Blooming marvellous! Iconic lakes are on track to be free of blue-green algae

Scientists are working on new ways of preserving and restoring New Zealand's iconic lakes with experimental trials in Lake Rotoehu.

Many lakes in New Zealand are adversely affected by nutrient enrichment. This is a legacy of land-use practices that cause run-off of fertilisers and animal effluent into the waterways that feed our lakes. An excess of nutrients enhances the growth of potentially toxic blue-green algae (cyanobacteria) in some lakes.

NIWA scientists began work trialling several phosphorus inactivation agents in Lake Rotoehu in autumn this year.

Phosphorus is released into the lakes from sediments on the lake bottom when there is little or no oxygen available. Oxygen levels are usually lowest in the summer. And indications of poor water quality, such as algal blooms, are common in summer.

"We are using 15 experimental water enclosures, or mesocosms, for these trials with different materials in each. Resource consent has been granted for these trials of all three products," says NIWA Freshwater scientist Dr Max Gibbs.

The scientists use capping agents to confine the phosphorus in the lake sediments. These capping agents are like a carpet laid over the phosphorus. Passive capping agents like sand or clay can bury the sediments using a thick layer (5 to 10 cm), locking the phosphorus into the sediment. Active capping agents, with a layer only 1mm thick, can achieve the same effect at a fraction of the cost.

NIWA and the University of Waikato, in a combined project with Bay of Plenty Regional Council have previously tested the capping agents to be studied on sediment cores from Lake Ōkaro and Lake Rotorua for effectiveness at removing phosphorus, and for any undesirable side effects.

The capping agents being tested are alum – an aluminium-based chemical commonly used in drinking water treatment and swimming pools; Allophane – a natural volcanic ash from past eruptions in the Central North Island volcanic plateau; and Aqual-P, a specially adapted zeolite clay mineral developed by Scion in Rotorua.

The mesocosm study will measure the effectiveness of different treatments with these products and the physical and biological interactions that occur with the capping agent materials in a controlled environment, according to Bay of Plenty Regional Council Lake Operations Manager Andy Bruere.

This work was funded by the Ministry of Science and Innovation and is supported by Bay of Plenty Regional Council.

For comment, contact:

Dr Max Gibbs
NIWA Freshwater scientist

Andy Bruere
Lake Operations Manager
Bay of Plenty Regional Council