Deep-seabed mining disrupts seafloor food web

Interventions on the seabed reduce the turnover of carbon

Deep-seabed mining is considered a way to address the increasing need of rare metals. However, the environmental impacts are considered to be substantial but remain largely unknown and clear regulatory standards are lacking. Researchers at the Max Planck Institute for Marine Microbiology in Bremen, Germany, together with colleagues from The Netherlands, Belgium, Portugal, Germany and the UK, now describe that mining-related disturbances have a long-term impact on carbon flow and the microbial loop at the deep seafloor.

The deep sea is far away and hard to envision. If imagined it seems like a cold and hostile place. However, this remote habitat is directly connected to our lives, as it forms an important part of the global carbon cycle. Also, the deep seafloor is, in many places, covered with polymetallic nodules and crusts that arouse economic interest. There is a lack of clear standards to regulate their mining and set binding thresholds for the impact on the organisms living in affected areas.

Poly­metal­lic nod­ules and crusts cover many thou­sands of square kilo­metres of the world's deep-sea floor. They con­tain mainly man­ganese and iron, but also the valu­able metals nickel, co­balt and cop­per as well as some of the high-tech metals of the rare earths. Since these re­sources could be­come scarce on land in the fu­ture – for ex­ample, due to fu­ture needs for bat­ter­ies, elec­tro­mobil­ity and di­gital tech­no­lo­gies – mar­ine de­pos­its are eco­nom­ic­ally very in­ter­est­ing. To date, there is no mar­ket-ready tech­no­logy for deep-sea min­ing. However, it is already clear that in­ter­ven­tions in the seabed have a massive and last­ing im­pact on the af­fected areas. Stud­ies have shown that many sessile in­hab­it­ants of the sur­face of the sea­floor de­pend on the nod­ules as a sub­strate, and are still ab­sent dec­ades after a dis­turb­ance in the eco­sys­tem. Also, ef­fects on an­im­als liv­ing in the seabed have been proven.

Disturbances of the food web

An international team of scientists around Tanja Stratmann from the Max Planck Institute for Marine Microbiology in Bremen, Germany, and Utrecht University, the Netherlands, and Daniëlle de Jonge from Heriot-Watt University in Edinburgh, Scotland, has investigated the food web of the deep seafloor to see how it is affected by disturbances such as those caused by mining activities.

For this, the scientists travelled to the so-called DISCOL area in the tropical East Pacific, about 3000 kilometres off the coast of Peru. Back in 1989, German researchers had simulated mining-related disturbances in this manganese nodule field, 4000 metres under the surface of the ocean, by ploughing a 3.5 km wide area of seabed with a plough-harrow. “Even 26 years after the disturbance, the plough tracks are still there”, Stratmann described the site. 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.”

“We looked at all different ecosystem components and on all levels, trying to find out how they work together as a team”, de Jonge explained who carried out the project as part of her Master’s Thesis at the NIOZ Royal Netherlands Institute for Sea Research and the University of Groningen, The Netherlands. The scientists 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.

Reduced throughput of carbon

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”, said Stratmann. “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. “Some animals seemed to do fine, others were still recovering from the disturbance. The diversity of the system was thus reduced”, said de Jonge. “Overall, carbon flow in this part of the food web was similar to or even higher than in unaffected areas.”

More vulnerable to climate change

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. The possible consequences of this will be part of de Jonge’s PhD Thesis, which she just started. “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”, she looks forward to the upcoming work. 

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

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