Lake and its trout starved of oxygen

By ROSALEEN MacBRAYNE

Lake Rotoiti's world-famous trout are in danger of dying because the lake is running out of oxygen, says an expert who has been studying the water quality of the Rotorua lakes for the past year.

Professor David Hamilton, who holds the Environment Bay of Plenty chair in lakes management and restoration at Waikato University, said the lake was threatened from the bottom up and saving it would require expensive and quite radical techniques.

Research he has been doing with the help of PhD students shows the problem of excess nutrients in the lake is worse than first thought.

Professor Hamilton said the level of nutrients flowing into the lake was now being exceeded by nutrient loads generated by lakebed sediments accumulated over decades.

That meant increased algal growth - algal bloom closed Lake Rotoiti for six weeks last summer - and subsequently the loss of oxygen from the bottom waters.

Professor Hamilton said the lake had been ailing progressively for three or four decades and could take as long to improve.

Nearly all the other Rotorua lakes were showing declines in oxygen.

An engineering option for cleaning up Lake Rotoiti might be to divert the water that flows through the Ohau Channel from Lake Rotorua, perhaps directing it straight to the Kaituna River. Chemical treatment to flock out nutrients and bind them into the lake sediment was another possibility.

Professor Hamilton said a trial was planned on the small Lake Okareka this summer.

The sheer size of Lake Rotoiti was the main drawback to any attempt at reoxygenation by bubbling oxygen into the bottom of the lake and diffusing it into the water. It would cost millions of dollars, the professor said.

Environment Bay of Plenty's regulation and resource manager, Paul Dell, said once the fundamental research was finished, all the data would be analysed carefully and options looked at late this year.

He said the toxic algal bloom was a big wake-up call.

"We need to take action but we can't just rush off and do something to make us feel better.

"If you are going to do something, you have to do it properly."