Fiordland has high levels of carbon stored in its seabed. Photo: 123rf
A NIWA report commissioned by Commissioner for Environment Simon Upton said about 2240 million tonnes of carbon was stored in marine sediment.
The report gives the first inventory of organic carbon stored in marine sediments in New Zealand's Exclusive Economic Zone.
The research was a step towards understanding whether the ocean could cope with the amount of greenhouse gases produced by humans, Upton said.
"Effective stewardship of marine carbon stocks is essential.
"In the same way that we need to keep carbon in the soil on land, we need to keep carbon in marine sediments - the ocean's soil," Upton said.
The report maps areas most at risk of disturbance by bottom trawling.
The report from the Parliamentary Commissioner for the Environment shows areas with the highest organic carbon stores. Photo: Supplied
Organic carbon is distributed unevenly in New Zealand seas, with 66 percent in waters deeper than 1500m, 26 percent on continental slopes and 8 percent in shallow environments.
Areas off south Westland, the Chatham Rise and from Kaikōura to Hawke's Bay have higher levels of carbon.
"The Fiordland fjords, which represent only a tiny fraction of New Zealand's waters, were found to contain 8 percent of our marine sediment carbon stocks, highlighting the value of this area as a carbon store," Upton said.
Marine organisms drew carbon from the atmosphere, but when they broke down and sank to the seafloor, they added carbon to marine sediments, the report said.
When the sea floor was disturbed by natural processes or human activities - such as building offshore structures, dredging or bottom trawling - carbon dioxide could be released into the water or atmosphere, depending on the water depth, it said.
Further discussion was needed on protect carbon stores, which could include include limiting or mitigating human activities in certain areas or making greater use of marine protected areas, Upton said.