Sea (Plastic) Levels are Rising!

Source:  Ostle, C., Thompson, R., Broughton, D., Gregory, L., Wootton, M., and Johns, D. Nat. Comm. 10, 1622 (2019). https://www.nature.com/articles/s41467-019-09506-1

 

While plastics have many useful contributions to society, single-use plastics are becoming a major pollution issue as a result of waste mis-management and their tendency to stick around in the environment for a very long time. It’s been estimated that this mis-management of waste leads to millions of metric tons of plastic entering the oceans each year.1 Despite the extraordinary amount of plastic entering the oceans, quantifying the amount of plastic in the oceans has been a real challenge, especially measuring how that amount has changed over time.

Figure 1. Macroplastics washed up on a beach. Source
Plastics of Different Sizes

Microplastics, or small pieces of plastic ranging from 100nm-5mm in size, have been all over the news lately for being found in human feces, drinking water, and the environment. But what are macroplastics? Simply said: big pieces. Macroplastics refer to pieces of plastic larger than 5mm, and essentially the remaining fraction of plastics found in the environment. These include your identifiable plastic bags, styrofoam, soda bottles, and fishing line… to name a few. Being larger, macroplastics are a lot easier to see than microplastics, but plastic pollution in our oceans and waterways is comprised of both. With the rise of plastic pollution as a major environmental issue, a group of researchers from Plymouth, UK recently used macroplastics as a way to estimate how the relative amount of plastics in the North Atlantic has changed with time. Since macroplastics usually eventually break down (due to sun exposure and abrasion) into microplastics, their results also indirectly inform us about microplastics.

How to measure plastic pollution? One scientist’s problem is another scientist’s data.

Accurate measurement of plastic concentration in the oceans is difficult—it’s a very large area to sample, and data collection needs to be uniform and unbiased to get the most representative results. Scientists have to develop creative ways to get around this problem, such as studying plastics swallowed by sea turtles.

Wanting to know how plastic concentration changes with time further complicates data collection, because it requires another layer of information. So, how did researchers find information on plastics in the ocean spanning six decades when this issue has only gained attention fairly recently? Apparently plankton in the ocean has been measured since 1931 using a torpedo-shaped device called a Continuous Plankton Recorder (CPR). This CPR has been dragged behind ships-of-opportunity (meaning volunteer boats that are already planning a voyage), following very standardized protocols (such as dragging depth and speed), across the Northern Atlantic for decades. Its routes are shown in grey in Figure 2 and cover over 6.5 million nautical miles. This provides a very large sample area over which to monitor ocean plastics.

Figure 2. Map of Continuous Plankton Recorder sampling (grey) and macroplastic detection (colored circles) in the Northern Atlantic Ocean and adjacent seas. Copied without modification from Ostle et al. under a Creative Commons License.

Occasionally, the CPR fails because it has been entangled in something floating in the water. This failure, and the reason for it, are then recorded in a log… which has been kept for 6 decades! By searching through this log, Ostle and coworkers could determine when the floating entangler was a piece of plastic. They counted the number of failures due to plastic over time, which was representative of the amount of plastic in the ocean. The same technique has been used previously to monitor plastic in the ocean prior to 1990,2 but this study was the first to investigate 1990-2016.

The amount of plastic in the ocean is increasing

Figure 3 compares the number of entanglements of the CPR with plastic (blue bars) to the number of entanglements with natural material (grey bars), such as seaweed. Clearly there has been an increase in the number of failures due to plastic over time, compared to natural materials (which have stayed the same). This shows that there is an increasing amount of plastic in the ocean. Furthermore, since commodity plastics take a very long time to degrade, and then only to smaller and smaller plastics, we can assume that the pieces of macroplastic found in the past are still in there! Therefore, the cumulative amount of plastic (shaded area) can be taken to represent the total plastic in the ocean, and it’s increasing. Big time.

Figure 3. Annual counts of macroplastics (blue bars), natural material (grey bars), and cumulative sum of macroplastics (shaded green; representing the buildup in the oceans) detected in the Northern Atlantic. Adapted from Ostle et al. under a Creative Commons License.

What can we do to slow the accumulation of plastic in the environment? Apart from advocating for new technologies and better waste management programs, we can do our best to manage our waste. This can involve: reducing our consumption of single-use plastic, recycling what we can, and picking up some litter in our neighborhood. Every little bit helps.

Other Sources:

  1. Jambeck, J., Geyer, R., Wilcox, C., Siegler, T., Perryman, M., Andrady, A., Narayan, R., and Law, K. Science 347, 768 (2015).
  2. Thompson, R., Olsen, Y., Mitchell, R., Davis, A., Rowland, S., John, A., McGonigle, D., and Russell, A. Science 304, 838 (2004).

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Mary Davis

I earned my PhD in Chemical Engineering from Princeton University in 2018, where my research focused on nanoscale polymer systems and how their properties change with geometry. I am now applying my background in polymers to environmental systems as a postdoctoral research associate at the U.S. EPA. This involves studying the breakdown of plastics and the generation of microplastics in the environment, as well as their interactions with other pollutants. When I’m not working in the lab, I enjoy crafting, cooking, and being outside.

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