“Marine snow” is the name given to a widespread natural phenomenon of the underwater world. Discovered and first reported by divers, and recorded by submarine cameras, it resembles a light snowfall — an endless stream of mostly white specks, swirling in the underwater currents like snowflakes in a breeze as they gradually sink into the ocean depths.

Only in recent years have scientists discovered that this “snow” includes a growing proportion of plastic bits and fragments, a little-understood aspect of plastic in the ocean.

Pieces of plastic between half a millimeter and five millimeters in size, though just barely visible to the eye, are known in the marine research world as microplastic, a term sometimes abbreviated to “MP.” According to Science Daily, one study estimates that there are 24.4 trillion individual pieces of microplastic in the world’s oceans, weighing up to half a million tons. The study also cautioned that the total is almost certainly much greater.

A Nutritious Snowfall

Watching flurries of white like snowfall may be charming for divers, but the natural components of marine snow are not a bit appealing to us humans. According to the Smithsonian Institution, it is largely made up of “waste, such as dead and decomposing animals, poop, silt and other organic items.”

Unappealing to us, perhaps, but marine snow is an important source of nutrition for marine life that lives in the intermediate and lower depths of the ocean, below the surface waters, as well as the inhabitants of the sea floor. As reported by NOAA‘s Ocean Service, studies of marine snow have found it rich in carbon and nitrogen, fundamental nutrients for all life, including the scavengers of the ocean depths.

This material drifts slowly downward and can take weeks to eventually reach the ocean floor. Much of the seabed under the ocean depths is covered by the ooze deposited by endless centuries of marine snowfall, amounting to as much as six meters over a million years.

Trillions of Bits and Pieces of Junk

When we think of the harmful effects of plastic in the ocean, reports Woods Hole Oceanographic Institution, we typically picture larger chunks of plastic, such as the ringed plastic containers for drinking cans. These notoriously can entangle marine mammals, seabirds and other large ocean dwellers, often with fatal consequences.

But surveys of the ocean surface have revealed that only a small fraction of the plastic that gets washed into the sea remains as large floating pieces. The rest is broken down into smaller pieces by sun exposure and other forces acting on it, eventually breaking up into trillions of individual microplastic fragments.

Other sources of plastic are already micro-scaled by the time the material reaches the sea. These small pieces, per Woods Hole, include “microbeads” incorporated into toothpaste and body-cleansing products, as well as “nurdles” produced by snap-off plastic tabs.

According to Pew Trusts, more than three quarters of microplastic pollution in the ocean may be from a source we don’t usually think of as plastic — namely, wear and tear from tires: bits of synthetic rubber (a type of plastic) that get washed down highway storm drains, eventually finding their way to the sea.

Plastic Snow Keeps On Falling

Plastic in the ocean comes from a wide variety of sources, and it also undergoes a complex cycle of interaction with marine life once it breaks down to the scale of microplastics fragments. For example, the bits of plastic may be ingested by fish or other sea life and subsequently excreted as fecal pellets, or they may provide anchoring points for colonies of marine microorganisms.

In turn, these processes can set the stage for a complex cycle. The scientific journal Nature reports on “the intriguing possibility that a widespread entrainment/release cycle dominates the transport of biologically active plastic particles.”

As a piece of microplastic becomes intermixed with living cells such as bacteria or plankton, a process called biofouling takes place. The result is a “plastic-organic aggregate” that has negative buoyancy — that is, it gradually sinks toward the bottom. As the microplastic sinks into deeper water, the pressure around it increases and can force chemical changes that “remineralize” the plastic, clearing away the biofouling.

The remineralized plastic, with its initial positive buoyancy restored, then drifts back upward in the ocean. Once it reaches the sunlit and life-rich upper waters (the euphotic zone) biofouling can again take place, and the whole cycle may repeat itself.

Searching for Clues on the Ocean Floor

All of these subtle processes, taking place largely in the dark depths of the oceans, are exceptionally difficult to observe. What comes through in research reports on marine snow and plastic in the ocean is how much we are still in the initial reconnaissance stage of our exploration.

The same Science Daily study that estimated there are 24.4 trillion pieces of microplastics in the world’s oceans also concluded that their combined weight was likely between 82,000 and 578,000 tons (“but probably much greater”). Another study, reported at FrontiersIn.org, found that what is rather oddly described as the “seafloor export potential” (that is, absorbing material into the seafloor ooze) could account for between 7,300 and 420,000 metric tons per year of plastic junk, or about 0.06–8.8 percent of the estimated total annual pollution of plastic in the ocean.

Those wide estimated ranges are a blinking yellow light reminding us that researchers are still finding their way around down there, and there is a lot we are only starting to learn.

The same caution applies to uncertainty about the possible effects of microplastics in the oceans, particularly on the health of ocean ecosystems, including fisheries, and on our own health. National Geographic notes that microplastics have been detected across the whole range of sea life, from plankton to whales. They have been found in seafood and also in drinking water, where standard drinking water treatment does not fully eliminate microplastics.

As Pew Trusts notes, microplastics don’t just have the potential to include harmful plastic components; they can also absorb and concentrate other substances from the environment, such as polychlorinated biphenyls (PCBs), which have been linked to humans. Pew also reports that microplastics produced by tire wear have been responsible for die-offs of salmon in the Pacific Northwest fishery, already regarded as fragile.

Woods Hole further observes that filter-feeding sea creatures, such as oysters and scallops, are exposed to microplastics, and as popular seafood items, they pass them along to humans. The full marine contribution to the human food chain is known only in the broadest outline — one more mystery of the deep that we are only beginning to tease out. Clearly a great deal more research is needed for us to fully understand the role of microplastics in the overall impact of plastic in the ocean.

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