Northland lake too sick for spray

By TONY GEE

A plan to spray the herbicide diquat on an oxygen weed infestation in Northland's biggest lake has been put on hold because of the lake's poor health.

Spraying the invasive weed in the 1200ha Lake Omapere was scheduled to start last month but now the operation has been put off because of low oxygen levels on the bottom and a significant algal bloom.

Northland Regional Council monitoring manager Tony Phipps said the bloom and poor oxygen level had slowed weed growth but spraying under such conditions could further reduce oxygen levels and worsen already poor water quality.

Council staff and contract divers surveyed the lake on Wednesday.

After discussions with the National Institute of Water and Atmosphere scientists who are advising the council on the project, it was decided that spraying should wait.

The decision will be reviewed next week after further tests.

Using the herbicide to kill about 100ha of weed in a 300ha area in the lake's centre will be the first step in the exercise.

The second will be to introduce another 20,000 weed-eating grass carp to control weed growth once the spraying residues have subsided.

Mr Phipps said that one option might be to release the carp instead of spraying first.

All the carp required for the job are on hand.