SOURCES OF MARINE DEBRIS
People and their actions, whether intentional or accidental, are the source of most marine debris. For this reason, it is important to identify and target the specific locations or types of activities that generate and convey materials that ultimately become marine debris. An extreme variety of
objects, materials, and particles comprise marine debris. These range in size from abandoned and derelict vessels, to fishing traps and nets, to single-use plastic bottles, to pre-production plastic pellets, and the breakdown products of larger debris items. Marine debris is made of many materials, including but not limited to cloth, glass, metal, plastic, processed lumber, and rubber.
The National Marine Debris Monitoring Program (NMDMP), a five-year national study of trash in the ocean and monitored debris on beaches in the United States, found that land-based sources are responsible for approximately 49 percent of marine debris items along beaches, ocean based sources for 18 percent and about 33 percent, as general source debris from either land- or ocean-based sources.
This does not consider floating and submerged marine debris in both the nearshore and open-ocean environments, and the relative importance of various sources of this unaccounted debris may differ from that suggested by debris found on beaches.
Because of ocean transport mechanisms, it may be very difficult to determine the source of marine debris, which is one reason for NMDMP’s general source category. An example of general source debris cycling in the open ocean is a location called the North Pacific Subtropical Gyre (the Gyre) made of the North Pacific, California, North Equatorial, and Kuroshio currents, along with atmospheric winds. Persistent and pervasive marine debris from both land- and ocean-based sources around the Pacific Rim aggregates in the currents of the Gyre. This debris may remain in the Gyre for long periods of time, becoming what is known as legacy debris. It is difficult to determine the age, origin, and source of legacy debris.
Land-based sources of MD may originate from coastal areas or farther inland. Waterfront areas, including beaches, piers, harbors, riverbanks, marinas, and docks are common land-based sources of marine debris. Debris also may originate from sources that carry material by precipitation runoff into waterways and, ultimately, the ocean. Debris may be the result of improper trash disposal, improper handling of materials, or inadequate reception facilities for waste.
Natural disasters such as tornadoes, floods, tsunamis, and hurricanes may all create large amounts of debris washed from near-shore areas that may end up in the marine environment because of high winds, waves, and storm surges. After the 2004 Indian Ocean tsunami, smothering by
debris was a principal cause of damage to coral. In the 2005 hurricane season along the Gulf of Mexico coast, an estimated nine million cubic yards of debris were spread over 1,770 acres of marsh in the Sabine National Wildlife Refuge in Louisiana alone.
The Pacific Rim has experienced significant event activity in recent years, with tsunamis occurring in American Samoa in 2009 and off the coast of Japan in 2011. Hurricane Sandy was also significant.
Litter, regardless of whether it is purposely or accidentally discarded or lost, has the potential to become marine debris.
Fishing gear, monofilament line, and other fishing-related items may also be introduced into the marine environment from waterfront areas and fishing piers. To address submerged debris in traditional fishing grounds, sonar over 1500 square nautical miles in the nearshore waters of Alabama, Mississippi, and Louisiana, over 7,100 objects were located, with some areas having a density of up to 200 objects per square nautical
Rising populations in coastal areas have increased the potential for introducing marine debris. Improperly disposed trash may wash into streams, sewer systems and storm drains, and eventually be carried into coastal and ocean waters. Combined sewer systems are older sewer systems that combine sewage and stormwater runoff into the same infrastructure. These systems may become overwhelmed during periods of heavy rain, and everything in the pipes, including street litter and sewage-related items (e.g., condoms, tampons, syringes), is diverted away from the treatment plant to the nearest receiving waters. Municipal separate storm sewer systems also have the potential to transport materials that may become marine debris as these systems often do little more than convey precipitation runoff down the storm drain and into the nearest surface water, bringing with the runoff all the remnants of human activity from around that storm drain.
The growth in coastal populations has also required expansion of waste repositories such as landfills and transfer stations. Overused and poorly managed landfill and transfer stations often may result in increased marine debris. Trash that is improperly covered during transport or
deposition into landfills may be carried by wind into the marine environment or into other aquatic systems that transport the trash to the marine environment.
Industrial facilities are another source of land-based marine debris. By-products from production, particularly persistent synthetic materials such as plastics, may become MD when dropped, washed, or blown away during transport to or from the factory or during production.
Plastic Pellets. While this was particularly true in the past during transport of pre-production plastic resin pellets, implementation of best management practices by industry has helped reduce this source of marine debris.
