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Divers join fight to save lake
Apr 15, 2004
Author: Mike Thorsen

Just how much effort is needed to eradicate an aquatic weed? After 5 years, over 325 hours underwater, boatloads of weed, and some innovative searching the East Coast Hawke’s Bay Conservancy feels it is getting close to the world’s first eradication of an aquatic weed from a large waterbody.

Lagarosiphon (an oxygen weed) was first found at Rosie Bay in Lake Waikaremoana in 1999, and was quickly recognised as a threat to the biodiversity values and recreational opportunities of the lake. The original plant probably arrived as a fragment on fishing gear or waterfowl from nearby lakes at Tuai.

An eradication plan was quickly prepared and the bay closed by a floating boom. Initial survey found Lagarosiphon was restricted to Rosie Bay, but one plant was since found and removed at another site.

To reduce the huge mass of Lagarosiphon at Rosie Bay fibreglass weed matting was laid over the main area of infestation and then covered with weighted black plastic. During this time divers would spend a week every month, sometimes with snow falling and howling winds, removing Lagarosiphon from areas of the bay not covered by matting and checking the security of the matting. When the matting was lifted after 3 years only mud was present, which was rapidly colonised by native aquatic plants. Currently divers spend one day a month, scouring the lake bed and removing the few plants that arise. It is hoped that these divers will soon be coming back empty handed.

A glass-bottom boat and a submersible camera are being used to scrutinise high-risk areas of Lake Waikaremoana to ensure the weed is not becoming established at any other sites. This boat and camera will be used next year to survey Lake Waikareiti, which is thought to be aquatic weed free.

Lagarosiphon (Lagarosiphon major) is also known as Curly Oxygen Weed.

The leaves of Lagarosiphon are up to 15mm long and are usually curled backwards. The leaves are arranged spirally around a slender stem.

Lagarosiphon forms dense clumps to the exclusion of all other vegetation and can completely choke waterways. It also harms recreation (e.g. boating, fishing, swimming) and can cause costly damage to hydroelectric companies.

Lagarosiphon is recorded throughout NZ except in the West Coast of the South Island. The species is still spreading and without intervention there is a risk that it will spread into all lakes and ponds.

Lagarosiphon was once sold throughout NZ for use in aquariums but is now an Unwanted Organism under the Biosecurity Act 1993. It is illegal to sell, propagate or distribute Lagarosiphon.

If you have Lagarosiphon in your garden pond or aquarium…
Put any Lagarosiphon in your compost bin, or bury it in the garden. If you have a large pond that is affected, contact DOC weeds staff for advice on management options.