Lake Rotoiti choking in algal grip

Richard Wilson remembers sitting down at the end of his jetty last summer and beginning to cry.

Where he used to spend long, sun-filled days hauling trophy-sized trout out of Lake Rotoiti, Mr Wilson was staring at water that looked like pea soup and smelled like wet hay.

Blue-green algae were growing in unprecedented numbers and health officials had deemed the entire lake unsafe for swimming.

"I just began to sob," Mr Wilson recalls. "I was thinking, God, what's happened to this beautiful lake."

In the mid-1990s, Rotoiti was the most heavily fished lake in New Zealand. Bagging a 7kg trout was not uncommon.

Last summer the average size was just 2kg.

The cause of the rapid decline had been developing beneath the water for more than 40 years. And Lake Rotoiti is not alone.

Nearby lakes Rotorua and Rotoehu are also suffering from an explosion of algae, and lakes Okareka, Okaro, Okataina, Tikitapu and Taupo are showing signs of beginning to falter.

Local body politicians and officials have known for decades that the health of the region's lakes was deteriorating. But last summer, when algal blooms covered Rotoiti entirely, made many people realise the seriousness of the problem.

Professor David Hamilton, appointed by Environment Bay of Plenty to lead research at Waikato University into the water quality of Rotorua's lakes, says scientists reported in the 1980s that Rotoiti was in trouble.

"Yet the brakes were not put on it then.

"I don't know whether people looked at the lake and said, 'Well, it still looks pristine to us'. But there were good scientific indicators ... that there was a serious decline."

Blue-green algae are found naturally in lakes worldwide but numbers are now out of control in some places because too much nitrogen and phosphorous are flowing into the water.

In Rotoiti's case, 70 per cent of those nutrients are coming from Lake Rotorua as its water flows through the Ohau Channel into Rotoiti before draining down the Kaituna River.

A small number of farms on the lake's northern shores also contribute nitrogen and phosphorous by way of animal effluent and fertilisers.

Septic tanks servicing 1000 or so residents and holiday homes add to the problem.

As a result, algal blooms have appeared periodically in parts of Rotoiti for a number of years. But their mere presence has unleashed another nutrient source.

When algae fall to the bottom and decay, oxygen is used up. A chemical reaction means nitrogen and phosphorous are able to escape, which in turn fuels more algal growth.

As a result, large quantities of phosphorous, in particular, have been released from the lake's sediments over the past 20 years.

For a lake that has been a popular holiday spot for generations of New Zealanders, the situation is now precarious.

Professor Hamilton, who describes Rotoiti as "a real mess", predicts that algal blooms will be widespread again in summers, when warmth and calm waters aid algae growth.

Local councils, scientists and officials are working hard to reduce the amount of nutrients entering the lake before it collapses altogether, killing all life within.

To date, 16km of fencing has been erected to keep stock away from the water's edge, 22,000 revegetation plants and 19ha of trees have gone in around Rotoiti.

But any intervention - such as replacing septic tanks with a reticulated sewage system, or retiring pastoral land - is likely to take decades to deliver noticeable results.

Major engineering and scientific work costing millions of dollars would be needed to restore the lake any faster than that, says Professor Hamilton.

Diverting the Ohau Channel so Lake Rotorua's water bypasses Rotoiti is one option being considered. Pumping oxygen into the lake's bottom waters to stop the sediments from releasing nutrients is another.

While the cause of the problem and likely solutions are known, many Rotoiti residents have argued for months that action is not being taken quickly enough.

Sally Brock, who holidayed at Lake Rotoiti for more than 20 years before moving there permanently in 2001, says residents have felt angry, confused and frustrated.

"We felt that Lake Rotoiti was almost in the too-hard basket; that the bureaucrats thought it had gone past the point of no return."

Mrs Brock remembers "amazing times" at the lake, when thousands of families descended over summer to waterski, fish and swim.

"There was never a holiday spent anywhere else for our family."

But the extent of last summer's algal bloom - which lasted in some bays for eight months - has prompted Environment Bay of Plenty and the Rotorua District Council to speed up efforts to save the lake.

A draft "action plan" outlining possible options will be ready by the end of the year - a move applauded by Mrs Brock.

"The councils are obviously facing a crisis situation now. They can't do anything else but move on it otherwise we're looking at a dead lake."

Richard Wilson, whose father built his family's home at Rotoiti in 1962, was another resident acutely aware of the lake's deterioration and the need for urgent action.

"Rotoiti used to be a trophy lake but in my opinion it's not any more," he says.

"Fishing has got harder in the last six or seven years ... Of the fish I caught last season, I put more than half back because they're either in poor condition or simply because they're too small."

Fish and Game's Eastern Region manager, Steve Smith, confirms that the lack of oxygen in Rotoiti's bottom waters due to algae growth is having a serious impact on the fishery, which is worth about $15 million to Rotorua's economy.

"The area that the trout have got to live in has been getting smaller."

Mr Smith says there is no doubt that the lake's plight justifies spending money to fix the problem and he is optimistic action will be taken as quickly as possible.

While residents are also hopeful that the work now being done on Rotoiti will bring results, Mrs Brock says she remains cautiously optimistic.

"We've got plans and we've got promises but we haven't got action yet.

"Let's get on with it and let's get it right."