Plastics in the Ocean: How Dense Are We?

Posted in the blog Scientific American.com by Harold Johnson | August 16, 2012

This is Curtis Cove, in Biddeford, Maine, a newly conserved & protected habitat, part of the Rachel Carson National Wildlife Refuge.

August 3, 2012, low tide

Curtis Cove lies at the end of a long, winding, dead-end road past private estates and private beaches. At the cove, there are no tourists with umbrellas, picnic baskets, and boogie boards. Yet I’ve collected all of this from a mere 150 feet of its shoreline since late winter:

5060 individual pieces of garbage; another 1717 pcs of fishing rope not shown

With very rare exception, all of this material washed in from the waves. Lobster trap vinyl scraps, bait bags, claw bands, bottle caps, coffee-cup tops, cable ties, plant pot fragments, dollhouse parts, inner tube chunks, a saw handle, coat hangers, a crate lid, an air filter, a car arm rest. On and on, anything you can think of.

Horrifying, that the Gulf of Maine is this fouled. But perhaps expected in a plastic, throwaway world.
What I didn’t expect was to find that if I were trawling the waters for surface debris, I would have missed about 95% of this.

Why? Because most of this material actually sinks in the ocean. Almost everything that’s washed up at Curtis Cove came from the bottom of the Gulf of Maine, not the top.

The range of consumer plastics straddles the density line of seawater. Seawater’s specific gravity (a common measure of density) hovers around 1.027. Generally speaking, anything lower than that will float, anything higher sinks. Polyethylenes and polypropylenes have specific gravity of about 0.900 to 0.970. They float. The other major plastics — styrenes, nylons, polyesters, polyurethanes, vinyls — range from about 1.050 to about 1.440. They sink.

Once wave agitation knocks off the air bubbles clinging to or inside them, Coke bottles sink. Water bottles sink. “SOLO” drink cups sink. Nylon fishing rope sinks. Bait bags sink. Vinyl upholstery scraps sink. PVC pipe sinks. Plastic toys — mostly styrene — sink.

In the relatively shallow continental shelves, auch plastics can bounce and roll along the seafloor for dozens or hundreds of miles, washing up in places like Curtis Cove. In the deep ocean, it’s a different story. In 1997 the container ship Tokio Express dumped 4,756,940 Legos into the sea off of Cornwall, England.

Many washed up in Europe, none confirmed beyond Europe. The reason? After the air bubbles are out of them, they sink. And once they tumble and fall off the continental shelf, game over. They’re 2 miles down, or more. They’re not likely coming back.

In fact, once it falls into the abyss, very little of this sinkable plastic is likely coming back. A 2007 study scoured beaches on Pacific islands, which of course lack a continental shelf.¹ The only plastics found washed up on their shores were polyethylenes and polypropylenes.

Moreover, another recent study shows that even lighter plastics tend to become more dense after time at sea, probably as a result of biofouling.² Perhaps some (most?) grow dense enough to sink too. (An interesting question is whether, once they start sinking, they get “cleaned” of their biofilm by scavengers and then float again. If so, they may be able to spread toxins up and down the water column over years or centuries.)

In a similar vein, research vessels don’t find many light polyethylene grocery bags floating in the deep ocean. Why? They’re bags — they collect sediment, get denser, and sink. Here is what the Rozalia Project is finding on the New England seafloor (Source).

Rozalia plastic bag

Down far from manta trawlers and the prying eyes of most ROVs, this polymer cascade leaches its plasticizers, maybe gets consumed, certainly changes abyssal benthic communities in ways that nobody has yet even seen, much less understood.

The problem of plastic pollution is becoming known. Which is good. By now most people are familiar with the scenes of Hawaii’s Kamilo Beach.

Postcard from paradise

The trouble is, Curtis Cove shows that when it comes to seeing what our plastic culture is doing to the ocean, places like Kamilo — and sobering reports from oceanic garbage patches — are literally the tip of the iceberg.

I must take a moment to give a nod of appreciation to Miriam Goldstein, whose ability to point me toward excellent & relevant plastic pollution research is unrivaled!

1 Rios, Lorena M., Moore, Ch. Jones, P.R. Persistent organic pollutants carried by synthetic polymers in the ocean environment. Marine Pollution Bulletin (2007) 54,1230-1237.
2 Morét-Ferguson, S., Lavender Law, K., Proskurowski, G., Murphy, E.K., Peacock, E.E., and Reddy, C.M. The size, mass, and composition of plastic debris in the western North Atlantic Ocean. Marine Pollution Bulletin (2010).

About the Author: Harold Johnson lives in Saco, Maine with his wife and young daughter. A freelance copyeditor and writer by trade, he spends his free time studying archaeology, earth sciences, and the ways the natural and manmade world have mingled across millennia. Since May 2010, he has written on marine debris and plastic pollution as The Flotsam Diaries. Follow on Twitter @FlotsamDiaries.