Lagarosiphon has been found in large quantities at Lakes Waitaki and Aviemore in South Canterbury. Photo: NIWA / SUPPLIED
Divers have found large infestations of an invasive lake weed in two South Island hydro lakes.
Lagarosiphon was found in Lakes Waitaki and Aviemore in South Canterbury, which were previously considered largely weed-free.
Land Information New Zealand biosecurity manager Tracy Burton said the discovery was a blow after 15 years of efforts to protect the lakes from lagarosiphon, which had been in the system since the early 2000s.
She said a goal to eradicate the weed from Lake Aviemore and keep Lake Waitaki weed-free was no longer achievable.
"We were always working against the flow to keep this weed from spreading downstream from its stronghold in the Ahuriri Arm of Lake Benmore. But our combined efforts allowed us to hold the line, keeping these lakes weed-free until now.
"Unfortunately, the size of these infestations means we will now have to change tack and do our best to manage lagarosiphon in these lakes and prevent it from spreading into other weed-free waterbodies."
Burton said the infestation was particularly large in Lake Waitaki - where it was 750 meters long - equivalent to 15 lengths of an Olympic-sized swimming pool.
"This shows just how quickly lagarosiphon can take hold in new habitats.
"All it takes for new plants to establish is a tiny fragment transferred downstream, by boat, fishing gear, or natural flow through the lake system," she said.
"Now the weed is in these lakes it's more important than ever that lake users check, clean and dry all gear when moving between waterways.
"It's essential we all do our bit to stop the spread of lagarosiphon as left unchecked it can very quickly take over and choke waterways, smother native plants and have a big impact on how we use and enjoy our lakes."
Control work and regular monitoring of the lakes was funded by Land Information New Zealand, Meridian Energy and the Canterbury Regional Council.
The council was working on a new management plan to help protect the lakes from the impact of lagarosiphon.
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