Deep-sea mining reduces number of seafloor animals by almost 40%

Over the past few decades, the deep sea has gained attention as a potential aid to the energy transition.

Lying at the bottom of the Pacific Ocean is what’s thought to be a vast reserve of minerals that could play a significant role in the production of batteries and green technology. But this region is also an ecosystem that’s remained almost unchanged for tens of millions of years. Plans to mine these areas leave many concerned about the potential impacts of deep-sea mining.

Actually assessing what those impacts might be has been difficult. This environment is so remote and so unknown that it’s hard to track the changes that could occur.

Researchers have now had a rare opportunity to explore the deep-sea sediment in the Pacific Ocean. They studied it before a mining test began and after it was completed, checking to see how animals living in the area were affected. After sifting through 80 samples taken during four expeditions, the researchers found that the abundance and number of species in the sediment declined by almost 40%.

Mining disrupts the seafloor, so this level of decline might not be surprising but this new study means there’s now solid data underpinning any future decisions.

Marine biologist Eva Stewart, a PhD researcher at the Natural History Museum, is trying to help us better understand the implications of mining the seafloor. She was involved in this latest study now published in Nature Ecology and Evolution.

“Being able to study these remote and poorly-known deep-sea regions is a huge privilege, and also extremely important as we consider the potential impacts of deep-sea mining,” says Eva. “These samples are very hard to collect and most of the species are unknown, meaning this is one of the largest abyssal studies ever to identify the species of every animal we collected.”

“We’ve been able to show that the number of species and number of animals didn’t change in the unimpacted areas, while in the area that was mined these decreased by nearly 40%.”

The hunt for minerals

It’s fairly well accepted that as the world transitions to green technology, the demand for certain minerals is only increasing. But how we fulfil that need is highly contested.

Some argue that we have already mined a lot of the resources we need, so we simply need to be better at recycling them and create a more circular economy. However, others have turned their eyes to the oceans.

In various regions, the seafloor is covered with fist-sized lumps of minerals known as polymetallic nodules. It’s estimated that these contain enough of substances such as nickel, cobalt and copper to make the energy transition.

But extracting this wealth of minerals has become incredibly contentious. While the deep sea is often thought of as fairly inhospitable, this couldn’t be further from the truth. As interest in these regions from industry has increased, so has our understanding of the sheer diversity of animals that thrive at these depths.

In the water column swim unusual-looking fish and shrimps. The nodules provide a solid substrate for hundred-year-old corals and crinoids, and the soft sediment is inhabited by endless species of worms, crustaceans and molluscs.

Mining will remove the nodules, compress the seafloor and create plumes of sediment in the water. It’s these impacts that, until now, have been difficult to quantify.

But Eva and her colleagues studied a site in a region of the Pacific Ocean known as the Clarion Clipperton Zone (CCZ) in the lead up to a mining test and immediately afterwards. This allowed them to better understand how much natural change there is in these ecosystems, before then comparing it to the impact that mining had.

The results were quite surprising.

Sedimental animals

Over a period of five years, researchers sampled the deep-sea floor of the CCZ three times before and one time two months after a mining test. They collected 80 samples of the deep-sea sediment to see what was living in it and how that changed over time and in response to mining activity.

The team of scientists sifted through this ancient mud, finding an extraordinary 788 different species of primarily worms, molluscs, and crustaceans. This makes it one of the biggest studies of what lives in these abyssal sediments to actually identify the species of every single specimen.

One of the most striking findings was the natural changes in the composition of the community of animals living in the sediment between the three samples taken before mining began.

“Historically it was thought that deep-sea ecosystems would be very stable and unchanging over time because of how remote they are from the ocean surface” explains Eva. “But we actually found there was quite a significant amount of natural change over the time we were studying”

“We found that the abundance of animals was changing naturally over time before the mining impact, which is potentially linked to natural climate cycles which affect the amount of food which reaches the seafloor. This is something we want to study more, but similar patterns have been found in other deep-sea communities too.”

But even when taking these natural changes into account, the results of the mining tests were clear. The number of species in the impacted samples declined by 32% while the overall number of animals dropped by 37%. In comparison, the samples from non-mined areas did not show any changes from immediately before to two-months after.

The researchers also looked at the impact of sediment plumes on seafloor species. The mining machine disturbs the sediment on the seafloor, creating large clouds that then settle away from the area that was mined. They found that whilst the number of species declined, the overall number of animals remained relatively stable. This is probably due to some species coping better with the disruption than others.

An uncertain future

This study focused on animals living in the sediment, which means we can’t say with certainty how mining activity impacts other animals in these deep sea environments. However, it seems likely that the nodule-living animals at least are even more heavily impacted.

What this means overall for the animals living in these deep-sea environments is uncertain.

“One critical gap is the knowledge of where species exist elsewhere,” explains Dr Adrian Glover, our deep-sea expert who was also involved in this latest study. “We don’t know what organisms live in the protected regions.”

“This makes it very hard to model the extinction risk for the mining zones if we don’t know what’s living in the protected areas and the degree of connectivity between the mined areas and the protected zones.”

By going back to the same site again in the future, scientists will hopefully be able to better understand how and if recolonisation occurs with these animals.