Estimating the amount of plastic in the ocean is very difficult. Currently, estimates focus on the amount of plastic floating on the surface of the ocean. This is because 60% of plastics are more buoyant than seawater and therefore float (Andrady, 2011). Perhaps the best estimate for the amount of floating plastic is provided by a group of scientists led by Marcus Eriksen. In their study, the team of scientists compiled all the current data measuring the concentrations of plastic at the ocean surface. This data is largely from ship observations where net tows collect the plastic at the ocean surface, before being sorted by sieving and then counted. To confirm plastic counts were accurate and to fill in missing data, a computer model of the ocean was used to simulate plastic movement. The team concluded that there are 5.25 trillion pieces of plastic weighing 268,940 tonnes floating in the world’s oceans.
However, a separate study estimating the input of plastic into the ocean suggested that in 2010 alone, 8,000,000 tonnes (8 million) of plastic entered the ocean. Therefore, there is a huge difference between the input of plastic (8 million tonnes) and the measured amount of plastic floating at the ocean surface (268,940 tonnes). This has been referred to as the ‘missing plastic problem’ (Ritchie, 2018, Lebretron et al, 2018). Two explanations for the missing plastic problem include:
1. Not all plastic is found at the ocean surface
The ‘missing plastic’ may be found in areas other the surface of the ocean. For example:
- Ingestion by marine organisms - As plastics breakdown into smaller pieces in the natural environment, they can mistakenly ingested by marine organisms (See: Why is plastic pollution an environmental problem?). Plastic may be stored in marine organisms rather than on the surface of the ocean.
- In the deep sea - There is evidence of plastic in sediment in the deep sea. For example, one study found plastic in seafloor sediments in the Southern Ocean, North Atlantic, Gulf of Guinea and Mediterranean Sea. Therefore demonstrating that plastic can sink to depths as low as 4,800 metres. A review article suggests that plastic sinks due to wide range of processes: E.g. vertical ocean currents, inclusion in faecal pellets of marine organisms and biofouling (a process where bacteria grow on plastic, increase the weight of the plastic and cause it to sink).
- In our shorelines - A study reviewing ‘missing plastic’ suggests that a large proportion of plastic that enters the ocean doesn’t reach the open ocean. Rather, plastic that reaches the ocean is transported by coastal processes (e.g. tides and wave action result in beaching) back to our shorelines where it is stored.
2. Estimates of ‘missing plastic’ may be wrong
Estimates of ‘missing plastic’ may be wrong if estimates of plastic input to the ocean (8 million tonnes of plastic) and observations of floating plastic (268,940 tonnes of plastic) are wrong.
- Estimates of plastic input could be wrong - Our current knowledge of how plastic enters the ocean is incomplete (See: How does plastic get into the ocean?). The exact amount of plastic available to enter the ocean and the exact dynamics of the processes which transport plastic to the ocean are not yet known (Horton et al., 2017).
- Estimates from ship observations (net tows) could be wrong - There are currently not enough ship tows which cover long periods of time and which sample enough of the ocean to be truly representative of the amount of plastic floating at the ocean surface (Lebreton et al, 2018). In addition, the mesh size of nets is typically 2mm. Therefore, if pieces of plastic are larger than 2mm, they will not be collected and produce lower estimates. Improvements in ship tow methodology has been shown to improved estimates: a 2018 study in the North Pacific used different nets in combination with aerial imaging and found observations of plastic to be 16 times higher than previous estimates.
Testing the validity of these explanations is central to providing an accurate estimate of the amount of plastic in the ocean. Therefore, the ‘missing plastic’ problem is currently an active area of scientific research.
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Eriksen, M., Lebreton, L.C., Carson, H.S., Thiel, M., Moore, C.J., Borerro, J.C., Galgani, F., Ryan, P.G. and Reisser, J., (2014). Plastic pollution in the world's oceans: more than 5 trillion plastic pieces weighing over 250,000 tons afloat at sea. PloS one, 9(12), p.e111913.
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