We humans are a pretty wasteful bunch, but our pathological inability to clean up after ourselves doesn’t just affect the land on which we live: tons of our waste plastic ends up in the sea too.

The impact of plastic isn’t just about six-pack rings choking adorable, photogenic animals – the floating plastic rubbish has actually formed huge floating islands of trash in the East and West Pacific.

But if we keep chucking more and more stuff into the ocean the plastic will surely continue to build and build, right?

Well maybe not: a multinational team of scientists who conducted a survey on floating plastic across the globe were surprised to find there was much less than they expected.

This poses a tricky question: where has all that plastic (tens of thousands of tons) gone?

So, what’s the point?

The fact that scientists found less plastic floating about than they expected is a good thing, isn’t it?

Well, as with a lot of things in science the answer is “we don’t really know” – at the moment it is unclear what is happening to it, and that could be either good or bad.

The scientists came up with four possible explanations:

1. Plastic is washed up on the beach (shore deposition).

2. Plastic is breaking up into tiny pieces too small to measure (nanofragmentation).

3. Plastic is covered with organic matter and is sinking to the ocean bed (biofouling).

4. Plastic is being eaten (but not broken down) by animals (ingestion).

All of these reasons are not necessarily good – essentially the plastic is being shifted from the ocean surface to somewhere else, and this could potentially cause harm to organisms and/or disrupt the balance of the ecosystem.

This means, for conservation efforts, figuring out where the plastic is going is a ‘matter of urgency’.

What did they do?

The research team surveyed 141 different sites around the globe, dragging a fine net through the sea and then analysing the amount of overall plastic and the sizes of the individual pieces.

From their study, they estimated that in total there is currently 7,000 – 35,000 tons of plastic floating around the surface of the sea. Although that is a heck of a lot of fizzy drink bottles, it was far less than they expected: They quote that in the 1970s, around 45,000 tons was being dumped in the sea per year.

Of course, humanity’s appetite for food wrapping and tacky souvenirs has increased since then (the authors claim we make about 5 times more plastic now than in the 1970s).

The scientists figured that the plastic must be going somewhere, so they tried to calculate how much of an effect each of their 4 proposed mechanisms was likely to have, taking into account the relative sizes of the plastic they skimmed from the sea and where they found most of it.

Did they find anything?

They found that most of the plastic had accumulated in ocean gyres. Essentially these are large, rotating currents that draw all the plastic from a huge area into their centres like a slow-moving whirlpool.

The North Pacific gyre has concentrated trash mostly thought to be from the Western US and East Asia into two gigantic ‘garbage patches’ in the open ocean.

They also found that tiny fragments of plastic (‘millimeter scale’) were far less abundant than expected when compared to other sizes, meaning that a lot of plastic of this size seemed to be missing.

They therefore figured that any process that results in the ‘degradation or permanent sequestration’ (trapping) of plastic must:

1. Be able to happen in the open ocean

2. Be ‘size selective’ i.e. must result in millimeter-sized fragments disappearing quicker than others.

Criterium 1 rules out shore deposition and biofouling (since biofouled plastic is only sequestered if it is trapped in the sediment on the sea bed – this is much more likely to happen in shallow water rather than in the open ocean).

Degradation from sunlight was ruled out as they claim the ‘rate of solar-induced degradation (is unlikely to have) increased since the 1980s.’

But bacteria could be eating the plastic, as a previous study has shown to be possible, and obviously smaller fragments are likely to be degraded faster than larger ones due as they have a larger surface area relative to their volume (so more bacteria can populate the fragment surface for every gram of plastic).

They also reckon that the size of the tiny plastic fragments is similar in scale to zooplankton, and so could be being eaten by the fish that feed on them.

Previous studies (1,2) have shown that many fish do indeed have plastic in their bodies, and so is effective stored in their bodies and the bodies of animals that then eat them.

Does it mean anything?

Finding that most of the plastic was collected into ocean gyres and that it was mostly small fragments that were absent helped the scientists to narrow down the possible fate of the plastic to two likely options:

1. Digested by bacteria

2. Eaten by animals

Obviously option 2 is not a good scenario, given that the plastic is unlikely to be properly degraded and could obviously harm the animals eating it.

Even option 1 isn’t great, because if the plastic essentially provides an extra food source for a particular group of bacteria then their population could increase, upsetting the balance of the ecosystem.

What the authors stress is that more study is needed, not just to better understand the two processes they highlighted, but also the sinking of larger plastic pieces, which they did not account for in their fragmentation model.

Only by better understanding what is happening to all this plastic can we be know best how to mitigate the potential damage it could cause to the environment. Either that, or we may just have to learn, as a species, how to clean up after ourselves.

Original article in PNAS Jun 2014

All images are open-source/Creative Commons licence.

Credit: Vberger (First); TSIC [adapted from am1969 and Gurdonark] (Second); NOAA (Third); epSos.de (Fourth).

Text © thisscienceiscrazy. If you want to use any of the writing or images featured in this article, please credit and link back to the original source as described HERE.

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