On Monday June 30 the prestigious journal Proceedings of the National Academy of Sciences (PNAS) published a scientific paper, led by Andrés Cozar from the University of Cádiz (Spain) and co-authored by one of us, C.D., which resulted from the global Malaspina Circumnavigation Expedition that C.D. leads. This paper provides a first estimate of the amount of plastic litter floating in the open ocean.
General circulation models predict that plastic particles should accumulate in the central areas of the oceans, known as the subtropical gyres of the ocean, of which there are five across the ocean, but only two, those in the northern hemisphere (N. Pacific and the N. Atlantic), had been sampled for plastic litter before.
We found plastic litter to be scattered all over the open ocean, as 88 per cent of the samples collected contained plastic debris, including significant loads around Australia. In particular, we surveyed all five gyres to confirm, in the paper cited above, that indeed all of them accumulate similar concentrations of plastic litter, with the load being somewhat larger in the S. Atlantic subtropical gyre. Whereas the mass media have popularised the plastic accumulation in the N. Pacific as the “great garbage patch” or the “oceanic plastic island”, this is a gross overstatement. The subtropical gyres support plastic litter loads of about 200 to 600 grams Km-2, certainly nothing that could be thought of as an island or even a dense patch. Globally, this represents an estimated load bracketed at between 6 and 35 thousand tons of plastic debris across the entire open ocean.
This result was striking as the expected load - based on the calculation that 0.1% of the global production of plastic reaches the ocean, half of which is buoyant and 2/3 of which reaches the open ocean – was in the order of 1 million tons or 100 times larger than the global load of plastic debris we found in the open ocean.
This may seem good news, but in fact, it means that, if our calculations are correct, we cannot account for the fate of 99% of the plastic litter entering the open ocean, a disturbing realization.
This means that, contrary to the assumption that the floating plastic debris remains in the open ocean for decades, there must be mechanisms in operation efficiently removing this debris. Moreover, these mechanisms must operate at a rate comparable to that of the current input of plastic into the oceans, as available time series show no evidence that the plastic debris has increased over the past decade.
Several mechanisms have been proposed, and discussed in our paper, as to what these removal mechanisms may be. First, current estimates of how much plastic enter the ocean could be wrong, as campaigns to avoid plastic use, recycle plastic, and improved water treatment systems may have reduced this load. While these actions have certainly helped, they do not seem sufficient to curve the trajectory of plastic load into the ocean provided global plastic production has continued to grow exponentially.
Multiple loss mechanisms are possible.:
- Microbial activity may be more effective than previously thought in breaking down the plastic, and a search for microbes able to chew down plastic litter is on going across the world.
- Plastic particles could simply fragment through the effect of solar radiation to result in plastic particles too small (< 0.2 mm) to be collected by the nets used to sample them, so that the missing particle could be present as nanoparticles.
Marine organisms grow on the plastic particles and, where these organisms may produce carbonate or silicate shells, ballast the particles so they would sink. However, these minerals dissolve below 2,000 m depth in the ocean so the particles would return to the surface.
Lastly, the particles may be ingested by marine organisms and enter the marine food web.
A piece of evidence suggests that the later is a plausible mechanism: analysis of the size distribution of the particles show that the abundance of particles increase as their size decreases according to the expectation from the fragmentation process, but only down to 4 mm in size. The abundance of particles is much less than expected in the range of 1 to 4 mm, where the losses seem to concentrate. This suggests that the process responsible for the removal of the plastics must be size-dependent, selectively removing particles within the 1 to 4 mm range.
The five great subtropical gyres of the ocean, the plastic patches of the ocean, are inhabited by mesopelagic fish, including lantern fish, light fish, and dragon fish, which – also as a result of the Malaspina Expedition – have been found to be the most abundance fish in the planet, contributing 10 times more biomass than the rest of all marine fish together.
The best solution would be, no doubt, cutting down the amount of plastic entering the ocean, which would require cutting down the consumption and production of plastic.
Mesopelagic fishes inhabit in precisely those areas of accumulation of plastic and feed with particles ranging from 1-4 mm, exactly where the loss of particles has been detected. Moreover, they migrate from the depths of 400 to 700 m where they stay during the daytime to the surface to feed at night, where they could encounter and ingest the plastic particles and transport them at depth, as shown in a couple of papers showing they contain plastic particles in the 1 to 4 mm range in their gus. These fishes are, in turn, predated by tuna, swordfish, squid and possibly some whales, opening up the possibility that the small plastic particles maybe transferred up the food web of the open ocean.
These are all plausible explanations, likely all operating to various extents that need be addressed to answer the burning question of where is the 99% of the “hidden” plastic?
Researchers at the Malaspina Expedition are busy trying to resolve this mystery and sampling areas not yet surveyed, such as the Arctic and the Mediterranean. We will report on our findings soon.
However, understanding where is the plastic will do little to solve the multiple impacts marine plastic pollution has on marine life, as the plastic waste entering the oceans is not easily degraded, is associated with organic pollutants that enter the food chain and causes major problems to all types of marine organisms, from copepods to whales.
The best solution would be, no doubt, cutting down the amount of plastic entering the ocean, which would require cutting down the consumption and production of plastic. Plastic is, indeed, a convenient and inexpensive material, but, after all, is the overconsumption of this material really necessary?
Take for example our supermarkets. Almost all products and veggies in our Aussie supermarkets are packaged or wrapped in plastic. So no wonder that today each human being in the planet consumes an average of 38 kg of plastic per year, a figure likely to be much higher in the case of Australian consumers.
Moreover, the scientific article Components of plastic: experimental studies in animals and relevance for human health, published in the Philosophical Transactions of Royal Society, reports that some components used in plastics, such as BPA, TBBPA or PBDE, which disrupt the endocrine system, have been identified in humans. Experimental investigations in animals indicate a wide range of effects associated with exposure to these compounds, causing concern about the potential risks they might have for human health. Do we really want to wrap our food in materials that may be potentially toxic?
Clearly, the solution to plastic pollution relies on curving our demand and consumption for plastic, de-plastifying our lives, to rely less and less on this material, propelling the search for innovative alternative materials by industry. This would do far more to solve the problem that discovering where the 99% missing plastic has gone, important as it is.
You too can be part of the solution: take action to avoid an ocean of plastic; de-plastify your life.
Carlos Duarte is director of the Oceans Institute at University of Western Australia. This piece was co-authored with Guiomar Duarte-Agustí, Digital Marketing Consultant. This article was originally published on The Conversation.
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