Manawatu not world's dirtiest

WARWICK SMITH/Manawatu Standard

WHITE WASH: Horizons Regional Council is rejecting claims the Manawatu River is one of the most polluted in the Western world.

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Horizons Regional Council has moved to "dispel myths" that the Manawatu River is the most polluted in the Western world.

But a freshwater ecologist concedes it is the most unhealthy river in the country.

The state of the river's struggling ecosystem has been a talking point since research from the Cawthron Institute showed, under a system measuring oxygen changes in water, that the river has by far the highest reading.

The river measured 107. Anything over eight is considered indicative of an unhealthy river ecosystem.

A measurement of 0-4 is considered healthy.

The measurement topped a list of 300 rivers and streams across North America, Europe, Australia and New Zealand.

But at a press conference in Palmerston North on Wednesday, Cawthron freshwater ecologist Roger Young said other factors needed to be considered when assessing water quality.

"There are many millions of streams throughout the world ... if you had 300 people in a room and you measured everyone's blood pressure, it's a big leap to say the person with the highest was the most unhealthy in the Western world."

Other factors, such as nutrient concentrations, water clarity, faecal bacteria and stream invertebrates were all indicators of poor river health, Dr Young said.

But the Environment Ministry had issued tables on those indicators which put the lower Manawatu in the bottom five out of 77 New Zealand rivers in each category, he said.

"So I think it's safe to say that, at the moment, it's the most unhealthy in New Zealand."

Horizons environmental scientist Jon Roygard said the danger it posed to humans varied from day to day.

"If it's clean and clear and hasn't rained for a few days, then generally you can swim in the river. I'd be quite happy putting my kids in the river, based on that rule of thumb."

Algal blooms, caused by nitrogen getting into the river from farming, were potentially toxic and a real concern in terms of recreation, Dr Roygard said.

Horizons chief executive Michael McCartney said ratepayers needed to decide what activities they wanted to do in the river, be it swimming or trout fishing, before the cost of cleaning it could be established.

Improving the state of the river would be a slow process, he said.

- © Fairfax NZ News