Dirty secrets of Taupo's big lake

Lake Taupo is often described as being one of the jewels in New Zealand's crown.

The image of its sparkling waters framed by snow-capped mountains attracts hundreds of thousands of tourists to Taupo every year.

But that image is under threat. Although the lake is still relatively healthy, its water quality has started to slip.

Scientists measuring the lake's water clarity could see only 6.7m below the surface at one stage last winter. The average visible depth is 14.2m but that average is slowing shrinking.

And as in Rotorua's lakes, algae have started flourishing in Taupo's waters because too much nitrogen and phosphorus is flowing in through streams and groundwater.

Temporary health warnings were issued for several bays in late March and local authorities and residents are concerned about the growth of invasive underwater plants.

"It is still at the cleaner end of the scale but the problem is it's not as clean as it used to be," explains Environment Waikato spokesman Tony Fenton.

Work to halt the decline in water quality has received top-level support.

In July, Environment Minister Marian Hobbs said the Government would form a partnership with local authorities and Tuwharetoa to tackle the issue.

Scientists know that the development of land for agriculture, forestry and the town of Taupo itself is to blame for the increase in nitrogen and phosphorus over the past 50 years.

Because groundwater carrying these nutrients takes several decades to reach the lake, the effects are only now starting to be seen.

In 2000, Environment Waikato publicised its concerns.

While septic tanks and stormwater runoff from urban areas were contributing to the problem, farmers were shocked to be singled out as the main culprits.

Environment Waikato believed the effluent of 1.5 million sheep, cows and deer being farmed in the lake's catchment area was responsible for 30 to 40 per cent of the nitrogen entering the water.

Major changes are required but finding a solution is far from easy.

"It's a very hard social problem because you're impacting on people's livelihoods," Mr Fenton said.

"If we want to have a sustainable clean lake then you've got to go down the hard road of saying we have to change land use."

Environment Waikato, in conjunction with the Government, the Taupo District Council and local Maori are forming a strategy to reduce the lake's nitrogen input by 20 per cent. Details of how that will be achieved will be released next month. Farmers will bear the brunt of the consequences and are likely to have to change farm management systems, retire substantial amounts of land and restrict stock numbers.

The economic effects for the catchment's farmers are so great that around 100 landowners got together three years ago and formed Taupo Lake Care to confront the issue.

The group's chairman, Bob Cottrell, said Taupo's farmers had a responsible attitude and were as keen as anyone to see the lake's heath improve. But they also wanted to ensure that their farms remained viable and flexible into the future.

"We want to manage this catchment by ensuring that everybody can see they're part of its future,"Mr Cottrell. "We don't want to be fixing one problem by creating another one."

Taupo Lake Care had met regularly with Environment Waikato to discuss possible solutions but the issues were complex.

Still under debate was the idea of placing a "cap" on existing nitrogen outputs from individual properties.

That meant farmers would not be able to increase stock numbers or change what sort of animals they farmed if it resulted in more nitrogen being produced, Mr Cottrell said.

"If sheep, for instance, became unprofitable then traditionally farmers changed to cattle. But cattle have a higher nitrate output than sheep so they'll no longer be able to do that.

"The cost to farmers will be enormous."

There were also concerns that some landowners whose forestry blocks or native bush plantings had benefited the lake would be unfairly punished by any nitrogen cap.

But Sue Yerex, secretary of Taupo Lake Care, said farmers were confident difficulties could be overcome.

"If we thought it was a waste of time we wouldn't be consulting with Environment Waikato," she said.

"Anything is achievable if we work together, but there's going to need to be some financial assistance to achieve land-use change.

"The economic impacts of a nitrogen cap or reduction are huge. The burden needs to be shared and money would have to be spent on research and development to lower nitrogen outputs from farming systems."

Discussions about who would give up what and who would pay still have a long way to go, but local farmers do not want to turn their backs on Taupo.

"Nobody is going to be walking away. There's too much at stake here," Sue Yerex said.

"We live here and we look at that lake every day. That's why we've got to get it right."

* This is the third in a five-part Herald series looking at the toxic threat to some of our magnificent lakes.