In this article, you'll learn:
- The gap between the estimated plastic entering our oceans and what surveys actually find
- What new evidence reveals about the surprising hiding spots for accumulated microplastics
- How fieldwork and advanced computer modeling help scientists track plastic pollution
About ten years ago, several studies offered the first robust estimates of the amount of plastic floating at the ocean’s surface. The results surprised scientists. Every year, an estimated 5 to 13 million metric tons of plastic were entering the ocean from rivers, long understood to be the primary source of ocean plastic. Yet surveys found only 0.007 to 0.25 million tons floating at the surface. Because most plastic floats, researchers expected a much larger accumulation. The gap between what should be there and what we can actually see became known as the “missing plastic paradox.”
How plastic pollution flows from land to rivers and the sea
To understand where the plastic might be going, scientists first examine how it travels. We have fairly good estimates of how much plastic is produced each year, how much is used, and how much ends up in landfills. From these figures, we can infer the amount of mismanaged plastic waste that enters the environment. What happens next is less clear. Plastic has many sources: fibers released from clothing, litter, tire wear, paints, and spills of manufacturing pellets called nurdles, to name a few.
Once in the environment, plastics reach rivers by runoff, sewage outfalls, and even atmospheric deposition. From there, they travel downstream into estuaries, where fresh and salty water mix, before entering coastal and open-ocean waters. Ocean currents transport plastic across vast distances, leading to its discovery in nearly every place researchers have sampled. Winds can also move synthetic fibers long distances and deposit them on the ocean surface. Despite this wide dispersal, the numbers still don’t add up — much of the expected plastic seems to vanish.
Where the missing plastic is hiding
Several theories attempt to explain this disappearance. The deep ocean, including the seafloor, is a likely destination, as plastics eventually sink after organisms such as bacteria or barnacles attach to them and increase their weight. Another possibility is that riverine plastic is intercepted before reaching the open ocean, trapped in estuaries, or deposited along coastlines. It is also possible that the paradox itself is overstated. Earlier studies may have either overestimated the amount of plastic entering the ocean or underestimated how much remains afloat. The challenge lies in working within a system as vast and variable as the global ocean.
How scientists track ocean plastic and microplastics
Solving the puzzle requires a diverse set of tools. Scientists collect samples from the water column and seabed, though analysis is difficult because plastic breaks into fragments too small to detect easily. We also rely on computer models to track how plastics move and transform, helping interpret observations and fill in gaps where no data exist.
Some of the most revealing work occurs at the intersection of models and field observations. In a study led by postdoctoral scholar Alex Lopez, we hypothesized that estuaries trap riverine microplastic particles less than 5 millimeters long and developed a computer model to support this idea. We then tested it in the field, sampling soils in a tidal marsh. A study led by graduate student Judy Bussarakum found microplastic accumulation dating back to the 1950s, when large-scale plastic production began.
At the global scale, direct observations and modeling have also provided possible solutions, though not always consistent ones. One study suggests most plastic resides in subsurface waters and ocean sediments. Another concludes there may be little paradox at all, estimating lower river inputs and higher surface-ocean concentrations than previously thought.
Why solving the missing plastic paradox matters
Ultimately, solving the missing plastic paradox is more than an academic exercise. Understanding where plastic goes will allow researchers to better predict the outcomes of different pollution-reduction strategies, from cutting production to improving waste management. But such predictions depend on models that can accurately simulate what has already happened, and those models need far more data. My opinion is that the greatest need is improved monitoring of both the vast ocean and the rivers that feed it.
Raymond Najjar is a professor of meteorology and atmospheric science whose research bridges oceanography, climate science, and hydrology. He is particularly interested in understanding how climate change and pollution are transforming coastal waters.
