Rangitikei lake algae clears in time for Christmas

The warning signs at Rangitīkei's Dudding Lake have come down. Photo / Stuart Munro

Algae levels at Rangitīkei's Dudding Lake have dropped massively - just in time for the holiday season.

The lake is just off SH3, between Bulls and Turakina. Warning signs there were taken down on December 20, Dudding Lake Reserve operations manager Bruce Gordon said.

Two weeks earlier there were 20 cubic millimetres of blue-green algae in every litre of water - twice the maximum recommended for swimming. Earlier this week it was down to 0.3 cubic millimetres - but that could change quickly.

The lake is now suitable for cautious swimming, but people are warned to avoid surface scum.

The densest algae often lie toward the water surface, and lake users can reduce cell density by stirring surface water into the layers below.

Despite the algal bloom scare, bookings for the reserve's campground are looking good, Gordon said. He keeps people up to date with its algae level through a Facebook page.

The lake has experienced an algal bloom before Christmas every year for the last six years, he said.

It has been monitored for algae for about the past eight years, and Horizons Regional Council's natural resources and partnerships manager Jon Roygard says the algal blooms are becoming more frequent.

On December 11 he warned the lake was close to "flipping" - from being dominated by exotic water weeds to being dominated by potentially toxic algae.

He said urgent action was needed, and the council would have a final plan ready in January.

The plan could include adding aluminium sulphate (alum) to the water, to clump algae together and let sunlight through to the plants.

Gordon, who is also the chairman of the regional council, prefers Rangitīkei District Council's more "natural" plan.

It is to introduce non breeding freshwater carp to eat the lake week, and freshwater mussels to filter the lake water.

Roygard has said carp are not a good option.

The lake is "a real little gem within the Rangitīkei", Gordon said.

"It's quite breathtaking, with freshly cut grass and the lake sitting out in front, with its sandy beaches, playground and hall."

He was one of a group of four Rangitīkei businessmen who begged the district council not to close the facility.

The council now gives about $20,000 a year for mowing and maintenance, and the men get grants and pay "out of their own pockets" for other improvements.

Gordon is not paid for his work as operations manager.

The lake is fenced off and has riparian planting. He doesn't believe the nutrients in it -which make algae grow - are a result of farming in the area.

Horizons is to talk to Rangitīkei District Council about how to improve the lake.

Meanwhile warning signs are still up at Whanganui's Lake Wiritoa, where until recently algae levels were twice the maximum recommended for swimming.