Where is the Missing Plastic?

In the current Plastic Age we’re living in, uneasiness about the levels of plastic pollution in our oceans is at an all time high. But there’s a massive gap between the plastic that’s being created and the comparatively little amount that we’re finding in our oceans [1].

Is the amount of plastic in the ocean being overestimated? Adapted from flicr.com

There are some theories attempting to explain why this mysterious vanishing of ocean plastic is occurring [2], but most are lacking the critical evidence. The most popular among them is the idea which treats the ocean floor as if it were a sink where plastic accumulates after a brief transit through its depths. Seems reasonable enough, but the assumptions needed to support this claim have to pull a lot of weight. There are several other theories concerning the whereabouts of the missing plastic that need brief mentioning, as well as learning about how errors in methods used to determine the levels of plastic can overestimate the amounts being discharged from rivers. It’s worth looking at how little we know about plastics to begin with before we can be confident in the data we collect.

The little that is known about plastic is enough to give one pause. There’s a possibility we’ve been entirely overestimating how much plastic is entering the ocean to begin with. It was thought that the residence time - how long something sticks around in the ocean before being moved or degrading - of plastics in the ocean was strangely short. “We know next to nothing about timescales of biodegradation” says the oceanographer Kara Law. Although support for research has been slow, researchers have learned of blunders that might be to blame for these miscalculations, which might prove promising enough to dispel the notion that there is a sink of plastic covering the ocean floor.

Inconsistent methods

 For example, the way in which mass fluxes- the speed of mass traveling through flow– are calculated (in this case plastic) can be error-prone. Turns out when it comes to plastic, their numbers are being inflated by two or three orders of magnitude. It’s imperative that estimations of plastic levels be accurate in order to determine where resources should be prioritized. Instead of focusing on the sink problem or various other ideas attempting to explain high levels of plastic, efforts need to be made in correcting errors which give us inflated numbers to begin with. 

The amount of unobservable pollution creates an additional challenge. The reasons that make plastic difficult to observe are various. The obvious challenge microplastics pose to being identified are their size, which makes it difficult to predict their movements [3]. If the nature of microplastics seems trivial, think again. What they lack in size, they make up for in quantity[4]. For example, a study of the Mediterranean Sea near the coast of Italy found an average of 1.9 million particles per square meter in the water. What’s more is that this is thought to be representative of surrounding areas [5]. Plastic in such quantities may even have influence on our planetary cycles [6].

Macro-plastics on the other hand are found to be elusive for entirely different reasons. Compared with plastics made in the 1950s, modern plastics tend to be heavier than seawater. This causes them to sink to the ocean floor, which masks their location. These types of plastics also have the distinction of eventually becoming microplastics [7] unless their fate takes it under the sea first.

Another source of error is the representative mass being used to determine plastic levels. Microplastics, which are plastics that range in size from being as big as 5 mm to being smaller than bacteria tend to be calculated in terms of their concentration levels in the water being sampled. Knowing the amounts of plastics in water depends on being able to convert this into concentration into mass. 

Global distribution of microplastics. Adapted from flikr.com

Finally, using different size parameters to illustrate the amount of plastic in rivers causes data to be unreliable. The techniques that are used to collect samples of water are inconsistent. Microplastics are normally gathered using plankton nets [8] but there are a number of studies that collect them by using a filtration system. The filter pore sizes are smaller than the smallest mesh in a plankton net though. This means much more data is consistently collected when using filtration techniques. Major discrepancies occur when such small microplastics are included in the same mass conversions as with those that are collected via nets, which explains the gross overestimation in levels of plastic.

Incomplete picture

These errors have resulted in several ideas about what happens to this unaccounted plastic. Some of these are that plastic is being ingested by fish with the environmental consequences from this being unsettled [9], its degrading and eventually becoming microplastics, microbial alteration, and the sun’s radiation causing it to dissolve. What all these lack is comparisons with their river borne counterparts. 

Taking a closer look at how overestimation of plastic can influence ideas about where it must go, some researchers imply that the residence time of plastics in the ocean must be short enough to explain the discrepancy in lack of plastic pollution being observed. Instead, it is thought they may wind up sticking around in the water longer than previously thought. 

