Media Release

Weed harvesting helps reduce nutrients in lakes

For immediate release: 18 August 2009

Environment Bay of Plenty has removed more than 3,000 tonnes of hornwort – one of the most damaging aquatic weeds - from Lake Rotoehu over the past few months. The 300 truck loads of hornwort equate to 7376 kilograms of nitrogen and 983 kilograms of phosphorus being removed – the lake’s entire nutrient reduction target for the year.

Nearly 130 tonnes of weed was also removed from Lake Rotoiti’s Okawa Bay for access and aesthetic reasons, although it did have an associated nutrient benefit. The equivalent of 370 kilograms of nitrogen and 80 kilograms of phosphorus was removed.

“We are really pleased with the weed harvesting in both areas and we’ve had positive comments from local residents about the success of the weed harvesting programme,” Environment Bay of Plenty Group Manager Land Management Warwick Murray said.

He warns that while harvesting hornwort is a valid means of reducing nutrients in the lake it’s not a quick fix solution to improving the lake’s overall health and reaching the water quality targets.

“The intention is for hornwort harvesting to achieve some in-lake nutrient reduction until catchment land use, land management changes and the creation of wetlands begin to take greater effect in the lake,” Mr Murray said.

The weed harvesting is one of the actions in the Lake Rotoehu Action Plan, developed as part of the Rotorua Lakes Protection and Restoration Action Programme, a joint project between Environment Bay of Plenty, Rotorua District Council and Te Arawa Lakes Trust.

Mr Murray said it is likely that Environment Bay of Plenty will carry out targeted weed harvesting in other bays of Lake Rotoiti and possibly some parts of Lake Rotorua in the future. He said the council would be happy to make the harvested weed available for commercial application or scientific investigation, providing there was no cross-contamination risk.

About Lake Rotoehu
Rotoehu means “murky water” which implies the lake was never crystal clear all of the time.
Lake size: 8km2
Catchment area: 57km2
Average depth: 8m
Deepest point: 13m
Formed 8,500 years ago

ENDS

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