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Sinking of marine snow and coprolites in the Bering Sea. (Andrew McDonnell/University of Alaska, Fairbanks via The New York Times)
(Science Times)
For as long as there has been marine life, there has also been marine snow, a relentless drizzle of death and debris falling from the surface to the depths of the sea.
Snow begins as specks, which clump together into dense, flocculant flakes that slowly sink and pass through the jaws (and mouthparts) of scavengers below. However, even marine snow that is eaten will most likely return to snowfall; the guts of a squid are just a rest stop on this long journey to the deep.
Although the term suggests the white color of winter landscapes, marine snow is usually rather brown or grayish and is made up mostly of dead things. For eons, these debris have been made up of the same components—flecks of plant and animal debris, feces, mucus, dust, microbes, viruses—and have carried carbon from the ocean for storage on the seafloor. Increasingly, however, microplastics are infiltrating marine snowfall: fibers and fragments of polyamides, polyethylene, and polyethylene terephthalate. Furthermore, this fake snow appears to be disrupting our planet’s ancient cooling process.
Every year, tens of millions of tons of plastic enter our planet’s oceans. Scientists originally assumed the material would float in garbage patches and ocean gyres, but surface surveys have only reported regarding 1 percent of the plastic estimated to be in the ocean. Recent modeling has found that 99.8 percent of the plastic that has entered the ocean since 1950 is buried deep below the top few hundred feet of sea level. Scientists have found 10,000 times more microplastics on the seafloor than in polluted surface water.
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Experimental “mesocosms” created by researcher Luisa Galgani and her team on the Greek island of Crete to mimic and observe marine snow. (Luisa Galgani, Chiara Esposito, Paraskevi Pitta via The New York Times)
Marine snow, one of the main connecting pathways between the surface and the deep, appears to be helping plastics sink. Scientists are just beginning to unravel how these materials interfere with deep-sea food webs and natural carbon cycles in the ocean.
“It’s not just that marine snow carries plastics or clumps together with them,” said Luisa Galgani, a researcher at Florida Atlantic University. “The thing is, they help each other to get to the deep ocean.”
The formation of marine snow
The sunlit surface of the sea turns green with phytoplankton, zooplankton, algae, bacteria and other tiny life forms, which feed on the sun’s rays or on each other. As these microbes metabolize, some produce polysaccharides that can form a sticky gel that attracts the dead bodies of tiny organisms, small fragments of larger carcasses, foraminiferal and pteropod shells, sand, and microplastics, all of which adhere and forms larger flakes. “They are the glue that holds all the components of marine snow together,” explains Galgani.
Marine snowflakes fall at different speeds. The smallest descend more slowly, “barely a meter a day,” explains Anela Choy, a biological oceanographer at the Scripps Institution of Oceanography at the University of California at San Diego.
Larger particles, such as dense fecal debris, can sink more quickly. “They plummet to the bottom of the ocean,” says Tracy Mincer, a researcher at Florida Atlantic University.
Plastic in the ocean is constantly degrading; even something as large and buoyant as a milk jug will eventually fall apart and break into splinters of microplastics, which form biofilms of distinct microbial communities – the “plastisphere”, explains Linda Amaral-Zettler, a scientist at the Royal Netherlands Marine Research Institute. who coined the term.
“We think of plastic as inert,” Amaral-Zettler said. “But once it enters the environment, it doesn’t take long for microbes to colonize it.”
Microplastics are capable of harboring so many transient microbes that they counteract the natural buoyancy of the plastic, which is why “the raft” sinks. But if the biofilms degrade on the way down, the plastic can float back up, leading to a relentless rise and fall of microplastics in the water column. Marine snow is anything but stable; as the flakes fall into the abyss, they constantly freeze and fall apart, torn apart by waves or predators.
“It’s not as simple as, ‘Everything is falling all the time,'” says Adam Porter, a marine ecologist at the University of Exeter, England. “It’s a black box in the middle of the ocean, because we can’t stay down there long enough to figure out what’s going on.”
