Niwa testing bloom on Lake Hayes

A structure which resembles a mussel farm has appeared on Lake Hayes this week, but those thinking that shellfish are being cultivated in the lake could not be more mistaken.

Two Niwa scientists, Dr Christopher Hickey and Max Gibbs, of Hamilton, installed the structure this week, hoping to understand the causes of the algal bloom in the lake and what can be used to treat it.

Ironically, Lake Hayes is the cleanest it has been for years, they said.

Dr Hickey said the "lake tubes" - 10m long bags - had been erected 50m out from the shore of the lake and extend to the lake floor.

The bags are joined together and weighted at the bottom ensuring the sediment of the lake was captured and "capped".

"We've done it to try and understand how much phosphate comes out of the bottom of the lake," Dr Hickey said.

"That's what drives the algal blooms in this lake.

"We can then try a number of different treatments.

"We've got a range of different products, like allophane clay and zeolite clay."

During the research, the water in the tubes would go anoxic - where the oxygen was removed - which would release the phosphate and ammonia in the lake, allowing algae to grow.

In summer, Lake Hayes had a tendency to become stratospheric, which allowed the algae to move up and down through the layers of the water getting its nutrients.

By trying several different treatments in the lake tubes, the scientists would see which treatments were most effective, and use the results of their research to develop models for restoring water quality in other lakes in New Zealand.

"If you're going to commit to this sort of process, you have to spend quite a bit of money and you have to know it's going to last a long time," Dr Hickey said.

"The approximate quantity of sediment-capping material needed for Lake Hayes ranges from 250 tonnes of alum to 550 tonnes of modified zeolite, with indicative material costs of $125,000 to $1.4 million."

The application costs would be added to that amount, with the "optimal capping blend of materials and quantity" to be determined by the trial, he said.