In the ocean, vessels of various sorts and structures are all potential vectors for the introduction of debris into the marine environment. Even with strict adherence to environmental regulations, accidental loss, especially in inclement or severe weather and the improper disposal of their trash at sea occurs. The type, magnitude, and impacts of vessel-generated MD differ according to vessel size, purpose, and their respective enforcement and compliance regimes. However, all vessels under United States jurisdiction are subject to the discharge regulations established under the Act to Prevent Pollution from Ships, which include the prohibition of disposal of plastic at all distances from shore; a prohibition against the disposal of any type of garbage within three miles of shore; and 12- and 25-mile minimum distance requirements for the disposal of other types of garbage.
Fishing vessels may introduce marine debris when nets, traps, monofilament, lines, light sticks, and floats are lost or discarded at sea. Derelict fishing gear including thousands of crab pots are lost each year due to severe winter storms that may move them many miles from their point of deployment, or because the floats tied to the pots are cut by passing vessels and the pots are no longer accessible for retrieval.
In the Pacific, debris may get trapped in the North Pacific Subtropical Gyre and accumulate along convergence zones that may transport debris to the remote islands of the Papahānaumokuākea Marine National Monument. From 1996 to 2007, 570 metric tons of derelict nets were removed from the Monument, which are known to act as a repository for marine debris. These nets may come from all areas of the Pacific Rim, get caught in the convergence zone, potentially stay in the convergence zone for many years, and end up in the Monument. In a 16-year study at French Frigate Shoals Tern Island (within the Papahānaumokuākea Marine National Monument), 23 percent of the total items collected originated from the maritime industry.
Other fishing related items, such as light sticks, buoys, and rope particularly constructed of plastic also demonstrate persistence in the marine environment.
Cruise ships and cargo carriers and other large, heavily regulated vessels are a potential vector for the introduction of marine debris. Due to their size, these ships are subject compliance inspections and garbage record book requirements, in addition to all regulations placed on smaller vessels. Cruise ships carry significantly more passengers and crew than cargo carriers and therefore are potentially more likely to create a larger proportion of domestic waste. Cargo ships may lose cargo or cargo containers at sea as a result of severe weather or poor loading practices. Industry experts estimate that anywhere from 2,000 to 10,000 containers fall off ships each year, less than 1% of the number of containers sent by sea annually.
Geography is another key factor; some cruise ships operate in environmentally sensitive habitats such as Caribbean islands or the Inside Passage of Alaska where marine debris may have a more significant impact. Some cruise ships have voluntarily developed advanced programs for
waste minimization, waste reuse and recycling, and waste stream management. Best practices to minimize container loss overboard were published in 2008 and distributed to containership owners and operators.
Recreational vessels are also a potential source of ocean-based marine debris. Vessels over 26 feet are subject to a MARPOL placarding requirement, and vessels over 40 feet must maintain a garbage management plan. Recreational fishing gear and domestic waste are likely components of marine debris contribution from these vessel types.
Derelict or abandoned vessels and off-shore materials and equipment are additional sources (e.g., research buoys, cables, aquaculture infrastructure). In high-wave conditions, severe storms events, or tsunamis, these vessels or structures may be broken up and strewn across the ocean floor, adversely affecting habitat and navigational safety. In pristine coral reef habitats, the iron enrichment from metal debris has been demonstrated to lead to algal blooms and to upset the ecological balance of the reef.
Offshore oil and gas platforms and drilling rigs are another potential ocean-based source. This may be the result of improper disposal of wastes or equipment, or loss during heavy weather that demonstrate that government oversight and intervention, as well as industry best
practices, cannot completely prevent the introduction of debris from regulated facilities. In 2005, the offshore oil and gas industry lost 117 platforms on the Outer Continental Shelf, and dozens more were significantly damaged as a result of Hurricanes Katrina and Rita. The Gulf sustained
another hit in 2010 with the Deepwater Horizon oil spill, which resulted in oiled debris on the shores.
IMPACTS OF MARINE DEBRIS
In addition to being unsightly, regardless of origin, debris entering the aquatic environment poses significant threats to ocean ecosystems, human health and safety, wildlife and the economy. Impacts of marine debris vary in scope and intensity depending on the type of debris (e.g., plastic
bags, miscellaneous plastics, derelict fishing gear, shipping containers, etc.) and its location (e.g.,floating in shipping lanes or resting on sensitive habitats). Some examples of these impacts are:
Ecological Impacts: impacts to the physical and biological environments including habitat destruction, wildlife entanglement, ingestion, and death, the spread of invasive species, and effects from chemical transport;
Human Health and Safety Impacts: endangering human health and safety and hazards to navigation;
Economic Impacts: reduced tourism, diminished aesthetic value, beach closures and clean-ups, and vessel damage.
