There are a variety of ways plastic gets into the ocean from the land (See: How does plastic get into the ocean from land?). However, the destination of plastic once it has entered the ocean from land is dependent on its buoyancy.
The majority of plastics (60%) are buoyant in seawater so remain on the ocean surface (Andrady, 2011). However, buoyant plastics are not evenly distributed across the surface of the ocean. They tend to be collected in ocean gyres that are referred to as plastic ‘garbage patches’ (NOAA, 2020). Ocean gyres are largely driven by the direction of trade winds and the Earth’s gravitational field (Pickard & Emery, 2007). There are five major gyres found in the subtropical regions containing varying amounts of plastic (estimates from Eriksen et al., 2014):
- North Pacific – 96 thousand metric tonnes (Kt)
- South Pacific - 21 Kt
- North Atlantic – 57 Kt
- South Atlantic – 13 Kt
- Indian Ocean – 59 Kt
Differences in the amount of plastic found in gyres can be explained by varying inputs of plastic from the land. For example, the greatest accumulation of plastic is within the North Pacific, containing 96 Kt of plastic mass. This is a consequence of connection to large plastic waste sources in Asia (Lebretron et al., 2018). It is estimated that China, Indonesia and Sri Lanka respectively account for 28%, 10%, and 5% of all plastic waste available to enter the ocean. From input into the ocean from land, plastic can reach the centre of gyres within a year (van Sebille et al., 2012).
The remaining proportion of plastics (40%) is non-buoyant and will sink in seawater (Andrady, 2011). Plastics are thought to sink via a wide range of processes including:
- vertical ocean currents
- inclusion in fecal pellets of marine organisms
- biofouling (a process where bacteria grow on plastic, increase the weight of plastic and cause it to sink)
This explains the discovery of plastic in seafloor sediments in Southern Ocean, North Atlantic, Gulf of Guinea and Mediterranean Sea (van Cauwenberghe et al., 2013). However, the large variation in plastic size, shape and chemical composition means it is very difficult to determine sinking processes (Kooi & Kolemans, 2019). Therefore, plastic sinking rates remain largely unknown and this contributes to the ‘missing plastic problem’ explained in: How much plastic is there in the ocean?
Andrady, A.L. (2011). Microplastics in the marine environment. Marine pollution bulletin, 62(8), pp.1596-1605.
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.
Kooi, M. and Koelmans, A.A. (2019). Simplifying microplastic via continuous probability distributions for size, shape, and density. Environmental Science & Technology Letters, 6(9), pp.551-557.
Lebreton, L., Slat, B., Ferrari, F., Sainte-Rose, B., Aitken, J., Marthouse, R., Hajbane, S., Cunsolo, S., Schwarz, A., Levivier, A. and Noble, K. (2018). Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic. Scientific reports, 8(1), pp.1-15.
NOAA. (2020) What is a Gyre? Available at: https://oceanservice.noaa.gov/facts/gyre.html (Accessed: 17 December 2020).
Pickard, G. and Emery, W. (2007). Descriptive Physical Oceanography: An Introduction. 5th ed. Oxford: Butterworth-Heinemann, pp.154-274.
Van Cauwenberghe, L., Vanreusel, A., Mees, J. and Janssen, C.R. (2013). Microplastic pollution in deep-sea sediments. Environmental pollution, 182, pp.495-499.
Van Sebille, E., England, M.H. and Froyland, G. (2012). Origin, dynamics and evolution of ocean garbage patches from observed surface drifters. Environmental Research Letters, 7(4), p.044040.