Accepting plastics’ role in a river’s carbon cycle

Rivers are a primary source of the plastics that clog oceanic ecosystems and contribute to the continent-sized Great Pacific garbage patch. Broken-down plastics cycle through ecosystems, accumulating nearly everywhere, from clouds to organisms to deep ocean trenches.

Stemming the source that spreads these pollutants is an important way to limit further harm, so scientists have turned to rivers to understand how plastics function in these ecosystems. Recently, researchers at Loyola University Chicago measured (Water Environ. Res. 2024, DOI: doi.org/10.1002/wer.11116) plastics that stick to rafts of sticks, leaves, and other debris on the Chicago River surface. Their work is an early step toward including plastic in carbon cycling and, ultimately, measurements of harmful carbon emissions contributing to climate change.

This approach is a mindset shift, requiring that we see plastics as a permanent part of ecosystems and includes, perhaps, “a degree of acceptance,” says Tim Hoellein, an aquatic ecologist who led the study. “We need to consider [plastic] as part of the environment, permanently, going forward, and measure it the same way we measure other components in the environment.”

Researchers selected a 500-meter stretch along the North Branch of the Chicago River known to have free-floating rafts of debris, then sampled surface waters up to 5 cm deep using a sieve. After rinsing, separating, drying, and weighing the samples, scientists used infrared-light spectroscopy to determine the content of the debris, including microscopic fragments of plastics and larger pieces, based on how light was absorbed or reflected by each material.

“What’s new here is quantifying carbon [from plastics] and natural organic matter together for the first time,” says Wilfred Wollheim, an ecosystem ecologist at the University of New Hampshire who has collaborated with Hoellein on other projects. Wollheim was not involved in this study.

Scientists found six polymers: acrylic, polyethylene, PET, polypropylene, polystyrene, and thermoplastic rubber—each with its own implications for future harm. Polystyrene, for example, was the most abundant plastic in the rafts—and it’s a cause for concern because it easily breaks down into smaller plastic “beads” that disperse quickly and are difficult to remove. But any plastic in rafts can be a hazard to small organisms and fish that are attracted to them, among other problems.

“Initially, this field of work was centered on oceans” and concerns about aesthetics, entanglement, and wildlife harms, Hoellein says of carbon cycling research. “Considering plastic as part of a carbon cycle in a river . . . as a part of the ecosystem and part of the carbon cycle is a step past thinking about its toxicological effects.”

Because currents and other river conditions vary widely, Hoellein says these early findings cannot yet be applied elsewhere.

“The study is an important step toward quantifying how much carbon there is in the environment from plastic,” says Lisa Erdle, director of science and innovation at the nonprofit 5 Gyres Institute, whose leaders seek to limit plastic pollution. While we’re far from quantifying total plastic in the environment, “with research like this, we can start to piece the story together.”