Marine debris

DEFINITION: Solid litter discarded by humans that finds its way into the oceans

The presence of large amounts of litter in the world’s oceans is a matter of major environmental concern. Such debris can threaten marine wildlife and ecosystems in numerous ways, such as by entangling animals, blocking the digestive systems of animals that ingest nonfood objects, and contaminating ocean waters with dangerous chemical compounds.

Much of human-made material that is discarded after use ends up in the oceans to become marine debris. Of this debris, plastic products are among the most troublesome because they are produced in vast amounts, are lightweight and hydrophobic (water repelling), and resist degradation. In large parts of the ocean, plastic debris can be more abundant than the natural prey of marine animals. Animals that ingest these items are exposed to high levels of chemicals and may suffer blockages in their digestive systems. Large pieces of debris can entangle and choke large marine animals, including endangered turtles and marine mammals.

Fates of Different Kinds of Debris

The stability and the density of marine debris are the most important properties that affect the fates of these materials. Degradable materials such as paper and wood do not remain marine debris for long because microbes readily break down these materials. Nonsoluble and nondegradable materials such as rubber, plastic, most metals, and concrete remain in water for years and therefore represent the majority of marine debris. The density of the debris determines whether it sinks to the bottom or remains in the surface waters. Dense materials such as metals, concrete, ceramic, glass, and some plastics to the bottom of the sea unless their shapes enable them to float. Materials that are less dense than seawater or equal to seawater in density, such as polystyrene and light plastics, drift in the surface currents of the oceans and often wash up on the shore with waves.

Large ocean gyres are vast areas where surface waters come together and downwell. This water movement causes light, nondegradable debris to concentrate in these areas. The amount of debris in these waters often exceeds the amount of living organisms, a condition known elsewhere only in landfills and some urban environments. At the North Pacific Gyre, the large accumulation of debris has been dubbed the Great Pacific Garbage Patch. Because the plastic materials have been broken down to small particles that are not easily seen from satellites, the size of this debris field is difficult to measure, but it is estimated to be approximately 1.6 million square kilometers and contains 1.8 trillion pieces of plastic. The debris is so abundant that some boat captains refuse to travel in the area because of the risk that their vessels could become entangled. In April 2010, the Associated Press reported that scientists had discovered similar plastic garbage patches in the Atlantic Ocean. This garbage patch, located off the coast of the southeastern United States, was believed to be the same relative size as its counterpart in the Pacific and also contained plastics and other debris. Several expeditions to study the garbage patch have been undertaken, including one in 2012 by the Sea Education Association. By 2016, it had become clear that the North Atlantic Garbage Patch was filled with more microplastics than the one in the Pacific, making it easier for marine life to ingest harmful substances.

Effects on Marine Wildlife

Large pieces of debris provide structures and serve as habitats for marine organisms. In fact, old bridges, barges, and boats are intentionally discarded in some places to create artificial reefs for the purpose of increasing the productivity and diversity of marine life. Scientific research regarding the colonization and utilization of artificial reefs has revealed how the arrangement and shapes of debris can optimize its utility as habitat. Even large floating objects such as plastic buckets, buoys, and crates are used as habitat in much the same way as sargassum seaweed is used as natural habitat.

Most of the effects of debris on marine wildlife are negative. Marine debris can entangle, strangle, and choke some kinds of creatures. “Ghost nets” are fishing nets abandoned or lost in the sea that continue to function as they were designed to function, entangling animals. Fish, reptiles, mammals, and even birds that become entangled in ghost nets usually die from starvation, stress, injuries, or suffocation. Debris that gets wrapped around the throats of animals can cause strangulation. Ingested debris can block the digestive tracts of animals, interfering with their ability to feed, digest food, or pass waste. These animals usually die of starvation.

Predatory marine animals often consume plastic debris because it resembles their natural prey. Floating plastic bags and balloons are mistaken for jellyfish by sea turtles. Small plastic nurdles (resin pellets from which larger plastic items are made) are mistaken for fish eggs or zooplankton. Because most of the nurdles and small particles that have been found in debris samples are blue, black, white, or green, researchers have hypothesized that yellow and red plastic particles are selectively eaten by marine organisms.

Even if ingested plastic does not kill animals by choking their digestive systems, it can still have significant sublethal effects on organisms of all sizes. Plastic particles concentrate hydrophobic compounds on their surfaces. These compounds interfere with the endocrine systems of animals, creating reproductive, developmental, and immunological problems for these animals. By concentrating these compounds by many orders of magnitude, even small plastic particles can negatively affect large animals by increasing their to endocrine-disrupting compounds, a problem that is already serious owing to the of compounds in long marine food webs.

Prevention and Mitigation

The 1973 International Convention for the Prevention of Pollution from Ships (along with the 1978 protocol that amended it, collectively known as MARPOL 73/78) and the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (also known as the London Convention) are international laws that aim to limit marine debris by prohibiting the disposal of certain kinds of wastes in the open oceans. Enforcing such laws has proven to be difficult, however. Within the United States, Congress passed the Marine Protection, Research, and Sanctuaries Act (MPRSA) or the Ocean Dumping Act of 1972. This piece of legislation aimed to regulate the disposal of materials into the ocean and allow for research on the effects of ocean dumping. In 2022, the Environmental Protection Agency (EPA) created an education program to explain how the Ocean Dumping Act had protected marine environments and human health over the previous fifty years.

Marine debris is removed in a variety of ways, from individual scuba divers gathering litter by hand to boat-mounted skimmers picking debris from surface waters and teams of people and machines picking up the that washes up on beaches. Given that approximately 80 percent of marine debris comes from continental sources, strainers placed on storm drains and across streams to retain litter are effective preventive measures to combat marine debris. One unique program meant to reduce the number of ghost nets in the world’s oceans is Hawaii’s Nets-to-Energy Program, which has collected 832 tons of derelict fishing nets and has used them to produce enough electricity to power three hundred homes for a year. Still in operation in 2024, this coordinated effort involves fishing companies, steel corporations, and energy companies, which provide their services free of charge to promote marine debris cleanup.

Scientists have also been studying microorganisms that digest plastic. Some have been found on plastic in the Garbage Patch. Spores of bacteria that eat plastic have been incorporated into plastic so the bacteria can get to work consuming the plastic when the plastic is composted, for example. Other researchers are working on similar biodegradable products.

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