Reuters/Aly Song

Deep-sea fish transfer carbon into the depths of the ocean.

Humans are pumping roughly 33 billion tonnes (36 billion tons) of carbon dioxide into the air each year. If all that showed up in the atmosphere, it would accelerate global warming even further. The ocean, however, absorbs around half of that CO2. Phytoplankton (essentially, microscopic plants) that live near the sea’s surface take in a lot of it as they photosynthesize, ultimately flushing the CO2 into the cold, dense depths, where it stays trapped for centuries.

How does it travel all those fathoms? Until now, scientists have chalked it up mainly to gravity—to the sinking of phytoplankton that have either died or been eaten and excreted by fish. But new research reveals that we have deep-sea fish to thank for transferring a lot of that carbon into the depths—and that sinking alone wouldn’t do the trick.

In fact, bottom-dwellers transfer more than a million tonnes of CO2 a year from surface waters of the UK and Ireland, helpfully storing between €8 million and €14 million ($10.9 million and $19 million) a year in carbon credit value, says a new study (paywall) by a University of Southampton team. Killing too many of those fishes, as well as the ones they feed on, risks damaging the ocean’s ability to store carbon, leaving more CO2 in the atmosphere.

From top to bottom

Though the cycle is complicated, there are two main fish groups in this story: those living in the sea’s middle swath, around 200-1,000 meters (650-3,300 feet) down; and the deep-sea set best known for fangs, underbites and Gollum-like looks.

Every night, billions of middle-layer fish head up to the giant plankton salad bar on the surface to chow down on plankton, small plankton-eating organisms, upper-level fish, and each other, swimming back down once they’re done. Scientists recently discovered (pdf) that these mid-level fish are likely just as critical as upper-dwelling species to the “biological pump,” as they call the mechanism that moves carbon deep into the sea. Presumably that’s because, like the plankton, these mid-level fish sink when they die, taking their CO2 load with them—but they’re much more efficient at moving that carbon down than if it had to sink all the way from the surface.

Here’s how scientists assumed carbon was reaching the ocean’s depths. earthobservatory.nasa.gov

As for deep-dwelling fish, though, everyone’s assumed they live mainly off nutrient crumbs falling from above. Since they’re more than a kilometer below the ocean’s surface, it’s hard to tell.

According to the new study, however, that’s not the case. A majority of deep-sea fish are cruising up to the middle layer during the day and tucking into the fish that live there, say the researchers. In fact, the deeper-dwelling the species, the more it tends to get its food from other fish.

The fact that these deep-sea critters are eating middle-level fish at far higher volumes than anyone previously realized explains how a lot of that mysteriously descending carbon gets so deep. If the researchers are right, this food web also means gravity alone isn’t nearly as critical to marine carbon storage as we’ve long assumed.

Which means, then, that overfishing threatens this biological pump, because it puts both layers of fish at risk. Though fishing vessels tend to target middle-level species, they also go for some deep-sea fish like blue ling, red seabream or black scabbard fish. And unwanted fish of both levels are also accidentally caught in huge volumes as “bycatch.

This post originally appeared on Quartz, an Atlantic partner site.

MORE FROM QUARTZ:

"No Broccoli, No Dessert," Right? Wrong.

Softbank's Humanoid Robot Will Be Great For Tending to Japan's Elderly

How America Fell Out of Love With Writing Checks

About the Author

Most Popular

  1. An illustration of the Memorial Day flood in Ellicott City, Maryland.
    Environment

    In a Town Shaped by Water, the River Is Winning

    Storms supercharged by climate change pose a dire threat to river towns. After two catastrophic floods, tiny Ellicott City faces a critical decision: Rebuild, or retreat?

  2. Environment

    A 13,235-Mile Road Trip for 70-Degree Weather Every Day

    This year-long journey across the U.S. keeps you at consistent high temperatures.

  3. Maps

    Visualizing the Hidden ‘Logic’ of Cities

    Some cities’ roads follow regimented grids. Others twist and turn. See it all on one chart.

  4. A line of stores in Westport, Connecticut
    Equity

    Separated by Design: How Some of America’s Richest Towns Fight Affordable Housing

    In southwest Connecticut, the gap between rich and poor is wider than anywhere else in the country. Invisible walls created by local zoning boards and the state government block affordable housing and, by extension, the people who need it.

  5. A woman walks down a city street across from a new apartment and condominium building.
    Design

    How Housing Supply Became the Most Controversial Issue in Urbanism

    New research has kicked off a war of words among urban scholars over the push for upzoning to increase cities’ housing supply.