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Previously untouched areas of Lake Benmore in North Otago have been infected by an invasive aquatic weed, but an operation to lower the lake and help halt the spread of the lagarosiphon weed started earlier this week.
Meridian Energy, which uses the lake to generate electricity at the Benmore Power Station, began lowering the lake on Monday to allow Land Information New Zealand (Linz) to carry out a spraying programme to control lagarosiphon.
Linz acting crown property manager John Hook said spraying would begin when the lake reached its lowest point - about 1.5m below its normal 359.5m operating range - some time between August 26 and September 2.
Mr Hook said the weed was first discovered in the Ahuriri Delta of the lake in 2003, but weather conditions in North Otago had limited Linz's ability to carry out work to control the spread of the invasive problematic weed.
''Approximately two years ago, an unusually high rainfall created turbid water conditions, which limited the level of control work we were able to carry out.
''It also made diver-based control work, using a mixture of hand-weeding or suction equipment to remove lagarosiphon, impossible.
''As such, lagarosiphon has colonised new areas of the lake, and spread in areas that have been well-controlled to date. ''The objective of the spraying programme is to enable us to reduce lagarosiphon to a manageable level.''
The lake level was expected to return to its normal level by September 6, he said.
A Meridian spokesman said the low lake level would not affect electricity generation, but people visiting and using the lake during this time should be aware that the low water level might mean fishing by boat and access to some areas of the lake could be a little more difficult for this short period.