Deep sea corals are being put to the test at NIWA to find out how they cope with sediment.

Deep sea corals are being put to the test at NIWA to find out how they cope with sediment.

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Think of corals and most of us imagine brightly coloured sunlit tropical reefs. But two thirds of the world's corals live in the deep sea.

That is more than 3300 species of coral that live in complete darkness, in temperatures as cold as minus 1° Celsius.

They can survive at depths of up to six kilometres.

And deep-sea corals are record holders - meet the oldest animals on earth, which can live for more than 4000 years.

And yes, they form reefs, although not quite in the style in the Great Barrier Reef. And these reefs provide important habitat for other marine creatures.

More than 110 deepwater species of coral have been described from the New Zealand region, and they feed on a rain of plankton drifting down from higher up in the water column.

Dr Malcolm Clark, from NIWA, says that on the Chatham Rise, which stretches out from Canterbury towards the Chatham Islands, you can find "quite spectacular reef-type structures" on the top of the rise, at depths of about 300 metres down to 1000 metres. Further down, at abyssal depths of up to three kilometres, you find sparser solitary corals

Living at the bottom of the sea does not mean that deep-sea corals are immune to human impacts. Fisheries trawling has been hugely damaging and in the past few years there have been proposals for deep sea mining and iron sand extraction that could also have an impact.

A team of marine ecologists at NIWA have been trying to gauge how resilient deep-sea coral ecosystems might be to these kinds of disturbance, especially sediment plumes or clouds stirred up by various activities.

"We know pretty well what happens when we put a trawl down," says Malcolm. "The footprint of the trawl damages the animals directly in its path, but it also stirs up this mud, and that spreads over a much wider area."

"The important thing about sediment is that it is like dust in the wind - it spreads out."

"We're talking about quite a wide depth range where human activities or natural events could disturb the seabed and create these clouds of sediment," says Malcolm.

"We think the animals may be less able to cope with disturbance because they're more adapted to a uniform, fairly constant environment."…

Go to this episode on rnz.co.nz for more details