Lake Taupo bloom is part of worldwide trend

By SIMON COLLINS science reporter

New Zealand can expect more algal blooms of the kind that hit Lake Taupo last summer if the world continues to pump out "greenhouse gases".

That is the message from Dr Ian Hawes, of the National Institute of Water and Atmospheric Research (Niwa), who is just back from a world conference on water in Japan.

He said other scientists reported the same blooms caused by cyanobacteria (blue-green algae) in lakes around the world.

"There are more and more blooms all over the world as a consequence of warmer water."

Dr Hawes was speaking at the NZ Antarctic Conference in Dunedin, where scientists debated the extent of global warming in Antarctica.

A US paper in the journal Nature last year found that Antarctica was the only continent to grow cooler in recent years, with an average drop in temperatures of 0.7C a decade.

But another group of NZ and US scientists led by Victoria University's Nancy Bertler reworked the figures to show that Antarctica has grown warmer over a longer period, and the latest cooling is due to the temporary effect of El Nino weather patterns.

Dr Hawes, a Niwa aquatic ecologist in Hamilton, said the Lake Taupo bloom in late summer was due to a combination of warmer water and more water stability, which created a layer of permanently warm water on the surface.

The problem was worsened by a long-term rise in nutrients flowing into the lake from houses and more intensive farming, including several farms converted to dairying.

As a result, the cyanobacteria, which had always been in the lake in small quantities, became noticeable throughout the lake this year.

"Two weeks ago it looked like snowflakes in the water.

"It's likely to happen again as long as the climate keeps changing and the lake becomes more stable."

The Japan conference, part of the third World Water Forum, set up a World Water and Climate Network of scientists to monitor the effects of global warming on lake quality. Niwa is a founding member.

Dr Hawes said that, while New Zealand could do little to affect global levels of carbon dioxide and other greenhouse gases, it needed to manage land uses in lake catchments to minimise the damage.

Meanwhile, Nancy Bertler's paper, based on temperature records at Scott Base since 1958, showed average increases in the 42 years to 2000 of 0.05 to 0.29C a decade. Another set of measurements from 1969 to 2000 at the Victoria Glacier near the Trans-Antarctic Mountains showed an average warming of 0.89C a decade.

Both sets of data confirmed a slight cooling trend since 1985. But Ms Bertler said this was because El Nino shifted a major low-pressure system, causing local cooling .

Other data showed temperatures still rising in other areas, especially in the Antarctic Peninsula south of South America, where a large ice sheet has broken up.