The ocean is full of tiny plastic particles – we found a way to track them with satellites

Plastic is the most common type of debris floating in the world’s oceans. Waves and sunlight break much of it down into smaller particles called microplastics – fragments less than 5 millimeters in diameter, roughly the size of a sesame seed.

To understand how microplastic pollution affects the ocean, scientists need to know how much there is and where it accumulates. Most of the data on microplastic concentrations come from commercial and research vessels towing plankton nets – long, cone-shaped nets with very fine mesh designed for collecting marine microorganisms.

But net trawling can only sample small areas and may underestimate the true concentrations of plastic. With the exception of the North Atlantic and North Pacific gyres – large areas where ocean currents rotate, collecting floating debris – scientists have done very little sampling for microplastics. And there is little information on how the concentrations of these particles vary over time.

To answer these questions, Madeline Evans, a research assistant at the University of Michigan, and I developed a new way to detect microplastic concentrations from space using the Cyclone Global Navigation Satellite System. from NASA. CYGNSS is a network of eight microsatellites launched in 2016 to help scientists predict hurricanes by analyzing tropical wind speeds. They measure how rough the wind makes the ocean surface – an indicator that we realized could also be used to detect and track large amounts of microplastics.

In search of smooth areas

The annual global production of plastic has increased every year since the 1950s, reaching 359 million metric tonnes in 2018. Much of it ends up in open and uncontrolled landfills, where it can flow into drainage areas. rivers and ultimately in the oceans of the world.

Researchers first documented plastic debris in the oceans in the 1970s. Today, it accounts for about 80-85% of marine litter.

CYGNSS satellite radars are designed to indirectly measure winds over the ocean by measuring how rough they are on the water surface. We knew that when there is a lot of material floating in the water, the winds make it less rough. So we tried to calculate how much smoother measurements indicated the area than it should have been if winds of the same speed were blowing over clear water.

This anomaly – “missing roughness” – is found to be strongly correlated with the concentration of microplastics near the ocean surface. In other words, areas where surface waters appear to be unusually smooth frequently contain high concentrations of microplastics. The softness could be caused by the microplastics themselves, or maybe something else associated with them.

By combining all the measurements taken by CYGNSS satellites orbiting the world, we can create time-lapse global images of ocean microplastic concentrations. Our images easily identify the Greater Pacific Waste Zone and secondary regions with high microplastic concentrations in the North Atlantic and Southern Oceans.

Monitoring of microplastics flows over time

Since CYGNSS constantly tracks wind speed, it allows us to see how the concentrations of microplastics change over time. By animating a year of images, we revealed previously unknown seasonal variations.

We have found that global concentrations of microplastics tend to peak in the North Atlantic and Pacific during the summer months of the northern hemisphere. June and July, for example, are peak months for the Great Pacific Garbage Patch.

Concentrations in the southern hemisphere peak during the summer months of January and February. The lower concentrations during winter in both hemispheres are likely due to a combination of stronger currents that break up plumes of microplastics and increased vertical mixing – the exchange between surface water and deeper water – which carries some of the microplastic below the surface.

This approach can also target smaller regions for shorter time periods. For example, we looked at episodic exit events from the mouths of the Chinese rivers Yangtze and Qiantang where they empty into the East China Sea. These events may have been associated with increases in industrial production activity or with increases in the rate at which managers allowed rivers to cross dams.

Better targeting for cleanings

Our research has several potential uses. Private organizations, such as The Ocean Cleanup, a non-profit organization in the Netherlands, and Clewat, a Finnish cleantech company, use specially equipped vessels to collect, recycle and dispose of marine litter and debris . We have started conversations with both groups and hope to eventually help them deploy their fleets more effectively.

Our spatial imagery can also be used to validate and improve numerical prediction models that attempt to track how microplastics move through the oceans using ocean circulation models. Researchers are developing several of these models.

While the ocean roughness anomalies we have observed correlate strongly with microplastic concentrations, our concentration estimates are based on the correlations we have observed, and not on a known physical relationship between floating microplastics and the roughness of the ocean. It could be that the roughness anomalies are caused by something else that also correlates with the presence of microplastics.

One possibility is that of surfactants on the surface of the oceans. These liquid chemical compounds, which are widely used in detergents and other products, move through the oceans similarly to microplastics, and they also have a damping effect on the roughness of the oceans due to wind.

Further study is needed to identify how the smooth areas we have identified occur, and whether they are caused indirectly by surfactants, to better understand exactly how their transport mechanisms relate to those of microplastics. But I hope this research can be part of a fundamental shift in the monitoring and management of microplastic pollution.

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