Seabed Mining Tracks Affecting Ecosystem After 26 Years

Deep-sea mining could satisfy the growing demand for rare metals. However, its potential environmental impacts have raised concern, further supported by a new study.

Researchers at the Max Planck Institute for Marine Microbiology and the Alfred Wegener Institute have studied simulated mining tracks made in the seabed 26 years ago. Their results, published in Science Advances, indicate that deep-sea mining-related disturbances have a long-term impact on ecosystem functions and microbial communities.

Polymetallic nodules and crusts cover many thousands of square kilometers 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 being explored. To date, there is no market-ready technology for deep-sea mining. 

Studies have shown that many organisms attach themselves to the nodules, and they remain absent decades after a disturbance in the ecosystem. The newly published research indicates that microorganisms inhabiting the seafloor would also be massively affected by mining. The team led by Antje Boetius from the Max Planck Institute for Marine Microbiology, travelled to the DISCOL area in the tropical East Pacific, about 3,000 kilometers off the coast of Peru, to investigate conditions of the seafloor. Back in 1989, German researchers had simulated mining-related disturbances at this site by ploughing the seabed in a manganese nodule area of three and a half kilometers in diameter with a plough-harrow, 4,000 meters under the surface of the ocean.

“Even 26 years after this disturbance, the plough tracks on the seabed were still clearly visible,” reports first author Tobias Vonnahme. “The bacterial inhabitants were also clearly affected.” 

Compared to undisturbed regions of the seafloor, only about two thirds of the bacteria lived in the old tracks, and only half of them in fresher plough tracks. The rates of various microbial processes were reduced by three quarters in comparison to undisturbed areas. “Our calculations have shown that it takes at least 50 years for the microbes to fully resume their normal function,” says Vonnahme.

The bio-geo-chemical conditions had undergone lasting changes, says Boetius. According to the researchers, this is mainly due to the fact that the plough destroys the upper, active sediment layer. It is ploughed under or stirred up and carried away by the currents. In these disturbed areas, the microbial inhabitants can only make limited use of the organic material that sinks to the seafloor from upper water layers. As a result, they lose one of their key functions for the ecosystem. 

Microbial communities and their functions could thus be suitable as early indicators of damage to deep-sea ecosystems caused by nodule mining – and of the extent of their potential recovery, says Boetius.

All mining technologies for manganese nodules currently being developed will lead to a massive disturbance of the seabed down to a depth of at least 10 centimeters. Commercial deep-sea mining would affect hundreds to thousands of square kilometers of seabed per year.

The researchers say the environmental impacts of mining the seafloor are still only partially known. In addition, there is a lack of clear standards to regulate mining and set binding thresholds for the impact on the organisms living in affected areas. 

Boetius says “Ecologically sustainable technologies should definitely avoid removing the densely populated and bioactive surface layer of the seabed.”