Deep-sea mining trials revealed significant but localized biodiversity loss in one of the planet’s least explored ecosystems.

Global demand for critical metals is rising sharply, prompting many countries to explore the possibility of extracting these valuable resources from the ocean floor. A new international study that uncovered large numbers of previously unknownspeciesat depths of 4,000 meters suggests that seabed mining may cause less overall environmental damage than once feared. Even so, the researchers found that species diversity dropped by about one-third in areas directly affected by mining equipment.

As part of a large collaborative effort, marine biologists from multiple countries set out to document life in one of the most remote and least studied regions on the planet, the deep-sea floor of the Pacific Ocean.

The research, published inNature Ecology and Evolutionwas closely tied to growing commercial and geopolitical interest in deep-sea resources, which made such an extensive investigation possible.

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“Critical metals are needed for our green transition, and they are in short supply. Several of these metals are found in large quantities on the deep-sea floor, but until now, no one has shown how they can be extracted or what environmental impact this would have,” says marine biologist Thomas Dahlgren, that together with Helena Wiklund, also at the University of Gothenburg, have participated in the research project.

160 days at sea

The work followed the International Seabed Authority’s (ISA) guidelines for baseline studies and environmental impact assessments.

Over five years, the research team surveyed deep-sea ecosystems and evaluated the effects of mining activity in the Clarion-Clipperton Zone, a vast region of the Pacific located between Mexico and Hawaii. Their results showed that the total number of animals declined by 37 percent in mined areas, while species diversity fell by 32 percent along the tracks left by the mining machinery.

“The research required 160 days at sea and five years of work. Our study will be important for the International Seabed Authority (ISA), which regulates mineral mining in international waters,” says Thomas Dahlgren.

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788 species

The examined deep-sea floor is 4,000 meters below the water surface, no sunlight reaches there, and this means that it is an extremely nutrient-poor environment where the sediment layer grows by one thousandth of a millimeter per year. A bottom sample from, for example, the North Sea can contain 20,000 animals. A corresponding sample from the deep-sea floor yields the same number of species but only 200 specimens.

The researchers have collected 4,350 animals larger than 0.3 mm that live in, and sometimes on, the sea floor. 788 species were identified. The animals found were mainly marine bristle worms, crustaceans, and mollusks such as snails and mussels.

But there is also a new solitaire coral that is described in another study.

Species distribution unknown

“I have been working in the Clarion-Clipperton Zone for over 13 years, and this is by far the largest study that has been conducted. In Gothenburg, we led the identification of marine polychaete worms. Since most species have not been described previously, molecular (DNA) data were crucial in facilitating studies of biodiversity and ecology on the seabed, says Thomas Dahlgren.

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During the inventory, the researchers observed that the seabed communities changed naturally over time, probably due to changes in the amount of food reaching the seabed. Exactly how widespread the different species are across the deep-sea floors of the Pacific Ocean is unknown.

“It is now important to try to predict the risk of biodiversity loss as a result of mining. This requires us to investigate the biodiversity of the 30 percent of the Clarion-Clipperton Zone that has been protected. At present, we have virtually no idea what lives there,” says Adrian Glover, senior author from the Natural History Museum of London.

Reference: “Impacts of an industrial deep-sea mining trial on macrofaunal biodiversity” by Eva C. D. Stewart, Helena Wiklund, Lenka Neal, Guadalupe Bribiesca-Contreras, Regan Drennan, Corie M. B. Boolukos, Lucas D.

King, Muriel Rabone, Georgina Valls Domedel, Amanda Serpell-Stevens, Maria B. Arias, Thomas G. Dahlgren, Tammy Horton and Adrian G. Glover, 5 December 2025,Nature Ecology & Evolution.
DOI: 10.1038/s41559-025-02911-4

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Support from the NHMUK Consultancy team (H. Rousham, R. Fryer and J. Herrington). E.C.D.S. was supported through funding from NERC (grant no. NE/S007210/1).

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