For as long as there has been marine life, there has been marine snow—an incessant drizzle of debris and dead beings that descend from the surface to the depths of the sea.
The snow starts in the form of specks, which aggregate to form dense flakes that gradually sink, passing through the mouths (and the like) of scavengers that live below. But even marine snow that is devoured is likely to revert to snow again. A squid’s innards are nothing more than a resting stop on this long journey into the depths.
The term “snow” may suggest something white and wintry, but marine snow is mostly brown or grayish, being composed mostly of dead material. From time immemorial, debris has always contained the same things: plant residues and animal carcasses, feces, mucus, dust, microbes and viruses, and they have always transported carbon from the ocean to be stored on the seabed.
Increasingly, however, marine snow is being infiltrated by microplastics: fibers and fragments of polyamide, polyethylene and polyethylene terephthalate (PET). And this snow of non-organic material appears to be altering our planet’s millennial cooling process.
Tens of millions of tons of plastic enter Earth’s oceans every year. At first, scientists thought the material was destined to float on large islands or eddies of rubbish, but ocean surface surveys reveal only about 1% of the estimated total plastic in the oceans.
A recent model concluded that 99.8% of the plastic that has entered the ocean since 1950 has sunk below the first few hundred feet of the ocean. Scientists have found 10,000 times more microplastics on the seabed than in contaminated surface water.
Marine snow, one of the main paths connecting the surface and the depths, appears to be helping the plastics sink. And scientists are just beginning to decipher how these materials interfere with deep-sea food webs and natural ocean carbon cycles.
“It’s not just that marine snow carries plastics or aggregates with plastic,” said Luisa Galgani, a researcher at Florida Atlantic University. “It’s just that snow and plastics can help each other get to the bottom of the ocean.”
The creation of marine snow
The sunlit sea surface is teeming with phytoplankton, zooplankton, algae, bacteria and other tiny life forms, all of which feed on the sun’s rays or each other. When these microbes metabolize, some produce polysaccharides that can form a sticky gel that attracts the bodies of tiny dead organisms, tiny bits of larger carcasses, foraminifera and pteropod hooves, sand and microplastics, which clump together to form larger flakes.
“They are the glue that holds together all these components of marine snow,” said Galgani.
Marine snowflakes fall at different speeds. The smaller flakes descend more slowly — “it could be as little as a meter a day,” said biological oceanographer Anela Choy of the Scripps Institute of Oceanography at the University of California, San Diego.
Larger particles, such as dense fecal bullets, can sink more quickly. “This material quickly sinks to the ocean floor,” commented researcher Tracy Mincer of Florida Atlantic University.
Plastic in the ocean is constantly being degraded; even a large, floating object like a milk jug eventually breaks down into microplastics. These plastics develop biological layers made up of distinct microbial communities — the so-called “plastisphere,” according to Linda Amaral-Zettler, a scientist at the Royal Netherlands Institute for Marine Research, who coined the term.
“We tend to think of plastic as being inert,” she said. “Once it enters the environment, it is quickly colonized by microbes.”
Microplastics can give so many microbes a lift that they end up neutralizing the plastic’s natural buoyancy, causing your “raft” to sink. But if the biological layers degrade on the way down, the plastic could end up floating up again, potentially leading to a microplastic purgatory describing a rise and fall in the water column.
Marine snow is anything but stable; As the flakes free-fall into the abyssal depths, they constantly freeze and decompose, torn apart by waves or predators.
“It’s not as simple as ‘everything falls down all the time,'” explained Adam Porter, a marine ecologist at the University of Exeter in England. “It’s a black box in the middle of the ocean, because we can’t spend enough time there to understand what’s going on.”
To explore how marine snow and plastics are distributed in the water column, Mincer began taking samples from deeper water, using a dishwasher-sized pump, filled with filters, that hangs from a cable attached to a research vessel. The filters are arranged from large mesh to small mesh so as not to allow fish and plankton to enter. Operating for ten straight hours at a time, these pumps have revealed fibers of nylon and other microplastics distributed throughout the entire water column below the subtropical gyre of the South Atlantic.
But even with a research vessel and its cumbersome, expensive equipment, it is not easy to retrieve an individual piece of marine snow from deep water in the real ocean. Bombs often disperse snow and fecal balls. And the flakes alone offer little clue as to how quickly some snow is sinking, information that is vital to understanding how long plastics stay, move up and down, or sink in the water column before settling. up on the sea bed.
“Is it decades?” asked Mincer. “Hundreds of years? With that information, we could understand what we’re facing here and what kind of problem this really is.”
instant sea snow
To answer these questions and work within a set budget, some scientists have produced and manipulated their own marine snow in the laboratory.
In Exeter, Porter harvested buckets of seawater from a nearby estuary and poured the water into bottles that roll continuously. Then he threw microplastics into the water, including tiny spheres of polyethylene and polypropylene fibers. The constant motion, plus a splash of sticky hyaluronic acid, encouraged the particles to collide and clump together, forming snow.
“Of course we don’t have a 300-meter-deep tube to make the snow sink in,” Porter said. “But by rolling the bottles, we’re creating an endless column of water through which particles can fall.”
After the bottles had been rolled for three days, he removed the snow and analyzed the number of microplastics in each flake. His team found that all types of microplastics they tested aggregated with sea snow and that microplastics such as polypropylene and polyethylene, normally too buoyant to sink on their own, sank readily when incorporated into sea snow. And all marine snow contaminated with microplastics fell much faster than natural marine snow.
Porter suggested that this potential change in the speed of snowfall could have immense consequences for how the ocean captures and stores carbon. Faster drops can deposit more microplastics into the deep ocean, while slower drops can make plastic-laden particles more available to predators, potentially impoverishing deeper food chains.
“Plastics are a diet pill for these animals,” said Karin Kvale, a scientist who studies the carbon cycle at GNS Science in New Zealand.
a feast of plastics
To understand how microplastics can move through deep-sea food chains, some scientists are looking for clues in creatures.
At the Monterey Submarine Canyon, Anela Choy wanted to understand whether certain species of filter-feeding organisms are ingesting microplastics and transporting them to deeper water food chains. “Marine snow is one of the main things that ties together ocean food chains,” she said.
Choy studied the giant larvacean Bathochordaeus stygius. This larvacean resembles a tiny tadpole and lives inside a sumptuous bubble of mucus that can reach up to a meter in length. “It’s worse than the biggest poop you’ve ever seen in your life,” Choy said.
When their “snot houses” become clogged with so much food, the larvaceans abandon them, and the heavy bubbles sink. Choy found that these mucus palaces are filled with microplastics, which fall to the depths along with all the carbon.
Giant larvaceans are found in every ocean in the world, but Choy noted that his work focuses on the Monterey Submarine Canyon, which is part of a network of marine protected areas and not representative of other, more polluted seas.
“It’s a deep bay on the coast of a country,” Choy said. “Now think on a large scale and visualize the vastness of the ocean, especially the deep waters.”
The individual flakes of marine snow are small, but they are many. A model created by Kvale estimated that in 2010 the world’s oceans produced 340 quadrillion marine snow aggregates, which can transport up to 463,000 tonnes of microplastics per year to the seabed.
Scientists are still investigating exactly how this plastic snow is sinking, but they know for sure, Porter said, that “everything in the ocean ends up sinking.”
Hell’s Vampire Squid (Vampyroteuthis infernalis) live, die, and over time turn into marine snow. But the microplastics that pass through them will remain, eventually settling on the seafloor in a stratigraphic layer that will mark our passage across the planet long after humans are gone.
Translation by Clara Allain