This Seagrass Traps Marine Plastic

Researchers find the Mediterranean species of seagrass collects plastics in fibrous balls that form from its fallen leaves

Posidonia oceanica seagrass
Researchers estimate this seagrass found in the Mediterranean could trap about 867 million plastic pieces per year in coastal areas. University of Barcelona / Jordi Regàs

New research suggests a type of Mediterranean seagrass naturally captures plastic pollution, reports Donna Lu for New Scientist.

The study, published this week in the journal Scientific Reports, estimates that beds of this type of seagrass, called Posidonia oceanica, may collect up to 867 million bits of plastic in the Mediterranean annually.

The marine plants accomplish this feat somewhat by accident, according to Agence France-Presse (AFP). When blades of P. oceanica fall or break off their fibers can form tangled masses called Neptune balls. These balls look a bit like brown clumps of steel wool, but researchers have found that Neptune balls have a knack for trapping small fragments of plastic and then washing ashore during storms.

“We show that plastic debris in the seafloor can be trapped in seagrass remains, eventually leaving the marine environment through beaching,” Anna Sanchez-Vidal, a marine biologist at the University of Barcelona and the study’s lead author, tells AFP.

Neptune balls
When blades of the seagrass break or fall off, their fibers can form tangled oblong masses that are called Neptune balls. Researchers discovered that these fibrous balls have a knack for trapping plastics. University of Barcelona

Around 8 million tons of plastic end up in the world’s oceans every year, according to the International Union for Conservation of Nature, enough to account for 80 percent of all marine debris. In the ocean, wayward plastics take decades to degrade and harm more than 800 species in myriad ways during their tenure, according to a 2016 United Nations report. Plastics can ensnare and trap marine life or end up in the stomachs of creatures large and small. Since these plastic fragments are indigestible, they can build up and cause starvation and death simply by blocking or filling up the gut.

The world’s growing awareness of marine plastic pollution has led to some multi-million dollar efforts to physically remove it, but, at least along the coast, this new research suggests seagrass could be a powerful and low-cost ally in the fight against ocean plastic.

To test the plant’s power to sequester debris, the researchers quantified the plastic collected in seagrass on four beaches on the Spanish island of Mallorca between 2018 and 2019, per New Scientist.

Half of the 42 samples of loose seagrass contained bits of plastic, with up to 613 individual pieces per kilogram (2.2 pounds), the researchers report. Though only 17 percent of the 198 Neptune balls contained plastics, the ones that did had a lot—each kilogram of seaball contained close to 1,500 pieces, almost three times more plastic-trapping potential than loose plant material.

Per AFP, the team arrived at their estimate of 867 million pieces of plastic potentially being captured by seagrass each year by building on prior estimates of seagrass fiber production in the Mediterranean.

Though the researchers focused on the Neptune balls that washed ashore, they can’t be sure if that’s where the bulk of them end up.

“We don’t know where they travel,” Sanchez-Vidal tells AFP. “We only know that some of them are beached during storms.”

Sanchez-Vidal tells New Scientist that her team’s findings suggest conserving these underwater meadows could provide substantial reductions in coastal ocean plastics in addition to seagrasses other benefits including absorbing carbon dioxide and providing habitat for fishes.

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