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What are the processes that transport plastic to the ocean? What are the main sources of plastic to the ocean?
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Plastic pollution in the ocean has either come from the land due to littering or poor waste disposal or directly from the ocean itself because of fishing and shipping. One study reviewing ocean plastic suggests that 80% of ocean plastic is from the land and the remaining 20% is from the ocean. Below are some examples of pathways from land and ocean-based sources that plastic takes to get into the ocean.  

Pathways from the Land

Any plastic that has the potential to enter the ocean from land is called mismanaged plastic waste - this is plastic which has either been littered or inadequately disposed of. The organisation  Our World in Data define the difference between littering and inadequate disposal as: 

  • Littered plastic – Plastic that has been dumped or disposed of without consent or in an inappropriate location.
  • Inadequately disposed plastic - Plastic which has the intention of being managed through waste collection or storage sites, but it is ultimately not formally managed. This includes plastic disposal in dumps or open, uncontrolled landfills; this means material can leak out or be transported into the natural environment. 

Plastic pathways from land to ocean are a new topic of scientific study. As a result, the pathways and transport processes are currently poorly understood. This is largely because the exact location of mismanaged plastic waste on land is poorly known and the effect of variables such as topography, land use, climate, vegetation and plastic shape and size on transportation are not yet known (Horton et al., 2017). Nevertheless, current understanding suggests the following pathways from land to ocean are important: 

From rivers  

Rivers are currently thought to be the main pathway by which plastic enters the oceans. One study estimates that between 1.15 and 2.41 million tonnes of plastic enters the ocean every year from rivers. Of this total, 67% of plastic entering the ocean can be attributed to just 20 rivers, which are mainly found in Asia. Different rivers across the world input different amount of plastic into the ocean based on population density, levels of urbanisation and industrial activity, the amount of rainfall and river discharge and the presence of dams which can act to hold back plastic.  

From beaches and Coast 

Coastal processes such as rainfall runoff, winds, tides and gravity help to transport mismanaged plastic waste downhill towards the ocean (Jambeck et al., 2015). All these processes (other than tides) occur further inland, but because they are further away from the ocean, they are less likely to transport plastic to the ocean. Therefore, any mismanaged plastic waste near to the coast is a risk because it is much more likely to enter the ocean. One study calculated coastal input of plastic to the ocean to be in the range of 4.8 to 12.7 million tonnes of plastic enter the ocean from the coast every year.  

From stormwater runoff  

Extreme weather events such as storms, storm surges and tsunamis can over short time periods, transport significant amounts of mismanaged plastic waste into the ocean. For example, the Japan Tsunami of 2011 was estimated to transport 5 million tonnes of waste into the ocean. 

From atmospheric fallout 

Plastic particles which are smaller than 5mm may be capable of being transported long distances within the atmosphere due to their low density. When the energy (primarily from wind) for transportation is lost, plastic particles ‘fallout’ to the Earth’s surface.  A study by Dris (2016) measured atmospheric fallout in France by collecting plastic samples in a funnel over a 3-month period at multiple sites. They measured an atmospheric fallout rate of between 2 and 355 particles per metre per day.  

Pathways from the Ocean 

The pathways from ocean-based sources are somewhat more obvious. The term mismanaged plastic waste is not used in reference to ocean-based sources because littering (both deliberate and indeliberate) is the only way in which plastic waste is generated. Ocean littering can come from all types of boats, ships and offshore platforms. In a large review of sources of plastic to the ocean by an organisation known as the Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESMAP)  summarised the ocean-based sources as follows:  

Shipping  

Large shipping vessels are constantly travelling across the world’s oceans. It is estimated that 90% of world trade is carried about the international shipping industry. These large shipping vessels have large crews and can travel for months. As a result, they generate significant amounts of waste daily such as wire straps, packaging materials, plastic sheets and boxes and sewage. This plastic waste can be accidentally enter the ocean when weather conditions are poor or inadequate waste storage can result in deliberate plastic disposal into the ocean. In addition, the regular cleaning of ships requires the use of plastic abrasives which directly released into the ocean.

Fishing  

Plastic fishing gear such as nets, traps, lines, ropes, buoys, bait boxes and bags are frequently found in the ocean due to accidental or deliberate disposal. The ways in which fishing gear is handled can depend on the fishing region, the type of fishery and the type and size of the fishing vessel.  A report by Greenpeace, suggests that plastic fishing gear makes up 10% of all the plastic waste in our oceans. This lost and abandoned fishing gear is often called ghost gear because it can continue to trap and entangle animals in the marine environment.  

References 

Dris, R., Gasperi, J., Saad, M., Mirande, C. and Tassin, B., (2016). Synthetic fibers in atmospheric fallout: a source of microplastics in the environment?. Marine pollution bulletin104(1-2), pp.290-293.

GESAMP (2016). “Sources, fate and effects of microplastics in the marine environment: part two of a global assessment” (Kershaw, P.J., and Rochman, C.M., eds). (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/ UNEP/UNDP Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). Rep. Stud. GESAMP No. 93, 220 p.

Greenpeace (2019). Ghost Gear: The Abandoned Fishing Nets Haunting Our Oceans. Available at: https://storage.googleapis.com/planet4-international-stateless/2019/11/8f290a4f-ghostgearfishingreport2019_greenpeace.pdf. (Accessed: 26 June 2020). 

Horton, A.A., Walton, A., Spurgeon, D.J., Lahive, E. and Svendsen, C., (2017). Microplastics in freshwater and terrestrial environments: evaluating the current understanding to identify the knowledge gaps and future research priorities. Science of the Total Environment586, pp.127-141.

Jambeck, J.R., Geyer, R., Wilcox, C., Siegler, T.R., Perryman, M., Andrady, A., Narayan, R. and Law, K.L., (2015). Plastic waste inputs from land into the ocean. Science347(6223), pp.768-771.

Lebreton, L.C., Van Der Zwet, J., Damsteeg, J.W., Slat, B., Andrady, A. and Reisser, J., (2017). River plastic emissions to the world’s oceans. Nature communications8, p.15611.

Li, W.C., Tse, H.F. and FOK, L., (2016). Plastic waste in the marine environment: A review of sources, occurrence and effects. Science of the Total Environment566, pp.333-349. 

Murray, C.C., Maximenko, N. and Lippiatt, S., (2018). The influx of marine debris from the Great Japan Tsunami of 2011 to North American shorelines. Marine pollution bulletin132, pp.26-32.

Ritchie, H. (2018). Plastic Pollution. Available at: <https://ourworldindata.org/plastic-pollution> (Accessed: 26 June 2020).

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