What can sea turtles tell us about the plastics in our oceans?
Source: Jung, M. R., Balazs, G. H., Work, T. M., Jones, T. T., Orski, S. V., Rodriguez, V., Beers, K. L., Brignac, K. C., Hyrenbach, K. D., Jensen, B. A., & Lynch, J. M. Polymer Identification of Plastic Debris Ingested by Pelagic-Phase Sea Turtles in the Central Pacific. Environmental Science & Technology 2018, 52, 11535-11544. https://pubs.acs.org/doi/10.1021/acs.est.8b03118
Properly managing plastic waste is a major global challenge, and its current mismanagement has severe ecological consequences. Ingestion of plastic fragments by sea turtles is an active area of research that can provide insight into what plastics are in our oceans and where they are located. For the first time, a recent study by Melissa R. Jung at Hawai’i Pacific University and collaborators linked the types of plastics consumed by sea turtles with the behavior and characteristics of different turtle species. In addition to providing a better understanding of which plastics pose the largest threat to sea turtles, identification of plastic type enables determination of general sources of mismanaged waste.
Not all plastics are created equal
Plastic materials are made up of long chain-like molecules called polymers. The properties and applications of a given plastic are largely determined by the type of polymer(s) it is made out of, also known as its composition. Different polymers can also vary in their ability to attract, and thus accumulate and transfer, pollutants. The table below lists some of the most commonly produced polymers, their resin identification code,* and a few (but certainly not all) of their applications.
*Side-note on resin ID codes and recycling: A resin identification code is the number you often see associated the recycling symbol on a plastic item. This identifies the composition of a plastic. It is important to note that, just because a plastic material has a resin ID code and recycle symbol, does not mean that it should be recycled in your area. For example, many biodegradable plastics, which fall under #7 (“other”), should not be placed in the recycling bin. In the United States, to see what types and forms of plastics can be recycled in your area, search “plastic” by your zip code here.
One property of particular interest in this study is density. Polymer density, relative to that of sea water, will largely determine whether a plastic floats or sinks in the ocean. This is important in determining the fate and transport of polymers in the ocean: floating plastics will travel far from their source and interact with sunlight, while sinking plastic will stay closer to their source and reside on the sea floor. Floating plastic is largely composed of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP), while polyethylene terephthalate (PETE) and polyvinyl chloride (PVC) usually sink. Polystyrene (PS) and polyurethane (PU) are denser than sea water, but they are often produced as foams (think styrofoam and seat cushions), which contain a lot of air pockets, so they can float or sink depending on how they are produced. However, interactions with ocean biota, debris, and currents can complicate the transport and buoyancy of plastic fragments… meaning the depth at which plastics reside in the ocean is difficult to predict.
What plastics are sea turtles eating?
Plastic samples were collected from the digestive tracts of 50 sea turtles (37 olive ridley, 9 green, and 4 loggerheads) caught as bycatch by longline fisheries in the central Pacific Ocean. These ingested plastics ranged from entire snack bags to microplastics, which comprised approximately 20% of the fragments collected. The ingested plastics were sorted by turtle species and examined in the laboratory to determine their composition. Since each of the sea turtle species forage at different depth ranges in the ocean, the researchers used these results to determine plastic composition profiles occurring at different depths, shown in Figure 2.
The predominant polymers ingested in all three species were LDPE, HDPE, and “unknown” PE (“unknown” just means they could not identify the type of PE from the sample)—all of which should float near the surface of the ocean. In fact, these various forms of PE, along with PP, combined to be 97.1% of the total plastics ingested, by mass. The dominance of PP and PE is consistent with previous measurements of plastic composition at the ocean surface. In addition, the olive ridley turtles, known to forage deepest of the three species, ingested slightly more sinking polymers than the green and loggerhead. These observations support the idea that the composition of plastics varies with depth in the ocean and is reflected by sea turtle “diet”.
Not all manufactured plastic is ingested
Interestingly, the composition of plastic ingested by sea turtles does not align with the composition of plastic produced. Figure 3 compares the plastic composition of European Market Demand and U.S. municipal solid waste (both reflective of polymer production) to that found in the sea turtles. The relative ratios of PE and PP ingested by sea turtles were much larger than those produced. Also, PETE, Nylon, and PVC represented a much smaller, if not negligible fraction of the ingested plastics. These differences are likely in part due to disproportionate use and release of PE and PP by the aquaculture, fishing, and shipping industries. The likelihood of denser plastics to sink in the ocean would also mean they are less available for ingestion by sea turtles. Finally, PP and LDPE are not commonly recycled in the US, which would account for more litter reaching the oceans.
What did the turtles tell us?
Identification of the plastics ingested by sea turtles showed us that floating plastics, such as PE and PP pose the greatest threat to surface-dwelling wildlife. Since some of these floating plastics (like LDPE and PP) are less-commonly recycled in the US, attention should go into improving the technology and infrastructure required to improve their recyclability and post-use management. Focusing our efforts on these plastics could help slow the accumulation of plastic in our oceans, prevent its ingestion by wildlife, and solve this waste management problem.