Algal bloom reaches a tipping point in Lake Omapere

The normally picturesque Utakura River, which drains Lake Omapere, was covered in a thick layer of foam on Friday evening.

Lake Omapere has been afflicted by a potentially toxic algal bloom that is threatening aquatic life and the health of people using the water.

Tests have yet to be completed, but land owners around the 1200ha lake are being warned not to use the water.

The bloom, caused by cyanobacteria that thrive in warm conditions, has been building up since late last year, but hit a tipping point on Friday evening. Locals say the water turned a lurid green almost as they watched, and the normally picturesque Utakura River, which drains the lake, was covered with a thick layer of vile-smelling white foam.

One of many dead eels in the Utakura River.

Neville and Linda Lewis, who own a farm by the river, were alerted by a neighbour on Friday evening, who said "something serious" was happening to the lake. Mrs Lewis checked the river, which supplied their drinking and stock water, and saw it had turned an intense blue and was covered in foam. There was an unpleasant smell that left a burning sensation in her mouth.

Later the colour changed to a bright blue-green and then dark green, as it did when the lake "flipped" 30 years ago. By Saturday afternoon eels were dying.

Mrs Lewis said the Northland Regional Council knew the lake was "turning" in January, but did not notify residents who depended on it for water.

Mr Lewis said the foam had gone but the smell remained.

"You don't feel that flash if you stand by the river for too long."

If they had been warned earlier they could have organised an alternative water source ahead of time, instead of in a panic on Friday evening, he added.

They were also concerned about people using the Twin Coast Cycle Trail, which passed Lake Omapere then followed the Utakura Valley to the Hokianga Harbour. They had put up their own signs warning cyclists not to swim.

Regional council environmental monitoring manager Jean-Charles Perquin said algal levels in the lake were monitored from November to April. The council became aware of increasing algal levels in November, and notified the district council and Northland DHB, although levels were not high enough to trigger concerns for human health.

Monitoring had been increased in January, but no toxins associated with cyanobacteria had been detected so far.

On Saturday concerned members of the public called the council's 24-hour hotline.

Samples were taken to check for toxins, but the outcome would not be known until later this week or early next week. Further samples were taken on Monday.

Regional council staff visited land owners on Saturday to reiterate earlier advice not to use water from the lake or river. That advice would remain in place until the bloom receded, which could take a long time in current warm conditions.

Conditions contributing to blooms included warm weather and increased nutrient levels, especially after run-off from heavy rain.

Bryce Smith, of the Lake Omapere Trust, said it was "devastating" to see what had happened to the lake. He called for more research into the causes rather finger-pointing, and full disclosure about the amount of water being taken from and discharged into the lake.

Lake Omapere is Northland's largest, but its lack of depth — 1.8m at its deepest point — makes it susceptible to blooms. The algae can produce toxins and consume all oxygen in the water, killing other aquatic life.

A restoration project underway since 2006 has included planting of lake edges and monitoring of aquatic life. The last bloom was in 2003-04.