Dirty lakes social problem

By JO-MARIE BROWN

A scientist studying the toxic algal bloom contaminating some of Rotorua's lakes says it is unbelievable that cattle are still wading in the water and locals are fertilising lawns.

Professor David Hamilton, who holds the chair in Lake Management and Restoration at Waikato University, is leading research into the cyanobacteria bloom which has flourished in several lakes where nutrient levels are too high.

Lakes Rotoiti, Rotoehu, and parts of Lake Rotorua have been off limits again this summer because of health risks associated with the blue-green algal bloom.

Professor Hamilton told a meeting of community leaders in Rotorua this week that it was "unbelievable" that some farmers still allowed their cows to wade in Lake Rotoiti, because their effluent was full of nutrients.

Local residents were also fertilising lawns leading right up to the lakes' edge and spraying superphosphate fertiliser across the surrounding hills which then seeped into the water.

"It's as much a social problem as anything. Some people do the right thing and some people don't," Professor Hamilton said.

"Humans are having such a big impact on these lakes that nothing else compares."

Professor Hamilton, who was funded by Environment Bay of Plenty and assisted by PhD students, said the bottom of the lakes such as Rotoiti and Rotoehu were being starved of oxygen each summer.

Water quality had declined dramatically over the past 40 years as the increasing level of nutrients such as nitrogen and phosphorus allowed the blooms to flourish.

The Rotorua District Council and Environment Bay of Plenty have formed plans to work with local communities to reduce the amount of nutrients entering the lakes.

But in response to a question from Rotorua Mayor Grahame Hall about what more could be done to speed up the lakes' recovery, Professor Hamilton said public education was critical.

Hundreds of thousands of residents could live around the lakes' edges and not cause any damage if septic tanks were replaced by sewerage systems, the land was not farmed and vegetation and wetlands were retained, he said.

Community leaders were told that it was not yet clear whether Lake Rotoiti would entirely recover from the bloom over winter or whether the problem was permanent.

Boaties have also reported seeing the blue-green algae in Lake Tarawera this year in higher concentrations than in the past, though the numbers are still low compared with other lakes in the region.

Professor Hamilton said one option to help restore the lakes' health might be to inject pure oxygen into the water.