Richard Robinson

The seafloor stores carbon, but trawling releases it

A new study reveals that New Zealand’s huge ocean territory contains two billion tons of carbon, locked in the seafloor. If we want to fight climate change, it might be a good idea to keep it there.

Written by       Photographed by Richard Robinson

More than two billion tons of carbon is buried in the seabed around New Zealand, but human disturbance has the potential to release some of it back to the atmosphere, a new report has found.

New Zealand’s exclusive economic zone (EEZ) contains an area of seafloor bigger than India, a vast mosaic of sand, gravel and mud. There, a slow rain falls from the surface: the remains of plankton, krill and other animals. This death and decay nourishes ecosystems and carries carbon from living creatures down to the sediment, where it stays.

If something disturbs the seafloor, however, that carbon can be re-suspended in the water column and perhaps even returned to the atmosphere.

The ocean stores around a quarter of the carbon dioxide pollution we produce, making it a vital sink in the fight to slow climate change. Overseas, studies have raised concerns that disturbance of the sea floor, particularly by bottom-trawl fishing nets, could release carbon back into the atmosphere.

Deep-water trawl fishers on board the Kaharoa in the Cook Strait. Every year, New Zealand vessels drag trawl gear across nearly 100,000 square kilometres of our seafloor.

A new report, commissioned by the Parliamentary Commissioner for the Environment and carried out by scientists from the National Institute of Water and Atmospheric Research (NIWA) pulls together what we know about seafloor carbon in New Zealand waters.

“We’ve always known that soil on land is a really big carbon storage reservoir,” says Geoffroy Lamarche, chief science advisor to the Commissioner, “but with New Zealand being 93 per cent under the water, there’s never been a study that quantified [it].”

The study gathered data from thousands of sediment samples, from the shallow Firth of Thames to abyssal waters thousands of metres deep. In addition, the researchers used statistical modelling to predict the amount of carbon in sediments from some parts of the EEZ.

The researchers found that New Zealand’s EEZ contains 2.24 billion tons of carbon—one per cent of the global store of seabed carbon.

Fiordland is particularly rich, holding up to eight per cent of our seafloor carbon—fiords were already known to be powerful contributors to carbon stores. “We can track that terrestrial carbon out to about 200 kilometres in the Hokitika Canyon off the West Coast,” says marine geologist Scott Nodder, the study’s lead author.

Other areas rich in carbon include the Chatham Rise and Bounty Plateau: parts of the sea where oceanic currents deliver food to masses of plankton, as well as fish, birds and sea mammals. Fishing boats target these parts of the ocean too, including bottom trawlers whose heavy gear rakes across the seafloor, churning up the organically rich sediment.

Not all sea floors are the same. Fiords help to lock carbon in sediment, as can be seen by the pink area near Fiordland. The purplish shading in the east shows the Chatham Rise and the Bounty Plateau, areas where ocean currents deliver higher concentrations of food to marine life.

Mapping carbon-rich areas against fishing patterns and the sediment composition of the seafloor allowed researchers to produce a map showing which parts of the seabed are most vulnerable to carbon release. South Westland, for example, is particularly at risk.

Around 11 per cent of the New Zealand seabed has been trawled over the past 32 years, with some carbon-rich areas, such as the Chatham Rise, receiving particular attention. The most heavily fished parts of our EEZ have been trawled the equivalent of 1.7 times a day for 32 years.

Fishing typically occurs only in water shallower than 1500 metres. “We don’t have any sort of idea of what disturbances there might be a little bit deeper than that,” says Nodder.

Where we trawl, displayed in colour: yellow represents the areas of highest fishing effort. The plume extending off to the east shows trawlers operating on the Chatham Rise, an area that also has higher levels of stored carbon.

Bottom trawling isn’t the only human activity that impacts seafloor carbon—anchoring, cable burial, dredging and deep-sea mining also disturb the seafloor.

In the deep ocean, natural events also play a big role in carbon turnover, particularly undersea avalanches called turbidity currents and landslides that occur after earthquakes. A single turbidity current can move millions of tonnes of sediment, stirring up buried carbon but also sequestering carbon by carrying it from shallow waters into the deep.

A trawl net rumbles along the seafloor in the Hawke’s Bay. The plume from one trawl area impacts an area between one and ten square kilometres in size, according to a 2021 study. The sediment suspended in the water remains for several days, altering the ocean’s biogeochemical processes.

“I think understanding the natural processes of disturbances is important to put some of these findings into context,” says Nodder. “Things like bottom trawling will re-suspend material on a more regular basis than more episodic events like turbidity currents, but equally waves and tidal currents might be doing the same thing. Trying to figure out the balance of those human activities versus the natural processes is something that we need to consider.”

The report is a good “first go” at mapping New Zealand’s undersea carbon stocks.

“It’s a baseline from which to work,” says Lamarche. “I think we should be careful in drawing too many conclusions in terms of decision-making. It’s more to say, there’s quite a bit of carbon there, let’s work on what those numbers mean and how we can protect it.” For instance: “Do we need to use these numbers as one of the many criteria to define and design marine protected areas?”

Andrew La Croix, a sedimentologist with the University of Waikato who wasn’t involved in the study, agrees. “Marine reserves aren’t just to protect biodiversity and organisms at the seafloor, but potentially are [also] for protecting things beneath the sediments—things that are ‘out of sight, out of mind’.”

La Croix points out that the Hauraki Gulf, an area that he studies, has also been identified as a carbon hotspot. “It’s a famous, treasured part of New Zealand for a number of reasons,” he says, “and this actually seems to me to be another reason.”

Read more about bottom-trawling here.

The 27-metre Sanford trawler Tengawai fishes in the Hauraki Gulf Marine Park off Aotea/Great Barrier Island. Currently, 73 per cent of the Gulf is open to trawling, which includes marine protected areas.

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