Climate change: New report reveals how much organic carbon stored in seafloor, where it is at risk of being released

For the first time, the amount of organic carbon stored in New Zealand's seafloors has been quantified, which scientists say will allow government regulation of activities like seabed mining and bottom trawling to prevent the release of CO2.

The sea absorbs carbon from the atmosphere, and plants and animals use that carbon to live and grow. When they die, some of their remains settle on the bottom and the carbon is stored.

The new report from NIWA, commissioned by the Parliamentary Commissioner for the Environment, reveals New Zealand's vast marine estate stores approximately 2,240 million tonnes of organic carbon in the sediments on the seafloor.

That's about 1 per cent of all carbon stocks in marine sediments globally.

These stocks play a vital role in regulating climate change by storing carbon for thousands to millions of years.

However, the report highlights some areas around Aotearoa are particularly vulnerable to having their organic carbon released as carbon dioxide by human activities, including the heavily-fished Chatham Rise, and the sea off Fiordland and south Westland.

Professor Abby Smith from Otago University's Department of Marine Science says it's probably not possible to stop all seafloor disturbance, but the government can decide to regulate areas where a lot of carbon is stored.

"Unfortunately, if bottom sediments are disturbed, some or all of the carbon can re-enter the ocean system or even the atmosphere," Smith says.

"The report here is about how much carbon is currently stored on the seafloor around Aotearoa, where the big deposits are, and thus where we should be more careful about disturbing the bottom with mining, trawling, dredging and anchoring."

In a statement to Newshub, Seafood New Zealand Chief Executive Jeremy Helson said Seafood NZ had not been provided the study for review, but wanted to reiterate the importance of any policy or regulatory decisions being underpinned by robust science and according to the law.

"Restricting fishing on the Chatham Rise will have a big impact on New Zealand’s ability to supply fish, so it is crucial that the science is right and properly peer reviewed so any decisions about where trawling occurs are based on the facts," he said.

"For example, a previous study comparing global trawling and aviation emissions has been proven to be scientifically flawed by a factor of 100 to 1,000, showing how important it is for robust science, rather than estimates or guesstimates.”

The report focuses on the impact of bottom trawling because that's the only seafloor disturbance activity with adequate data to measure, but scientists say the next logical step is to look at the impact of dredging, anchoring and mining to understand the consequences of our activities in shallow water inlets, harbours, estuaries and fiords.

Senior Lecturer and head of the Sedimentary Environments and Analogues Research Group, Te Aka Mātuatua - School of Science, University of Waikato, Dr Andrew La Croix, says the new report is a welcome baseline dataset about the carbon storage potential of marine sediments in New Zealand.

"With a rapidly warming climate and its associated environmental changes, there is an increase in the urgency to understand every detail about how, where, and why carbon is cycled through our oceans and sediments," he says.

Chief Science Adviser to the Parliamentary Commissioner for the Environment, Dr Geoffroy Lamarche says without quantifying this carbon stock, we cannot address the risks it may face.

"This first assessment, despite a high level of uncertainty, marks significant progress and opens the door for important discussions on this poorly understood topic."