Posted: June 18th, 2024

Ocean Circulation and the Global Distribution of Marine Plastic Pollution

Ocean Circulation and the Global Distribution of Marine Plastic Pollution

Plastic debris has contaminated the marine environment on a global scale. From the surface waters to the deep seafloor, no area of the ocean remains untouched by this persistent pollutant. The distribution patterns of marine plastic waste reveal a strong connection to prevailing ocean circulation regimes. Investigating this relationship enhances scientific understanding of how plastic litter disperses across vast distances and accumulates in certain oceanic regions.

Ocean Currents as Conveyor Belts

Ocean currents act as conveyor belts, transporting marine plastic over immense areas. Buoyant plastics readily enter surface currents and can travel thousands of miles before washing ashore or becoming trapped in convergence zones (Hardesty et al., 2017). The perpetual clockwise rotation of subtropical gyres draws in and concentrates floating plastic debris, creating areas with staggeringly high concentrations nicknamed “garbage patches” (Lebreton et al., 2018).

The Great Pacific Garbage Patch between Hawaii and California contains an estimated 1.8 trillion pieces of plastic, representing the largest known accumulation worldwide (Lebreton et al., 2018). Similar but smaller garbage patches occur in the subtropical gyres of the Atlantic and Indian Oceans. As plastic fragments into smaller pieces, the potential for dispersal by complex current systems increases, spreading pollution across entire ocean basins (Hardesty et al., 2017).

Coastal Environments and Plastic Hotspots

While oceanic gyres act as long-term sinks, coastal regions represent plastic pollution hotspots closer to shore. Densely populated coastlines contribute high plastic loads while complex current patterns facilitate debris accumulation (Hardesty et al., 2017). For example, the Kuroshio Current Extension running eastward from Japan becomes a trap for plastic waste originating across the Northwest Pacific rim (Isobe et al., 2015). Along the U.S. East Coast, the Gulf Stream redistributes buoyant debris northwards, concentrating plastics off Cape Hatteras where this powerful current separates from the coastline (Hardesty et al., 2017).

Ocean currents carry floating plastics far from their input sources. The Arctic Ocean contains proportionally high levels of plastic pollution compared to its limited Arctic littoral population (Cózar et al., 2017). This contamination results from the Thermohaline Circulation conveying plastic northwards on surface currents from the Atlantic and Pacific (Cózar et al., 2017). Similar long-range transport occurs in the Antarctic Circumpolar Current, carrying plastic litter into the remote Southern Ocean (Eriksen et al., 2014).

Sinking of Marine Plastics

While buoyant plastics travel far on surface currents, dense plastics follow different pathways as they sink through the water column. Bottom currents may resuspend settled plastics while downwelling gyres can contribute to the export of debris into deep waters and the seafloor (Hardesty et al., 2017). In the Northeast Atlantic, sinking debris accumulates along submarine canyons and slopes shaped by powerful boundary currents (Pham et al., 2014).

The Mediterranean Sea represents a plastics accumulation zone due to its semi-enclosed nature and downwelling currents causing debris to settle in deep basins (Woodall et al., 2014). While less studied than surface distributions, the downward transport of plastic from the upper ocean represents a growing area of research to map seafloor litter. Dense plastic concentrations on the seafloor may relate to sinking rates overcoming lateral transport by bottom currents (Hardesty et al., 2017).

Impacts on Marine Ecosystems

As plastics disperse throughout the global ocean, the potential for ecological impacts increases across taxa and habitats. Floating plastics can entangle or be ingested by seabirds, turtles, marine mammals and fish. Plastic debris physically alters seafloor habitats like coral reefs and acts as a reservoir for the accumulation of toxic substances like persistent organic pollutants (Hardesty et al., 2017). Marine plastic pollution presents an interdisciplinary challenge that requires integrating oceanographic data with ecological research.

Plastic inputs continue rising with over 8 million tons estimated to enter the ocean annually (Jambeck et al., 2015). As the problem persists, understanding the mechanisms driving global plastic distributions becomes crucial to mitigating this environmental threat. Incorporating ocean circulation patterns, currents, and processes helps predict where marine plastic pollution will accumulate and delineates areas facing disproportionate impacts from this persistent contaminant (Hardesty et al., 2017). Recognizing the relationship between oceanic transport and plastic debris can also inform policies managing marine litter on regional and global scales.

The ocean circulation exerts a controlling influence over the pathways and distribution patterns of marine plastic pollution worldwide. Buoyant plastics can travel immense distances via surface currents and accumulate within oceanic convergence zones. Densely populated coastal areas represent major input sources leading to local hotspots and long-range dispersal. Sinking plastics follow bathymetric contours and downwelling currents to accumulate on the seafloor. As plastic contamination continues growing, interdisciplinary study of this oceanographic relationship becomes vital to assessing ecological impacts and developing solutions. Mapping the global distribution and transport of marine plastic debris will remain a key scientific priority in the years ahead.

References:

Cózar, A., Martí, E., Duarte, C.M., García-de-lomas, J., van Sebille, E., Ballatore, T.J., Eguíluz, V.M., González-Gordillo, J.I., Pedrotti, M.L., Echevarría, F. and Troublỳ, R., 2017. The Arctic Ocean as a dead end for floating plastics in the North Atlantic branch of the Thermohaline Circulation. Science Advances, 3(4), p.e1600582.

Eriksen, M., Maximenko, N., Thiel, M., Cummins, A., Lattin, G., Wilson, S., Hafner, J., Zellers, A. and Rifman, S., 2014. Plastic pollution in the South Pacific subtropical gyre. Marine Pollution Bulletin, 68(1-2), pp.71-76.

Hardesty, B.D., Hararing, P., Isobe, A., Lebreton, L., Maximenko, N., Potemra, J., van Sebille, E., Vethaak, A.D. and Wilcox, C., 2017. Using numerical model simulations to improve the understanding of micro-plastic distribution and pathways in the marine environment. Frontiers in Marine Science, 4, p.30.

Isobe, A., Kubo, K., Tamura, Y., Kako, S.I., Nakashima, E. and Fujii, N., 2015. Selective transport of microplastics and mesoplastics by drifting in coastal waters. Marine Pollution Bulletin, 89(1-2), pp.324-330.

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. Science, 347(6223), pp.768-771.

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.

Pham, C.K., Ramirez-Llodra, E., Alt, C.H., Amaro, T., Bergmann, M., Canals, M., Davies, J., Duineveld, G., Galgani, F., Howell, K.L. and Huvenne, V.A., 2014. Marine litter distribution and density in European seas, from the shelves to deep basins. PLoS One, 9(4), p.e95839.

Woodall, L.C., Sanchez-Vidal, A., Canals, M., Paterson, G.L., Coppock, R., Sleight, V., Calafat, A., Rogers, A.D., Narayanaswamy, B.E. an

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