Deep-sea mining threatens life, food webs in the ocean’s dim ‘twilight zone’

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Deep-sea mining could unleash a silent disaster in a vital, hidden part of our oceans. That's the message from a new University of Hawai'i at Mānoa study, the first to truly look at the impact of mining waste. Researchers found that more than half of the tiny animals, zooplankton, forming the ocean's food building blocks in the "twilight zone" (a vital region 200-1,500 meters below sea level) could be harmed, risking bigger creatures further up the food web.

Researchers discovered that waste discharged from deep-sea mining operations in the Pacific's biodiverse Clarion-Clipperton Zone (CCZ) could disrupt marine life in the midwater twilight zone. The study finds that 53% of all zooplankton and 60% of micronekton, which feed on zooplankton, would be impacted by the discharge of the mining waste, which could ultimately impact predators higher up on the food web.

"When the waste released by mining activity enters the ocean, it creates water as murky as the mud-filled Mississippi River. The pervasive particles dilute the nutritious, natural food particles usually consumed by tiny, drifting zooplankton," said Michael Dowd, lead author of the study and oceanography graduate student in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST). "Micronekton, small shrimp, fish and other animals that swim, feed on zooplankton. Some migrate between the depths and near surface waters and they are consumed by fish, seabirds and marine mammals. Zooplankton's exposure to junk food sediment has the potential to disrupt the entire food web."

The study examines the content and effects of mining waste released during a 2022 mining trial in the midwater CCZ, an expansive area of the Pacific Ocean targeted for the extraction of deep-sea polymetallic nodules, which contain critical minerals, including cobalt, nickel and copper. Researchers collected and tested water samples from depths where the mining waste was discharged, finding that these particles had far lower concentrations of amino acids-a key indicator of nutritional value-than the naturally occurring particles that fuel life in these depths.

The twilight zone hosts a staggering diversity of life, including tiny krill, fish, squid, octopus and gelatinous species such as jellyfish and siphonophores. By rising toward the ocean's surface every night, then swimming back down again, these creatures support the transport of carbon to greater depths in the ocean, which is critical to ocean and human health. These creatures either feed on the particles in the twilight zone or prey on those that do, creating a tightly linked food web that connects the surface ocean to the abyss.

"Our research suggests that mining plumes don't just create cloudy water-they change the quality of what's available to eat, especially for animals that can't easily swim away," said Jeffrey Drazen, co-author, SOEST oceanography professor and deep-sea ecologist. "It's like dumping empty calories into a system that's been running on a finely tuned diet for hundreds of years."

Urgent concerns with commercial mining

The findings raise urgent concerns about long-lasting, system-wide effects if large-scale commercial mining proceeds without strong environmental safeguards. Pacific tuna fisheries, for example, operate in the CCZ, which means that deep sea mining waste could impact fish that land on dinnerplates globally.

"Deep-sea mining has not yet begun at a commercial scale, so this is our chance to make informed decisions," said Brian Popp, co-author, SOEST earth sciences professor, and expert in marine stable isotope biogeochemistry. "If we don't understand what's at stake in the midwater, we risk harming ecosystems we're only just beginning to study."

"This isn't just about mining the seafloor; it's about reducing the food for entire communities in the deep sea," said Erica Goetze, co-author, SOEST oceanography professor and expert in marine zooplankton ecology. "We found that many animals at the depth of discharge depend on naturally occurring small detrital particles-the very food that mining plume particles replace."

The study comes as some countries ramp up their efforts to meet growing global demand for metals needed for electric car batteries and other low-carbon technologies. Currently, about 1.5 million square kilometers of the CCZ are under license for deep-sea mining.

Disrupting a system tuned to scarcity

During the deep-sea mining process, nodules are collected from the seafloor, along with seawater and sediments, and transferred through a pipe to a collection ship for separation of nodules from sediment waste. This seawater-containing sediment waste, as well as pulverized very small nodule particles, must be returned to the ocean. Although the release depth for this waste is currently unclear, some mining operators have proposed midwater mining discharge within the twilight zone. Until now, the impacts of this waste on midwater communities was poorly understood. These impacts will be critical for the establishment of regulations around waste discharge, which don't yet exist-a gap in governance over the industry.

The authors hope their findings will inform international regulatory decisions currently being shaped by the International Seabed Authority, as well as the National Oceanic and Atmospheric Administration, which is responsible for reviewing environmental impacts of U.S.-led deep-sea mining initiatives. They also call for expanded research to protect the full vertical extent of ocean ecosystems.

"Before commercial deep-sea mining begins, it is essential to carefully consider the depth at which mining waste is discharged," said Drazen. "The fate of these mining waste plumes and their impact on ocean ecosystems varies with depth, and improper discharge could cause harm to communities from the surface to the seafloor."

Additional authors on the study include oceanography graduate students Victoria Assad and Alexus Cazares-Nuesser, and oceanography professor Angelicque White.