A Dunedin environmental consultant says there will be more didymo in the lower Waitaki River if Meridian Energy Ltd builds its proposed power scheme, but Meridian challenges the claim.
Consultant Bill Chisholm, said increased nutrient leaching from land development, including dairying, would make didymo more prolific, but Meridian consultant Ned Norton was "more optimistic" and said didymo levels could be less than what he was expecting.
Both were giving evidence at an Environment Court hearing in Oamaru yesterday on conditions for four resource consents to grant water for the $1 billion north bank tunnel scheme.
Mr Chisholm based his projection on more recent reports and studies that had emerged after evidence was presented to the court during a hearing last year.
However, Mr Norton maintained there was nothing new in the information to change his views on didymo; in fact, he said, it indicated the problem might not be as bad as predicted.
Meridian legal counsel Jo Appleyard said conditions to help control didymo had been changed after discussions with the Central South Island Fish and Game Council, Lower Waitaki River Management Society and Waitaki First.
Changes included additional flushing flows if average didymo cover exceeded a trigger level, more didymo research before the scheme is commissioned and a process for changing flushing flows as a result of that research.
Mr Chisholm said land intensification, particularly dairying, continued along the lower river and the information suggested didymo was "likely to be more prolific" after the scheme was commissioned.
Flushing flows to remove didymo needed to be higher and more frequent than proposed.
The flows should not just remove didymo mats but also turn over the river bed.
That would give longer relief before didymo spread again, he said.
Mr Norton said there was uncertainty over the effects of higher nutrient levels on the invasive alga.
"In some cases didymo has increased, in some cases it has decreased. Changes in nutrient levels in the future will cause changes in didymo - it may be positive or it may be negative."
Mr Norton said none of the new information provided by Mr Chisholm took the issue any further since the court's interim decision in September in which it said it was likely to grant water consents for the scheme.
