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Plastics in the Ocean: How Dense Are We?

The views expressed are those of the author and are not necessarily those of Scientific American.


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

Postcard from paradise
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
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
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.

August 3, 2012, low tide

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).

Harold Johnson 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.

The views expressed are those of the author and are not necessarily those of Scientific American.






Comments 5 Comments

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  1. 1. podboq 8:15 pm 08/16/2012

    Oh well! I’m not giving up convenience for health! live fast, die young, take as many out with you as you can manage.

    Link to this
  2. 2. kienhua68 12:20 pm 08/17/2012

    Nature is giving us ample warning. Ignorance is the biggest
    enemy on the planet. I read thousands of comments in all
    aspects of life. What I can perceive from most of the science
    related comments, is a very shallow understanding, to complete and total lack of any notion about real issues involved.
    There is now and from history a message that what you don’t know can hurt you and your successors.
    At some point, which we are now beginning to experience, our presence is having a nonrecoverable effect that will
    cause severe suffering of all sorts of life on this planet.
    Sure you can take it lightly or just pretend its not that
    serious. The younger you are now the more relevant all this
    will become.
    I wish, as an early baby boomer, to express an apology for
    my generations disregard for all that we’ve done to bring
    this horrible mess to this point. Just as my generation
    managed to destroy our economy, we have proved ourselves to
    be a very selfish, self-centered and seriously ignorant.

    Link to this
  3. 3. IslandGardener 6:11 am 08/20/2012

    I’d like to see three changes at individual, local, national and international levels:

    We make things in truly sustainable ways, using truly recyclable materials, and following the guidelines set out by Michael Braungart and William McDonough in their book ‘Cradle to Cradle: Re-making the way we make things’.

    We make all packaging compostable and biodegradable.

    We all take responsibility for our stuff. We never throw things ‘away’ (there is no away to throw things to – the Earth is the only home we’ve got). And we pick up any litter we see and put it in recycling bins or litter bins.

    In the meantime, well done to Harold Johnson and all the other people* who remind us what our disgusting behaviour is doing to our beautiful planet. We’re fouling our own nest.

    * for example Rebecca Hosking – see trailer for her ‘Message in the Waves’
    http://www.youtube.com/watch?v=eeBuHqomufk

    Link to this
  4. 4. JTGreen 3:56 pm 08/20/2012

    “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 assume you mean these are “literally” the tip of the iceberg in the same sense that you “literally” did research for this blog, that you are “literally” interested in the science and that you “literally” choose your words carefully.

    Link to this
  5. 5. SacoHarry 4:02 pm 08/20/2012

    I was going to go with “indubitably,” but that gets tedious when repeated back many times in a paragraph.

    Link to this

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