Toitū Te Whenua Land Information New Zealand (Linz) announced yesterday the presence of a "large infestation" of the invasive water weed in Lake Waitaki for the first time.
Divers undertaking regular monitoring and control work made the discovery in March.
Both Lake Waitaki and Lake Aviemore had largely been considered "weed-free" until now, Linz biosecurity manager Tracey Burton said.
Small lagarosiphon infestations were previously found in Lake Aviemore, on the North Otago-South Canterbury border.
Ms Burton said the recently discovered 750m-long infestation in Lake Waitaki showed how quickly it could spread.
It came "as a blow" after 15 years’ effort to protect the lakes from lagarosiphon.
"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."
The newly discovered Lake Waitaki infestation was the equivalent of 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."
The monitoring and control work was being undertaken on behalf of Linz, Environment Canterbury (ECan) and Meridian Energy.
Ms Burton said the goal was previously to eradicate the weed from Lake Aviemore and keep Lake Waitaki weed-free.
"This is no longer achievable.
"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 water bodies."
Because of that it would be "more important than ever" for boat users and other lake users to check, clean and dry all gear.
ECan principal biosecurity adviser Laurence Smith said an updated management plan would now try to protect the lakes from the impact of lagarosiphon and support their natural biodiversity values.
— APL
