15 Apr 2013
What Does the Garbage Patch Really Look Like?
Editor’s Note: Is the Garbage patch a surreal floating island of plastic garbage the size of Texas? Nothing so dramatic —or concrete. What really happens to plastic out in the ocean is that it breaks down into smaller and smaller fragments, gets dispersed by wind and currents all over the ocean’s surface and into the depths—but never goes away. Thus far, the vastness of this problem—the United Nations Environmental Program estimates 13,000 visible pieces of plastic per square kilometer—has defied any plan to contain it, yet alone to clean it up.
The first time I heard the term “garbage patch” was in the mid-2000s while preparing for a four-week expedition as the chief scientist of the SSV Robert C. Seamans. This 134-foot tall sailing ship and state-of-the-art oceanographic research vessel would carry 25 undergraduates from colleges and universities around the country participating in Sea Education Association's SEA semester program to the open ocean, where they would learn to sail the ship and conduct independent research on the oceanography of the eastern North Pacific.
Prior to the research cruise a journalist had written to
inquire whether or not we were likely to encounter the “fabled ‘Eastern Garbage
Patch’” on our transit from Honolulu, HI to San Francisco, CA. Not familiar with such an oceanographic
feature, I did a quick search through the scientific literature for reports
about the type of garbage and the geographic area in which it had been
observed. I found few papers describing
any kind of floating man-made debris, and those publications that did reported relatively
sparse measurements of mostly small fragments of plastic collected from the sea
surface using plankton nets. Was this the
At Sea Education Association (SEA), where we have taken students to sea to study the ocean for more than 40 years, the occurrence of small floating bits of plastic in our plankton net tows trawling the sea surface was not unusual. In fact, dozens of SEA Semester students had chosen to study the distribution of plastic debris for their research projects.
However, the student projects were typically carried out aboard SEA’s sister ships in the Sargasso Sea region of the western North Atlantic, where former SEA scientist Jude Wilber first began documenting floating plastic debris in the early 1980s (Wilber, 1987). Wilber noted that plastic debris was generally found within the surface ocean currents moving in a clockwise pattern around the North Atlantic Ocean termed the ‘subtropical gyre’.
Since 2001, SEA students and scientists had been conducting the same surface plankton net tows in the North and South Pacific during annual transits from the U.S. west coast to Mexico, Tahiti, Hawaii, and back to the west coast. In general, very little floating plastic had been collected anywhere on these routes except between Hawaii and the west coast, the track that transited the subtropical gyre of the North Pacific Ocean. But was this the “fabled ‘Eastern Garbage Patch’”?
As it turns out, the answer was ‘yes’. The ‘garbage patch’ is not a dense collection of trash like you might find in garbage cans, in landfills, or as litter on beaches. Rather, most of what floats in the open ocean consists of irregular bits of plastic smaller than your pinky fingernail that are very difficult to spot even from the deck of a ship, and whose origin is unknown and, at least currently, unknowable.
We do know the most common plastics collected at the sea
surface consist of polyethylene (HDPE and LDPE), polypropylene (PP), and
polystyrene foam (PS), materials that float in seawater. You may recognize these materials according
to their plastic resin code, more commonly referred to as the ‘recycling code’,
stamped on the bottom of many consumer goods (#2–HDPE; #4–LDPE; #5– PP; #6–
PS). While it is very likely that many
of the heavily weathered and fragmented bits of plastic collecting far offshore
in the subtropical gyres originated from consumer goods, we simply cannot track
the source of these tiny bits – whether from land-based litter; lost or
discarded fishing or aquaculture gear; catastrophic events such as tsunamis,
floods or hurricanes; or intentional or non-intentional (albeit illegal)
dumping of plastic at sea from ships and at-sea platforms.
At the time of that first inquiry about the North Pacific ‘garbage patch’, we realized the scientific value of the unique and unparalleled floating plastic data set that had been diligently collected by thousands of SEA students in both the Atlantic and Pacific Oceans. In 2010 a map of the extent of floating plastic debris in the western North Atlantic from more than 6,600 plankton net trawls was published (Figure 1, Law et al., 2010), and a similar map for the eastern North and South Pacific Oceans from more than 2,500 net trawls is currently in preparation.
