Invasive Species of New Zealand 2020. To view Click here
[2] NIWA (2015) Spraying as a Solution. To view Click here
[3]LINZ (2020) Biosecurity Annual report 2019/20. To view Click here
[4] DOC (2002) Assessing the Prospects for Biological Control of Lagarosiphon (Lagarosiphon major (Hydrocharitaceae)). To view Click here
[5] NIWA (2016) Ten Year Management Plan for Lagarosiphon at Lake Dunstan: 2016 to 2025. To view Click here
[6] NIWA (NO DATE) Diquat Summary - Use & Safety. To view Click here
[7] NIWA (NO DATE) Options. To view Click here
[8] NIWA (NO DATE) Diquat: Frequently Asked Questions and Answers. To view Click here
[9] LINZ (2019) Biosecurity Control Programme 2018/19 Annual Report: Otago region. To view Click here
[10] NIWA (2016) Ten Year Management Plan for Lagarosiphon at Lake Wanaka. To view Click here
[11] NIWA (NO DATE) Submerged Macrophytes. To view Click here
Main image: NIWA
Images in order: ORC; Crux; NIWA; NIWA; Lakes water Quality; NIWA; NIWA; LINZ
[1] NIWA (2020) Freshwater Invasive Species of New Zealand 2020. To view Click here
[2] NIWA (2015) Spraying as a Solution. To view Click here
[3]LINZ (2020) Biosecurity Annual report 2019/20. To view Click here
[4] DOC (2002) Assessing the Prospects for Biological Control of Lagarosiphon (Lagarosiphon major (Hydrocharitaceae)). To view Click here
[5] NIWA (2016) Ten Year Management Plan for Lagarosiphon at Lake Dunstan: 2016 to 2025. To view Click here
[6] NIWA (NO DATE) Diquat Summary - Use & Safety. To view Click here
[7] NIWA (NO DATE) Options. To view Click here
[8] NIWA (NO DATE) Diquat: Frequently Asked Questions and Answers. To view Click here
[9] LINZ (2019) Biosecurity Control Programme 2018/19 Annual Report: Otago region. To view Click here
[10] NIWA (2016) Ten Year Management Plan for Lagarosiphon at Lake Wanaka. To view Click here
[11] NIWA (NO DATE) Submerged Macrophytes. To view Click here
Main image: NIWA
Images in order: ORC; Crux; NIWA; NIWA; Lakes water Quality; NIWA; NIWA; LINZ
