Life in the Deep Ocean

UH scientists reveal the shifting world beneath the waves

Four kilometers beneath the Pacific, there is no sunlight, no sound, and no sign of stillness.

In that pitch-black world, tiny creatures glow with bioluminescence, flickering like stars against the dark. It is a place most people will never see, but new research from the University of Hawai‘i shows that even the planet’s deepest waters are full of movement and change.

A recent study led by UH Mānoa doctoral candidate Gabrielle Ellis found that zooplankton drifting just above the seafloor are far from static. Their communities shift with the seasons, responding to pulses of organic matter that sink from the surface. The findings from this study challenge the long-held idea that the deep sea is lifeless and unchanging. It also raises new questions about how human activity, especially deep-sea mining, could throw those delicate rhythms off balance.

“People would be surprised by how alive the deep sea really is,” said Dr. Craig Smith, professor of oceanography at UH Mānoa and a co-author on the study. “It is a huge reservoir of biodiversity. The abyssal region, between four and six thousand meters, covers about half of Earth’s solid surface. It is the planet’s dominant habitat.”

Smith explained that even in complete darkness, life there still depends on light. “Most species use bioluminescence,” he said. “They make their own light to find mates, prey, or to stay hidden from predators.”

The team’s work focused on the benthic boundary layer, a thin zone of water right above the seafloor where life, sediment, and nutrients mix. Using autonomous pumps mounted on free-falling landers, the researchers gathered samples from 4,000 meters down. The technology has to survive crushing pressure, freezing temperatures, and months alone on the seafloor before surfacing again.

What they brought back showed something unexpected. As seasons changed at the surface, the composition of zooplankton below changed too. Some species vanished while others appeared, suggesting a deep-sea community more dynamic and connected to surface conditions than scientists had assumed.

“It is really the pathway by which a lot of animals disperse,” Smith said.

Many species that live fixed to the seafloor as adults rely on these drifting larvae to colonize new areas.

“If mining were to go ahead and disturb huge areas of the seafloor, those areas would have to be recolonized by larvae drifting through this layer,” Smith said.

That concern is not hypothetical. The Clarion-Clipperton Zone, a vast region of the Pacific targeted for its mineral-rich earth, is already under exploration by private companies. Smith warns that industrial mining there could devastate ecosystems scientists barely understand.

“It is basically strip mining,” he said. “They will drag dredges the size of a house across the seafloor, sucking up the top few centimeters of sediment, the nodules, and everything living on them.”

Each mine could disturb up to 500 square kilometers of seafloor each year, and dozens of mines are being proposed.

“If all the areas under contract were mined, we would damage an area nearly half the size of Europe,” Smith said.

The destruction would not end at the seafloor. Mining stirs up thick plumes of sediment that drift through the water, smothering filter feeders and blocking the faint organic particles that deep-sea life depends on.

“These are the clearest waters on the planet,” he said. “Add sediment, and it clogs their feeding systems and dilutes the food. It is deadly.”

Supporters argue that deep-sea mining could supply metals like cobalt and nickel for electric vehicle batteries, reducing the need for land mining. Smith disagrees.

“Mining in the deep sea will not replace mining on land,” he said. “It will only expand the footprint of mining across the biosphere. You are not going to save the rainforest by mining the seafloor.”

For Smith, Ellis, and the rest of the research team, the deeper issue is how little humanity knows about what is at stake. He said every expedition to the Clarion-Clipperton Zone brings back hundreds of species that are new to science. Many are identified only through DNA because they are too fragile to survive the trip to the surface.

“We are talking about ninety percent of what we collect being completely unknown,” he said. “It is probably the most biodiverse and least understood ecosystem on Earth.”

After decades of studying the deep sea, from Antarctica to the equator, Smith still sounds like someone in awe of it.

“Everything you bring back is fascinating,” he said. “We are still finding new habitats every year. It is a major portion of the biosphere, and we have barely mapped it.”

He hopes more students and voters in Hawai‘i start paying attention.

“The deep sea is full of unique forms of life, some beautiful, some strange, and they are all part of the planet’s story,” he said. “It is not out of sight or out of mind. What happens down there affects what happens up here.”