The Rapid Rise of Plastics
Since the start of commercial production after World War II, global plastic production has experienced extraordinary growth, surpassing 400 million metric tons in 2015. This growth reflects the nature of this exceptional class of materials, whose endlessly versatile properties have brought unprecedented societal advancement and, with it, a reliance on plastics.
An estimated 8.3 billion metric tons of plastic have been produced in total since 1950, 70% of which have become waste (most within a year of their production and use). Less than 10% of this waste has been recycled, and 12% has been incinerated. Together with plastics currently in use, 90% of all plastics ever made are still in existence on the planet (Geyer et al. 2017).
Sources of Plastics to the Marine Environment
Plastics enter the marine environment when they are carried by wind and water, including in rivers, streams, and wastewater and stormwater outflows. Plastics are also directly deposited on shorelines or in the ocean when items are lost or littered, and waves and tides can carry plastics from the beach into the ocean.
Plastic consumer items – such as straws, cups, plates, utensils, bags, bottles and caps, and food packaging – may be littered, accidentally lost, or dumped in the environment. Fishing and aquaculture gear, including nets, fishing line and traps, can continue “ghost fishing” when they are lost at sea. Major storms, such as hurricanes, floods and tsunamis, displace massive amounts of all kinds of debris (not just plastics) into the environment. Some of the most memorable losses of plastics to the ocean, such as bath toys, Nike sneakers, Lego bricks and bike helmets, occur when container ships lose their cargo in rough seas. And the smallest plastic debris, including microfibers shed from clothing and other textiles, can be carried not only by water but also by winds from land to the ocean (and back!).
The Accumulation of Plastics in the Environment
We often hear the term “plastic” used as if it were a single material, when in fact “plastics” are a class of materials composed of many distinct polymers combined with a wide variety of chemical additives. Although there are probably hundreds, or perhaps thousands, of types of plastic, the majority (nearly 77%) of plastics produced to date have a common chemical structure (a carbon-carbon backbone) that makes these materials strongly resistant to biodegradation, or breakdown by microorganisms in the environment.
From the perspective of a chemical engineer or product designer, this may be an important and even desirable characteristic, especially for products designed to be used for years or decades. However, if these plastics are lost to the natural environment, their resistance to biodegradation will lead them to persist, and therefore accumulate, in the environment for a very long time. Plastic debris now contaminates terrestrial, freshwater, and marine environments around the globe.
What are Ocean “Garbage Patches”?
Despite misconceptions about massive floating islands consisting of plastic snack wrappers, bottles, flip flops and other common household items, the vast majority of floating plastic in the open ocean consists of microplastics, or plastic particles smaller than 5 millimeters in size. Most microplastics are fragments or fibers broken apart or shed from once-larger objects.
Ocean garbage patches are regions of the open ocean, far from land, where floating microplastics accumulate in high concentrations. Larger plastic items also accumulate here, but are far fewer in number than microplastics. These accumulations occur when ocean surface currents, which carry the debris, slow down and converge in the center of subtropical ocean gyres, which flow in circular motions around each of the five major subtropical ocean basins: North and South Atlantic, North and South Pacific, and Indian Oceans. Therefore, ocean gyres (or garbage patches), are essentially dead-end zones for floating plastics that accumulate in the ocean.
Daily photos taken from the deck of the SSV Robert C. Seamans while sailing through the North Pacific Ocean from Hawaii to California in May and June, 2020. While many microplastics were found in surface plankton net tows, and large debris was spotted in the distance, the plastic debris did not occur in a massive floating pile commonly imagined to be a “garbage patch”.
Why Do We Care About Microplastics?
Microplastics are not only the most numerous form of plastic debris in the marine environment, but they are also the most widespread, found globally on beaches and coastlines, in the water from the sea surface to the seafloor, frozen into sea ice, and buried in ocean sediments.
Microplastics are also readily ingested (eaten) by wildlife as small as plankton and as large as whales. Although the effects of eating microplastics (or inhaling them through gills) are not fully known, there is evidence that the particle itself, or the chemicals associated with it, can cause harm to marine animals.
Many studies have also documented microplastics in many items humans consume (food and drink) and in the air we breathe. Although microplastics have been found in different parts of the human body (e.g., lung, blood, placenta), we do not yet know what effects they might be causing.
How Do Scientists at SEA Collect Data on Ocean Plastics?
Ocean plastics are typically measured from SEA ships by collecting them in a plankton net towed at the sea surface. The “neuston net” has a rectangular mouth (0.5 x 1.0 meter in size) with a fine mesh (0.335 mm) designed to sample marine organisms and debris in the upper 25 centimeters of the water column.
SEA ships tow the neuston net at the air-sea interface for 30 minutes while moving at a ship speed of two knots, giving a tow length of one nautical mile. Students count the plastic particles collected in the net and divide by the surface area sampled (tow length * 1-meter wide net mouth) to report plastic concentration in units of particles per square kilometer.
Plastic debris in the ocean occurs in sizes ranging from microscopic to tens of meters, and is distributed from the sea surface to the sea floor. Depending on the size of the debris and the sampling location, other methods used to measure plastic debris include: filtering a smaller volume of water through smaller mesh filters (some even < 1 micron!); towing nets deeper in the water or along the seafloor; and deploying cameras from ships, on Remotely Operated Vehicles, or from aircraft. And scientists are working on new ways to measure plastics floating in the ocean from satellites in space!