To date, high plastic concentrations have been more extensively documented in the northern hemisphere basins, although floating plastic has been observed on individual cruises in the southern hemisphere (5gyres.org), consistent with accumulations of floating debris predicted using numerical simulations of surface ocean circulation (Maximenko et al., 2012; Lebreton et al. 2012; van Sebille et al., 2012). Outside of these convergence zones floating plastic concentrations are likely to be quite low.
While scientific attention to plastic as an ocean contaminant has increased in recent years, many of the most basic questions about the amount, distribution, lifetime, and ecological impacts of ocean plastic remain. The sources of plastic to the ocean are well known, but very difficult to quantify. The areal extent of the plastic accumulation zones as predicted by numerical models has yet to be confirmed with ocean observations; even the best surveyed ocean basins are woefully under sampled, and will likely remain so given the vast distances and cost of ship time. The smallest particles (< 0.3 mm in size) that pass through the mesh of most plankton nets used to trawl the sea surface cannot be easily collected and identified as plastic, leaving a potentially large reservoir of floating plastic currently unaccounted for. And while all these questions remain about plastic that floats, we know far, far less about what lies on the seafloor.
Research efforts to study the basic science behind ocean plastic are increasing. At SEA we are beginning a study of the thousands of plastic pieces that have been carefully archived since the 1980s in order to determine the plastic materials present, and to identify signatures of physical and chemical degradation that may reveal clues about the age, or lifetime, of plastic in the ocean. We are also using measurements of wind speed collected in conjunction with plastic concentration data to estimate the amount of buoyant plastic that is mixed by the wind tens of meters below the sea surface. This analysis will provide more refined estimates of plastic concentration than measurements from surface-trawling plankton nets alone. Studies such as these are just the tip of the iceberg in a scientific discipline that is in many ways still in its infancy.
Given what little we do know to date, should we be worried about the impacts of plastic on the marine ecosystem? Yes. Some of the impacts of plastic debris are well known, such as the entanglement of marine animals by lost nets, cargo packing straps, and beverage six-pack rings. Since the 2011 Japanese tsunami there has been an increased public awareness of the impacts of large debris as a navigational hazard to ships and a threat to coastal ecosystems from the transport of non-native species. But even small bits of plastic harbor complex communities of microbes and encrusting organisms taking advantage of and traveling with this new substrate.
Foraging seabirds are known to ingest smaller pieces of plastic and feed them to their young, and as the size of plastic particles decreases as a result of environmental degradation, plastic debris is accessible to an increasing cohort of marine organisms from fish to invertebrates. In addition, plastics contain known toxins from manufacture than can leach into seawater or potentially into tissues of organisms that ingest them, and plastics also act as a sponge for other persistent pollutants already present in seawater such as PCBs and DDT. While the weight of scientific evidence is not yet in, there is reason to be concerned about such toxins making their way into and up the marine food web where humans might ultimately be exposed via seafood consumption.
In short, there is good reason to care about the plastic we are putting into the ocean and into terrestrial environments as well. We have become accustomed to an increasingly disposable culture thanks in large part to the low cost and high utility of a wide variety of plastics. Now we must, as consumers, face the true cost of our actions.
Law et al., 2010. Plastic accumulation in the North Atlantic subtropical gyre. Science 329, 1185-1188.
Lebreton et al., 2012. Numerical modeling of floating debris in the world’s oceans. Mar. Poll. Bull. 64, 653-661.
Maximenko et al., 2012. Pathways of marine debris derived from trajectories of Lagrangian drifters. Mar. Poll. Bull. 65, 51-62.
van Sebille et al., 2012. Origin, dynamics and evolution of ocean garbage patches from observed surface drifters. Env. Res. Lett. 7, 1-6.
Wilber, 1987. Plastic in the North Atlantic. Oceanus 30, 61-68.