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Deep-sea mining shown to leave decades-long carbon cycle disruption

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Deep-sea mining can leave a devastating mark on the seafloor that lasts for decades and negatively impacts the ocean carbon cycle, scientists have said.

To determine the findings, an international team from the Max Planck Institute, University of Edinburgh, and others, travelled to the DISCOL area in the tropical East Pacific, about 3,000km off the coast of Peru.

Back in 1989, German researchers had simulated mining-related disturbances 4,000m under the surface of the ocean, by ploughing a 3.5km-wide area of seabed with a plough harrow.

“Even 26 years after the disturbance, the plough tracks are still there,” said researcher Tanja Stratmann. Previous studies had shown that microbial abundance and density had undergone lasting changes in this area.

“Now we wanted to find out what that meant for carbon cycling and the food web of this deep ocean habitat,” she added

The scientists looked at different ecosystem components to determine how they worked in conjunction with each other.

They quantified carbon fluxes between living and non-living compartments of the ecosystem and summed them up as a measure of the “ecological size” of the system.

Carbon fluxes are the amount of carbon exchanged between Earth's carbon pools (typically the oceans, atmosphere and land), and living things.

They found significant long-term effects of the 1989 mining simulation experiment. The total throughput of carbon in the ecosystem was significantly reduced.

“Especially the microbial part of the food web was heavily affected, much more than we expected,” Stratmann said.

“Microbes are known for their fast growth rates, so you’d expect them to recover quickly. However, we found that carbon cycling in the so-called microbial loop was reduced by more than one third.”

The impact of the simulated mining activity on higher organisms was more variable. While some animals seemed to do fine, others were still recovering from the disturbance, even decades later.

“The diversity of the system was thus reduced,” researcher Daniëlle de Jonge said. “Overall, carbon flow in this part of the food web was similar to or even higher than in unaffected areas.”

The simulated mining resulted in a shift in carbon sources for animals. Usually, small fauna feed on detritus and bacteria in the seafloor. However, in the disturbed areas, where bacterial densities were reduced, the fauna ate more detritus.

“Future climate scenarios predict a decrease of the amount and quality of detritus reaching the seafloor. Thus this shift in diet will be especially interesting to investigate in view of climate change,” de Jonge said.

“You also have to consider that the disturbance caused by real deep-seabed mining will be much heavier than the one we’re looking at here.

“Depending on the technology, it will probably remove the uppermost 15cm of the sediment over a much larger area, thus multiplying the effect and substantially increasing recovery times.”

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