Scientists Warn of Potential Impacts of Deep-Sea Mining on Midwater Ecosystems 


Lying between the seabed and the sunlit zone near the surface of the ocean, the vast deep midwater ecosystems are home to an abundance of strange, otherworldly creatures.

These creatures include the bristled worms of the genus Tomopteris, typically transparent; they assume the color of the prey they have eaten. Or Rhabdosoma minor, a crustacean whose head is shaped like a spear. These unusual animals are uniquely adapted to life in the midwater, which represents more than 90 percent of the biosphere, contains fish biomass 100 times greater than the global annual catch, and is one of the least understood places on Earth. 

However, a paper published in the Proceedings of the National Academy of Science (PNAS) in July argues that deep-sea mining poses significant risks to the midwater ecosystems, which must be properly evaluated. Although deep-sea mining extraction is not yet a reality, it is fast approaching. The International Seabed Authority (ISA) – the intergovernmental body responsible for regulating deep-sea mining in areas beyond national jurisdictions – is developing mineral exploitation regulations, which could be finalized later this year. 

Midwater animal biodiversity: Squid, fish, shrimp, copepods, medusa, filter-feeding jellies, and marine worms are among the midwater creatures that could be affected by deep-sea mining. Photos by E. Goetze, K. Peijnenburg, D. Perrine, Hawaii Seafood Council (B. Takenaka, J. Kaneko), S. Haddock, J. Drazen, B. Robison, DEEPEND (Danté Fenolio), and MBARI.

The ISA has also authorized 30 deep-sea exploration contracts to a mix of state-owned and private companies. Covering some 1.5 million square kilometers of seabed, the contracts permit the exploration of manganese nodules on the abyssal seafloor, polymetallic sulfides near hydrothermal vents and cobalt-rich crusts on seamounts.

While the potential impacts vary depending on the type of resource being mined, the paper suggests that the dispersal of mud and chemicals caused by mining could affect marine life hundreds of kilometers from the extraction site.

“What we are concerned about in the midwater environment is the discharge of all the unwanted mud, seawater, and potential chemicals, which would come up with the ore,” said Jeffrey Drazen, a professor in the Department of Oceanography at the University of Hawaii at Manoa and the paper’s lead author.

Deep-sea mining strategies will involve ore and sediment being lifted off the seafloor, put on to ships, and separated during a process known as dewatering. Leftover waste would then be discharged back into the ocean.

“Mining companies differ in what they propose to do, but [dewatering waste] could be discharged anywhere in the water column because there are no regulations on where they have to discharge it,” Drazen added.

Deep-sea mining could impact tuna, commercial fisheries and the human seafood supply. Some tuna species range in deep waters and migrate vertically through the water column, with bigeye tuna diving to depths up to 600 meters to feed. Toxic particles discharged into the mesopelagic zone – which is also known as the twilight zone and lies between 200 and 1,000 meters below the ocean’s surface – could make their way into the food web and potentially impact human health through bioaccumulation.

Another concern is how discharged mud particles could impact filter-feeding species such as gelatinous animals and larvaceans, a type of free-swimming tunicate with transparent tadpole-like bodies. Essential to food chains, these animals live in water that is very clear apart from marine snow – an organic detritus that includes dead animals and fecal matter. Marine snow is an important food source for filter-feeding creatures.

Mining-generated sediment plumes and noise have a variety of possible effects on pelagic taxa. (Organisms and plume impacts are not to scale.) Image credit: Amanda Dillon (graphic artist)

Drazen said that these animals “make or use very fragile filter meshes to filter particles out of the water” and that those “filters may become clogged” with mud particles produced by deep-sea mining.

“Animals that work through the water column and pick out the marine snow organic particles may have to sift through a lot of particles that are just mud and have no nutritional value,” explained Drazen.

Animals that use visual communication and bioluminescence signaling in the deep midwaters could also be affected by seabed mining.

As Drazen explained: “[In the mesopelagic zone] there is still enough light for animals to see one another and play all kinds of visual trickery with bioluminescence. Most of the organisms create their own light, either to interact with one another or to play some of these visual trickeries and camouflage in the dim light of this environment.”

Sediment plumes produced by collector vehicles on the seafloor could reduce these animals’ ability to use visual communication and bioluminescence signaling, which they rely on to catch prey and reproduce.

The authors of the paper argue that precautionary management measures must be taken now to avoid harm to the midwater ecosystems caused by deep-sea mining.Kristina M Gjerde is the senior high-seas advisor to the IUCN Global Marine and Polar Programme, executive board member of Deep Ocean Stewardship Initiative and a co-author of the paper. She said, “There are many different types of ways that people suggest you exercise precaution, and they run the range of don’t do anything to just doing research or a little bit of experimentation. They do not suggest that one goes full-tilt ahead and adopts an adaptive management approach because often with these sorts of harms, you cannot repair them after.”

The authors contend that studies must include the entire water column, especially the bathypelagic and abyssopelagic zones located between 1,000 meters and just above the seafloor. According to the paper, midwater sampling to measure ecosystem impacts are included in the ISA’s recommendations. However, the data collected by contractors appeared to be very limited. Nonetheless, Drazen said that some mining companies are starting to include midwater research in their environmental baseline studies.

Meanwhile, in May, the Deep Sea Mining Campaign and MiningWatch Canada published a joint report examining the potential impacts of mining polymetallic nodules in the Pacific Ocean, where more than 50 percent of exploration contracts have been awarded.

Analysing more than 250 peer reviewed scientific articles, the report argues that mining would destroy habitats at extraction sites as well as potentially impacting midwater ecosystems.

The report also warns how little is known about the environmental, social and economic impacts of mining the seafloor, with the authors calling for a moratorium on deep-sea mining, stating that until the risks are fully understood, it is the only responsible way forward.

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