Estuaries in decline, study finds

ALEX FENSOME
Last updated 05:00 10/11/2012

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Parts of two major Southland estuaries are on the verge of environmental collapse, an Environment Southland-commissioned report says.

Algae growth in the New River estuary in Invercargill and the Jacobs River estuary at Riverton has increased exponentially, increasing sediment and loss of biodiversity, members of Environment Southland's Environmental Management Committee were told yesterday.

In a presentation on behalf of the Wriggle Coastal Management group, which has monitored the estuaries for the council since 2000, coastal consultant Greg Larkin said the Waihopai arm of the New River estuary could be eutrophic - dominated by algae and practically devoid of life, within 10 years, and both estuaries faced much the same fate as the Waituna Lagoon.

Mud, nitrates and phosphorous were being washed into the estuaries, leading to mass blooms of algae, loss of habitat and sediment anoxia, all of which were further increasing the rate of eutrophication, Mr Larkin said.

In 2000, the New River estuary was in good condition, with eutrophic areas totalling only 0.3 per cent of its area.

Now, large areas of algae and deep, polluted sediment were smothering life in the Waihopai Arm and Daffodil Bay, Mr Larkin said.

Three hectares of the Waihopai Arm was eutrophic in 2000. This has increased to 123ha this year.

Mr Larkin said sediment had accumulated so much in the Waihopai Arm inside a year that monitoring plates put in in 2011 were now buried and the sediment was 200mm higher than in 2007.

Beneath the sediment in the Waihopai Arm, there was no life except highly pollution-tolerant organisms, Mr Larkin said.

Sea grass, which was abundant in the estuary even in 2007, was almost non-existent.

The eutrophic areas made up 5.8 per cent of the total area of the estuary, but the amount of growth in the last five years was extremely concerning, he said.

In the Jacobs River estuary at Riverton, there are extensive areas of degraded sediment and algae cover which have also expanded significantly since 2000.

Mr Larkin said the Pourakino Arm appeared to be taking in nutrients and sediment washed down the Aparima River through tidal movement leading to similar issues as New River.

Council chairman Ali Timms said the report was graphic and troubling but not unexpected.

"We knew Waituna was not the only place we were going to have some issues."

Ms Timms said the council's efforts to improve water quality under its Water and Land 2020 programme would take time to arrest the decline, but was encouraged something was being done.

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Cr Jan Riddell asked what was likely to happen in the next 10 years.

Mr Larkin said the entire Waihopai arm could be eutrophic by 2020.

Cr Brian Mason asked if the growth of Invercargill was contributing to the decline of the New River estuary.

Mr Larkin said the next stage of the investigation would be to work out where the sediment and nutrients were coming from and assessing how much could be assimilated by the estuaries.

- The Southland Times

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