Both the physical and biological environments are affected including aquatic ecosystems, such as coral reefs, wetlands, fish habitats, beaches, and migratory species breeding grounds and pathways. Marine debris may impact species directly, such as through entanglement or smothering of species, or indirectly, such as through changes to habitat.
Depending on the type of marine debris, abandoned nets, plastic tarps, fishing gear and other debris may smother and crush sensitive ecosystems and their bottom dwelling species.
Derelict fishing gear, as well as other large blanketing debris, may damage coral reefs by smothering, breaking apart, or abrading corals. It modifies the structure of the reef by damaging the coral substrate. After net debris snags on a coral reef, wave action acting on that debris breaks the coral heads on which that debris is fixed, freeing it to subsequently snag and similarly damage additional corals. This action continues until it is removed or becomes adequately weighted with broken corals to sink. The damage to corals caused by such nets can be substantial and continuous.
Ghost fishing occurs when marine species become trapped in lost or abandoned pots or nets that continue to catch prey without being retrieved by fishermen to harvest. Ghost fishing does not discriminate: target and non-target species, as well as local and migratory species including those protected under the Endangered Species Act (ESA). In many situations, animals captured in derelict traps serve as attractants for other animals resulting in a self-baiting ‘ghost’ fishing cycle.
Fishing line, nets, rope and other debris may entangle, maim, and even drown many wildlife species by encircling or ensnaring the animals. The entanglement may occur accidentally or when an animal is attracted to the debris as part of normal behaviour or out of curiosity. Diving seabirds, such as albatross, may be caught, entangled, and subsequently drowned by debris. Animals may incur lacerations or other wounds from debris, potentially leading to infection and debilitation. When marine species become entangled within debris, their mobility is limited. Constricted movement may inhibit the animal’s ability to collect food or breathe and may lead to starvation, suffocation, exhaustion, and increased predation. It is typical for marine animals such as the endangered Hawaiian monk seal to investigate foreign items in their local marine habitat, which may lead to injury, drowning, or suffocation in nets, line (including monofilament), straps, or plastic items. In the Northwestern Hawaiian Islands between 1982 and 2006, 268 entanglements of the monk seal were documented. This figure likely underestimates actual entanglement rates because it only reflects those seals that became entangled but were still mobile enough to reach shore at a time of year when humans were able to find them.
In some cases, debris acts as habitat. Species are attracted to in situ debris for both shelter and for the food sources trapped within. This can complicate moving or removing objects as endangered species or predators may also be attracted.
Although large debris items, such as derelict fishing gear, may have severe and highly visible impacts, smaller debris items such as bottle caps, lighters, and plastic pieces are also hazardous to wildlife. Some debris may be mistaken for food by animals. Once ingested, these materials may cause starvation and/or choking. Seabirds are known to ingest small debris items along with their food. Northern fulmars and other marine birds that ingest plastic debris do not have the capacity to regurgitate the indigestible material. Ingestion of marine debris may lead to starvation or malnutrition because the ingested items may collect in the animal’s stomach and lessen the desire to feed. In addition, ingestion of sharp objects may damage the mouth, digestive tract, or stomach lining and cause loss of nutrition, infection, sickness, starvation, and even death. Ingested items also may block air passages and cause suffocation. Ingestion may occur accidentally, but often animals feed on marine debris because it resembles their food. For example, sea turtles have been known to ingest plastic bags instead of their target prey, jellyfish. In a study of green sea turtles, 23 of 38 animals were shown to have ingested anthropogenic debris. In addition, some debris items may leach harmful chemicals when ingested. The extent to which this occurs is unclear. Several efforts are investigating the potential for debris, especially plastic
debris, to be a vector of chemicals to marine systems and organisms.
An indirect impact of marine debris on shoreline habitats occurs on beaches as a result of debris reduction and removal efforts. Mechanical beach raking, accomplished with a tractor or human labor, is used to remove debris from the shoreline and may help to remove floatable material from beaches and marine shorelines. However, beach raking may also be harmful to aquatic vegetation, nesting birds, sea turtles, and other types of aquatic life. A study by the U.S. Fish and Wildlife Service (FWS) on the effect of mechanical beach cleaning on threatened piping plovers found that such practices harmed nesting birds by destroying potential nesting sites, crushing nests and chicks, and removing the natural wrack-line feeding habitat. To minimize this impact, FWS suggested that beach raking should not be conducted during nesting season.
