Outlook grim for Taupo's waters

By SIMON COLLINS science reporter

Lake Taupo looks doomed to suffer algal blooms for the next half-century, after a discovery that water takes up to 80 years to percolate into the lake from surrounding farmland.

The discovery shows that water entering the lake today from the Whangamata Stream at Kinloch originally fell on nearby land in about 1923.

For 80 years it has been stored in huge underground aquifers, working its way slowly upwards to emerge in the springs feeding the Whangamata.

One bay to the west, water in the Kawakawa Stream has been dated at 60 years. Water in streams on the western side of the lake is around 40 years old.

Environment Waikato scientist Bill Vant says the finding means that we are only now seeing a "signal" in nitrogen levels in the lake caused by manure and urine from sheep and cows that lived decades ago.

"What that means is that we can expect the nitrogen levels to continue to go up before they go down.

"There is still a lot of water entering from the streams to the lake that is more than 40 years old," he said.

He told the New Zealand Geographical Society conference in Auckland last week that the amount of tiny, free-floating algae in the lake was rising steeply.

Plant material in water is measured by chlorophyll A, the green pigment in plants that is used for photosynthesis. Since 1994, it has risen from around 0.5 milligrams in a cubic metre of water to an average of around 1.3mg in the last few years.

Last summer, chlorophyll peaked at 2.6mg/cu m, producing a visible "bloom" that led to a swimming ban in much of the lake in March.

Mr Vant said the bloom was caused by hot, settled weather which failed to mix up the water, so it would not reappear every year.

But the algae were feeding off increasing levels of nitrogen and phosphorus being washed into the lake from farms established in the 1960s and 70s.

Dissolved inorganic nitrogen in the Whangamata Stream had risen from about 0.3mg per litre of water in the 1970s to about 1.2mg today.

He said the length of time the water stayed underground before entering the lake was a surprise.

"Even for someone like me, after years on the job, you don't tend to think about water hiding underground for so long before it pops out.

"So it's going to get worse before it gets better, which is why Environment Waikato has a strategy to be announced later this year," he said.

The agency has said it wants to change farming practices and land uses to cut the amount of nitrogen entering the lake by 20 per cent.

AgResearch's science leader for land and environmental management, Dr Liz Wedderburn, said there were several options for achieving this, such as building concrete platforms to replace winter grazing, or planting lucerne instead of pasture.

Environment Waikato resource information manager Tony Petch said the agency was awaiting Government comment before releasing a lake strategy in the next month.

The strategy will be open for public comments for several months and the agency will then draw up detailed proposals for changes to its regional plan.

Lake at risk

Lake Taupo is one of New Zealand's deepest and clearest lakes, 158 metres deep and usually visible from the surface down to about 15m.

Visibility has declined slightly in recent years and was blocked dramatically last summer by a widespread algal bloom.

More blooms are likely because of growing inflows of nitrogen from animal urine and manure on surrounding farms.

Water can take up to 80 years to get from farms through underground aquifers to the lake.

Taupo Info