To explore how marine snow and plastics are distributed in the water column, Mincer has begun sampling deeper waters with a dishwasher-sized pump full of filters hanging from a cable attached to a research ship. Filters are arranged from large to small mesh to filter out fish and plankton. With these pumps running for 10 hours straight, nylon fibers and other microplastics have been detected distributed throughout the water column under the subtropical gyre of the South Atlantic.
But even with a research ship and its expensive and unwieldy equipment, it’s not easy to retrieve a whole chunk of marine snow from the deep waters of the real ocean. Pumps often churn up snow and disperse fecal debris. And flakes alone offer little insight into how quickly some snows sink, which is critical to understanding how long plastics stay more or less in the same place or up and down before sinking into the water column. to settle on the seabed.
“Is it decades?” Mincer asked. “Is it hundreds of years? So we can understand what we are dealing with and what kind of problem it really is.”
Instant sea snow
To answer these questions, but without going over budget, some scientists have made and manipulated their own marine snow in the laboratory.
At Exeter, Porter collected seawater from a nearby estuary and poured it into bottles that rolled continuously. He then sprinkled microplastics on them, such as polyethylene beads and polypropylene fibers. Constant agitation and a gush of gooey hyaluronic acid caused the particles to collide and stick together to form snow.
“Obviously, we don’t have 300 meters of pipe for them to sink,” Porter said. “By rolling it, what you do is create an endless column of water in which the particles can move.”
After the bottles had been rolling for three days, he pipetted the snow and analyzed the number of microplastics in each flake. His team found that all types of microplastics they tested clumped together with sea snow, and even microplastics such as polypropylene and polyethylene, which are normally too buoyant to sink on their own, sank safely once incorporated into sea snow. Furthermore, all marine snow contaminated with microplastics sank much faster than natural marine snow.
Porter suggested that this potential change in the rate at which snow falls might have important repercussions for how the ocean captures and stores carbon: faster snowfalls may store more microplastics in the deep ocean, while faster snowfalls Slower speeds might make charged plastic particles more accessible to predators, which might theoretically starve deeper food webs. “Plastics are like a diet pill for these animals,” says Karin Kvale, carbon cycle scientist at GNS Science in New Zealand.
a plastic banquet
To understand how microplastics travel through deep-ocean food webs, some scientists have turned to sea creatures for clues.
In the Monterey Bay canyon, Choy wanted to know if certain species of filter feeders were ingesting microplastics and transporting them to deeper-sea food webs. “Marine snow is one of the main elements that connect food webs throughout the ocean,” he noted.
Choy focused on the giant Bathochordaeus stygius larva. This larvacean resembles a tiny tadpole and lives inside a palatial bubble of mucus that can reach up to a meter in length. “It’s worse than the grossest booger you’ve ever seen,” Choy said. When their mucus houses become clogged as a result of feeding, the larvae burst out and the heavy bubbles sink. Choy discovered that these snot palaces are packed with microplastics, which in turn are funneled down to the depths along with all their carbon.
Giant larvaceans are found in all the world’s oceans, but Choy stressed that his work focused on the Monterey Bay canyon, which belongs to a network of marine protected areas and is not representative of other, more polluted seas. “It’s a deep bay on one country’s coastline,” Choy said. “Go up the scale and think regarding how vast the ocean is, especially the deep waters.”
Marine snowflakes are small, but other substances are added to them. A model created by Kvale estimated that in 2010 the world’s oceans produced 340 trillion marine snow aggregates, which might transport up to 463,000 tons of microplastics to the seabed each year.
Scientists are still exploring how this plastic snow reaches the seafloor, but what they do know for sure, according to Porter, is that “everything eventually sinks into the ocean.” Vampire squids will live and die and eventually turn to sea snow. But the microplastics they come across will remain and end up being deposited on the seafloor in a stratigraphic layer that will mark our passage through the planet long following humans have disappeared.
