Today, researchers at the University of Michigan have developed a new way to detect and track microplastics in the world’s oceans over time, providing a daily timeline of where they enter the world, the water, how they move, and where they tend to congregate. This approach is based on the Cyclone Global Navigation Satellite System (CYGNSS) and can move or zoom in small areas to obtain a high-resolution image of microplastic releases from one location.
This technique is a significant improvement over current monitoring methods, which rely primarily on intermittent reports of plankton net trawlers catching microplastics with their catch.
said Chris Rove, Frederic Bartmann Professor of Climate and Space Sciences at UM, Director of Surveys at CYGNSS and lead author of a recently published paper on the work.
The season changes in the Great Pacific Garbage Patch
The team found that global concentrations of plastic particles tend to vary seasonally, peaking in the North Atlantic and Pacific Oceans during the northern hemisphere summer months. June and July, for example, are the peak months for the Great Pacific Garbage Patch, a contiguous region in the North Pacific where plastic particles accumulate in huge quantities. Concentrations in the Southern Hemisphere reach their peak during the summer months of January and February. Concentrations tend to be lower during the winter months, possibly due to a combination of stronger currents penetrating columns of microplastics and increased vertical mixing that pushes them below the water’s surface.
The data also showed several short spikes in the concentration of plastic particles at the mouth of the Yangtze River – long considered the main source.
“Being wary of the source of microplastic contamination is one thing, but seeing it happen is another,” Rove said. “Data on microplastics that were available in the past was extremely scarce, and only brief snapshots could be reproduced.”
The researchers produced visualizations showing concentrations of plastic particles around the world. Areas of accumulation are often the result of prevailing local currents and areas of convergence, the Great Pacific Garbage Patch being the most extreme example.
“What makes plumes in large estuaries so noteworthy is that they are a source in the ocean, as opposed to places where microplastics accumulate,” Rove said.
Rove says the information could help organizations that clean up microplastics deploy ships and other resources more efficiently. The researchers are already in talks with Dutch cleaning organization The Ocean Cleanup to work together to validate the team’s initial findings. Single release data may also be useful for the United Nations agency, UNESCO, which sponsored a working group to find new ways to track the release of microplastics into the world’s waters.
Hurricane-tracking satellites set their sights on plastic pollution
Developed by Ruf and UM Madeline C. Evans, the tracking method uses current data from CYGNSS, a small eight-satellite system launched in 2016 to monitor weather conditions near the core of large storm systems and improve predictions of their intensity. Rove leads the CYGNSS mission.
Key to this process is the roughness of the ocean’s surface, which CYGNSS actually measures with radar. The measurements were mainly used to calculate wind speeds near the eyes of hurricanes, but Rove wondered if they had other uses as well.
“We took these radar measurements of surface roughness and used them to measure wind speed, and we knew that the presence of the materials in the water alters their response to the environment,” Rove said. “So I came up with the idea of doing everything in reverse, using changes in response to predicting that there are things in the water.”
Using independent measurements of wind speed from NOAA, the team looked for places where the ocean appeared to be less choppy than it should be given the wind speed. They then matched these regions with actual observations of plankton trawl nets and ocean current models that predict microplastic migration. They found a significant correlation between areas that are softer and those with more plastic particles.
converging ocean currents
Rove’s team believes that changes in ocean roughness may not be directly caused by the microplastics themselves, but rather by surfactants – a family of oily or soapy compounds that reduce surface tension at the surface of a liquid. Surfactants tend to accompany microplastics in the ocean, because they are often released along with microplastic particles and because they likewise travel and accumulate once in water.
“There are areas with high concentrations of microplastics, such as the Greater Pacific Garbage Region, because they are found at the confluence and eddies of ocean currents. Microplastics are transported by the movement of water and eventually gather in one place,” Rove said. Surfactants do the same way, and it’s very likely that they act as a kind of microplastics tracker.”
The research team is currently testing this hypothesis, working with marine engineering and assistant professor of marine engineering Yulin Pan to conduct experiments in a wave-generating tank at the Marine Hydrodynamics Laboratory at Aaron Friedman.
“We can see the relationship between surface roughness and the presence of microplastics and surfactants, so the goal now is to understand the exact relationship between the three variables, as well as the reasons behind them,” Ban said. “The wave tank and its ultrasonic sensors allow us to focus on these relationships by taking measurements under very precisely monitored surfactant and microplastic conditions.”