Didymo is not disappearing from the lower Waitaki River, Dunedin environmental consultant Bill Chisholm told an Environment Canterbury hearings panel in Christchurch yesterday.
Chemical control of didymo in the river was probably not feasible because of the river's size and flows, he said.
It would also need prolonged use, which could have a toxic effect on the river.
Mr Chisholm was giving evidence on behalf of Waitaki First to the panel which is considering four resource consent applications by Meridian Energy Ltd for its north bank tunnel concept (NBTC) power scheme between the Waitaki dam and Stonewall, near Ikawai, on the north bank of the Waitaki River.
Recent research on methods to control didymo had identified chelated copper (Gemex) to be the most effective, Mr Chisolm said.
However, testing the chemical showed prolonged use was needed because of the unique composition of didymo which made it difficult to control.
The dense, mat-like spread, which he described as like wet cotton wool, was difficult to penetrate with chemicals.
Mr Chisholm said recent reports suggested didymo was apparently disappearing from the Waitaki, Hakataramea and Maerewhenua Rivers, but Biosecurity New Zealand had said this was due to a natural cycle, algae blooming then receding before blooming again.
Mr Chisholm said amounts of didymo had been visibly removed.
But research by Meridian showed a 60% cover of didymo still remained at research sites and infestations would be back to usual densities in a fairly short period.
Meridian consultant Ned Norton said studies into didymo on the river had continued, including monitoring its extent and the effect it had on invertebrates, which were vital as fish and bird food.
Didymo still existed and was greater than 60% at test sites established on the river.
However, he observed from a jet-boat that, visually, significant amounts of didymo had gone from some areas recently.
This had been caused by didymo being washed away or buried under gravel transported by the river.
Studies were also being carried out into the effect didymo had on invertebrates, compared with research carried out before didymo infested the river.
A final report would not be available for some time, he said.