It needs to be restated how little we understand how much plastic is in our oceans. So much so that the latest numbers suggest that as much as 66.7% of plastic floating in the ocean today is unobservable [10]. Next to the frontier of space, the ocean remains largely a mystery. It’s imperative that we strengthen our knowledge on this front if we’re to mitigate our effect on it. One wonders why this isn’t being reported about in the media, but it might simply be that it’s doesn’t make quite as good of a story to report on what we don’t know. But the age-old headlines that speak to the levels of plastic in the ocean are built on outdated assumptions. 

We presume there is a sink of plastic pollution at the bottom of the ocean, because the sheer amount of plastic that’s determined to be discharging into coastal waters is so vast, that it becomes a game of clue. Perhaps we should look inland at the rivers, since they’re the source of nearly all microplastics entering the ocean [11]. The second largest river in Europe has more plastic in it than fish [12]. The bottom line is that humanity produces 300 million tons of plastic per year [13], with no signs of stopping in the near future. Finding where and how much is mismanaged will go a long way towards improving our understanding of its fate. 

References 

[1] A. Cózar et al., “Plastic debris in the open ocean,” Proceedings of the National Academy of Sciences of the United States of America, vol. 111, no. 28, pp. 10239–10244, 2014.

[2] L. C. Woodall et al., “The deep sea is a major sink for microplastic debris,” R. Soc. open sci., vol. 1, no. 4, p. 140317, Dec. 2014, doi: 10.1098/rsos.140317.

[3] S. Liubartseva, G. Coppini, R. Lecci, and E. Clementi, “Tracking plastics in the Mediterranean: 2D Lagrangian model,” Marine Pollution Bulletin, vol. 129, no. 1, pp. 151–162, Apr. 2018, doi: 10.1016/j.marpolbul.2018.02.019.

[4] M. Poulain et al., “Small Microplastics As a Main Contributor to Plastic Mass Balance in the North Atlantic Subtropical Gyre,” Environ. Sci. Technol., vol. 53, no. 3, pp. 1157–1164, Feb. 2019, doi: 10.1021/acs.est.8b05458.

[5] “Solving the mystery of missing ocean plastic. - EBSCO.” https://discovery.ebsco.com/c/3czfwv/viewer/pdf/w6pznwdoab (accessed Jun. 29, 2022).

[6] C. M. Rochman and T. Hoellein, “The global odyssey of plastic pollution,” Science, vol. 368, no. 6496, pp. 1184–1185, Jun. 2020, doi: 10.1126/science.abc4428.

[7] A. ter Halle et al., “Understanding the Fragmentation Pattern of Marine Plastic Debris,” Environ. Sci. Technol., vol. 50, no. 11, pp. 5668–5675, Jun. 2016, doi: 10.1021/acs.est.6b00594.

[8] K. L. Law et al., “Distribution of Surface Plastic Debris in the Eastern Pacific Ocean from an 11-Year Data Set,” Environ. Sci. Technol., vol. 48, no. 9, pp. 4732–4738, May 2014, doi: 10.1021/es4053076.

[9] R. C. Thompson et al., “Lost at Sea: Where Is All the Plastic?,” Science, vol. 304, no. 5672, pp. 838–838, May 2004, doi: 10.1126/science.1094559.

[10] A. Isobe and S. Iwasaki, “The fate of missing ocean plastics: Are they just a marine environmental problem?,” Science of The Total Environment, vol. 825, p. 153935, Jun. 2022, doi: 10.1016/j.scitotenv.2022.153935.

[11] R. Dris et al., “Beyond the ocean: contamination of freshwater ecosystems with (micro-)plastic particles,” Environmental chemistry (Online), vol. 12, no. 5, pp. 539–550, 2015, doi: https://doi-org.colorado.idm.oclc.org/10.1071/EN14172.

[12] A. Lechner et al., “The Danube so colourful: A potpourri of plastic litter outnumbers fish larvae in Europe’s second largest river,” Environmental Pollution, vol. 188, pp. 177–181, May 2014, doi: 10.1016/j.envpol.2014.02.006.

[13] L. C. M. Lebreton, J. van der Zwet, J.-W. Damsteeg, B. Slat, A. Andrady, and J. Reisser, “River plastic emissions to the world’s oceans,” Nat Commun, vol. 8, no. 1, p. 15611, Aug. 2017, doi: 10.1038/ncomms15611.