Storm events, such as hurricanes and tsunamis, often mobilize marine debris, impacting various species and habitats as it moves throughout the water column. Marine debris may also indirectly damage the environment if it causes vessel accidents that spill oil or hazardous materials.
Indirect impact also occurs through alien species transport and the introduction of invasive species such as those carried on a floating dock that washed ashore in Oregon in June 2012 and was confirmed to be from the March 2011 tsunami that struck Japan. It carried a biofouling community that included over 90 marine species that were not native to the west coast of North America. Some of these species were known to be invasive and could cause ecosystem and economic harm.
Human Health and Safety Impacts
Beachgoers may be injured by stepping on broken glass, cans, needles or other items. Swimmers and divers may become entangled in abandoned netting and fishing lines. Vessels may directly strike floating or submerged marine debris, which may lead to human injury or severe damage to the vessel.
Passengers may be injured or killed if the vessel is damaged or disabled. Grocery and trash bags, fishing line, nets, rope and other debris may wrap around boat propellers and clog seawater intakes, causing costly damage to vessels and becoming a safety hazard. This may impact movement and navigation by disabling the vessel, and ultimately endangering human lives.
In 1993, derelict fishing gear contributed to the sinking of the Korean passenger ferry M/V SeoHae, which resulted in the deaths of numerous passengers. Recreational boaters have also been subject to stranding due to engine fouling from plastic bags blocking intake valves or
derelict fishing nets or lines becoming entangled around propellers.
Medical and personal hygiene debris may enter waterways when sewer systems fail or overflow. These items often contain harmful bacteria and pathogens. Syringes, broken glass and other hazardous items pose obvious dangers to barefooted beachgoers. Sharp debris objects such as broken glass, rusted metal, or medical debris, on beaches or the ocean floor. In the late 1980s, beaches in New York and New Jersey were closed to protect the public from medical waste, including syringes and bandages from hospitals that washed ashore. SCUBA divers become entangled in lost
or abandoned fishing line and nets. While this is a rare occurrence, entangled divers may be seriously injured or killed.
Direct economic losses from marine debris may be measured by analysis of impacts on tourism, losses in catch revenues, loss of fishing gear, damaged vessels, and human injuries.
Marine debris may be detrimental to the tourism industry by creating unsightly, dangerous beaches. Beach closures, often a direct result of marine debris, may have particularly serious economic ramifications in coastal areas dependent upon tourism. In addition, the costs associated with cleanups and proper disposal of debris may be significant. Cleanup-related costs may include the cost of restoring the habitat impacted by marine debris, beach cleanup costs, the costs to clean piers, harbors, marinas, docks, and other waterfront areas, and the costs associated with at-sea cleanups.
Environmental contamination from debris in the marine environment, both onshore and in local fish habitats, may also have significant economic impacts. For example, loss in tourism was estimated to be between $706 million and $2,977 million (in 2008 U.S. Dollars) as a result of medical debris wash-ups in New Jersey in 1988.
Commercial fishery revenues may be adversely impacted due to by catch of target fish or shellfish in lost nets or other types of “ghost” fishing gear. For example, an estimated 200,000 pounds of Dungeness crab are killed in derelict crab pots every year in Puget Sound, an amount worth approximately $335,000. Within the European Union, it is estimated that 1,500 demersal cod/turbot gill nets are lost each year in the Baltic Sea fishery, removing anywhere from 0.01 to 3.2 percent of the commercial harvest. Such by catch not only reduces the standing stock of fish or shellfish available to a fishery but also may reduce reproductive capacity and thereby the long-term viability of the stock.
Vessels adversely impacted by marine debris may incur economic costs. Marine debris has the potential to disable vessels through collisions, or by wrapping around propellers or blocking intakes. In 1992 Japan estimated their fishing industry spent U.S. $4.1 billion in boat repairs resulting from damage caused by marine debris. In addition to property damage, marine debris may cause “lost opportunity” costs. For example, fishermen may lose opportunities to fish if they are forced to stop operations as a result of entanglement or vessel damage incurred from marine debris. This opportunity cost may have a range of economic impacts on communities dependent on fishing revenues. Additionally, it may
impose costs to locate, mark, and remove debris that could pose a hazard to navigation.