Alarm has been raised that Palmerston North proposals to remove more phosphorus from its wastewater discharge could make the Manawatu River sicker, not better.
City councillor Chris Teo-Sherrell told a committee of council meeting this week that changing the mix of nutrients in the river could encourage the growth of algae that release toxic cyanobacteria, which sometimes kills dogs.
City councillors have added a note of caution to a proposed project to install a $2.9 million disc filter at the plant to remove particulate phosphorus.
Its draft Annual Plan, to be released for public consultation next month, will include a caveat that the project depends on further research on the consequences of changing the balance of phosphorus and nitrogen in the river.
Cr Teo-Sherrell had wanted the spending removed from the coming year's budget. He was supported by Cr Rachel Bowen.
"It is alarming to think we could do more harm than good by accident."
Both referred to comments made by freshwater scientist and Resource Management Act commissioner Kate McArthur at a seminar at the weekend.
She said the river was complex, and it was simplistic to target one thing in isolation. While the focus in the past had been on removing phosphorus, it was now clear that nitrogen was important too.
The city council had no plans to remove nitrogen from its discharge.
Council water and waste manager Rob Green said after the meeting that issues about the nitrogen-phosphorus ratio and the potential to encourage cyanobacteria in the river were not new.
They were taken into account when the recommendation was made for the council to implement the disc filter system, he said.
The city council may carry out additional research.
Dr McArthur said there was a whole community of different types of algae growing in the river, and they were all capable of responding to stress in different and unpredictable ways.
"We can get ecological surprises and unexpected consequences."
Toxic algae were one of those unpredictable life forms.
Changing the balance of nutrients in the river could increase the dominance of blue-green algae, which under certain circumstances produced a strong neuro-toxin that was especially deadly for dogs.
"There is some evidence it can get its own phosphorus from sediment and has a competitive advantage.
"If we want not to have toxic algae, the answer is to manage both nutrients - phosphorus and nitrogen."
Dr McArthur said it was difficult to manage the balance of nutrients in an already complex river system without removing the discharge altogether.
Horizons Regional Council is reviewing the conditions of the city council's discharge consent at the moment, with submissions closed and dates for a hearing yet to be set.
- © Fairfax NZ News
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