Johns Hopkins Water Institute Magazine
June 18, 2014
Contamination of our oceans by Marine plastic has likely occurred since the advent of plastic, but it was not until the early 1970’s when reports began describing plastic fragments floating in extremely remote water in the Northern Hemisphere. Research and awareness efforts from groups, including Algalita Marine Research Foundation, have been instrumental in addressing this long-standing issue; in fact, supplemental research confirms that plastic pollution accounts for an astonishing 60-80% of marine litter by mass.
Originating from land, plastic pollution makes its way into the marine environment via a number of ways. Plastics that do not reach the landfill are often swept to nearby ditches and streams by wind, only to be carried out to sea by storm water runoff. Additionally, tidal flows and coastal winds sweep plastic litter directly from the shoreline out to sea. Finally, maritime activities and illegal dumping also contribute heavily to the plastic pollution burden. With respect to maritime activity, severe storms claim a significant amount of cargo along international shipping routes.
Once in the ocean and due to positive buoyancy, plastics tend to remain at or near the water’s surface where they are subjected to the forces of large rotating surface currents called gyres. Due to the convergence of surface waters, gyres have the effect of concentrating plastics and other floating debris over time. Importantly, plastics undergo essentially no biodegradation, but rather photo-degradation. That is, instead of being biologically transformed through metabolism, they are simply divided into smaller and smaller polymer chains by the effects of UV sunlight; their ultimate fate to become only a smaller version of the original material, namely, plastic. As a result, plastics persist and accumulate indefinitely in ocean gyres and in the environment, in general. In the subtropical gyre of the North Pacific Ocean, a rotating “garbage patch” twice the size of Texas has been estimated.
Plastics are more than an aesthetics problem. As many plastics float near the surface, they are often mistaken for food by marine mammals, reptiles, birds, and fish. Plastic ingestion can lead to choking and starvation of marine species. Such species can also become entangled by debris and consequently drown. In the North Pacific, approximately 100,000 marine mammals annually die due to plastic entanglement and ingestion. Worldwide, studies examining the stomachs of various bird species showed that 50 - 100% of individuals had ingested plastic fragments. Furthermore, offshore plastic-to-plankton mass ratios of 6:1 have been reported in the North Pacific, with some studies reporting ratios as high as 16:1 and 20:1.
Perhaps less obvious than the direct effects on marine species are the effects plastic contaminants have on biodiversity. Given their buoyancy and consequently the ease with which they become “hijacked” by ocean currents, plastics can serve as vehicles for introducing invasive species to new habitats. Though the specifics are less documented, the adverse effects of invasive species are well understood.
In addition to environmental impacts, marine plastic pollution has direct implications to human health. Due to their hydrophobic properties, plastics serve as nuclei onto which other hydrophobic substances sorb. Among these hydrophobic sorbents are many toxic persistent organic pollutants (POPs), including polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs), as well as organo-chlorine pesticides such as dichloro-diphenyl-trichloroethane (DDT) and hexachlorobenzene. By way of ingestion, these plastic particles, or nuclei, introduce toxicants into the marine food chain, becoming concentrated in fish and other marine species over time.
Low density plastics are often eaten by birds and pelagic fish at the sea surface while high density plastics which settle to the bottom are ingested by bottom dwelling zooplankton and benthic fish. In general, smaller plastic fragments have both the highest mass ratio of contaminants as well as the greatest likelihood of being ingested by small marine species and ultimately passed on to large fish and mammals. It is through human consumption of these larger marine species that plastics pose a threat to public health.
The public health implications of the bioaccumulation of methyl mercury through the aquatic food chain is both well documented and well understood. In a similar manner, POPs are biomagnified such that concentrations are ultimately measurable in human blood. That plastics are highly contaminated by POPs and are increasingly adulterating the diets of marine organisms could mean increased human exposure to POPs through seafood consumption. At this time, the extent this situation is occurring remains uncertain due to limited investigation and sequential research or literature. However, the implications of increased POP exposure should not be overlooked; POPs are widely known to cause endocrine disruption, mutagenicity, and carcinogenicity, and have consequently been banned from production in many nations.
In the United States alone, the average citizen disposes approximately 186 pounds of plastic each year. A portion of this plastic litter then enters the oceans and other environmental media; however, some efforts are attempting to curb this environmentally harmful plastic redistribution Laws are widely in place that incentivize recycling and discourage littering. Additionally, city-wide efforts to control the use of plastic bags have been implemented in California and other states. Besides policy changes, awareness campaigns seek to encourage consumers to avoid purchasing plastic products if substitutes are available and to avoid purchasing excessively packaged goods.
Other efforts have advocated for the use of reusable grocery bags and water bottles in place of their single-use counterparts. Many areas efforts have had positive outcomes in decreasing plastic usage; however, on a worldwide- and somewhat national level- the use of plastic is still rampant. Currently, plastic pollution is largely attributable to the disproportionate consumption levels within developed nations; however, the effects of this problem are globally shared. For this reason, policy efforts by low and middle income countries should be also developed and applied in the future, perhaps in the form of a proposed multilateral agreement.
Finally, if marine plastic contamination and subsequent animal and human health hazards are to be mitigated, particularly in the face of increasing population growth, serious and widespread efforts by both grassroots organizations and governmental agencies will become increasingly necessary.