Underwater seagrass in coastal areas appears to trap plastic pollution in natural bundles of fibre known as “Neptune balls”, researchers have found. The swaying plants, anchored to shallow seabeds, may collect nearly 900 million plastic items in the Mediterranean alone every year, according to a study published in the journal Scientific Reports.
“We show that plastic debris in the seafloor can be trapped in seagrass remains, eventually leaving the marine environment through beaching,” lead author Anna Sanchez-Vidal, a marine biologist at the University of Barcelona, told AFP. This clean-up “represents a continuous purge of plastic debris out of the sea,” she added.
The study adds to the long list of services that seagrass provides for ocean ecosystems and humans. They play a vital role in improving water quality, absorbing CO2, and exuding oxygen, and are a natural nursery and refuge for hundreds of fish species. By anchoring in shallow waters, they help prevent beach erosion and dampen the impact of destructive storm surges.
To better understand the plastic bundling capabilities of seagrass, Sanchez-Vidal and her team studied Posidonia oceanica, a species found only in the Mediterranean. In 2018 and 2019, they counted plastic particles in seaballs that had washed up on four beaches in Mallorca, Spain. Plastic debris was found in half of the loose seagrass leaf samples, up to 600 bits per kilogram of leaves, while only 17% of the tighter bundled Neptune balls contained plastic, but at a much higher density—nearly 1,500 pieces per kilogram.
The oval orbs form from the base of leaves shredded by ocean currents but remain attached to stems called rhizomes. As they are slowly buried by sedimentation, the damaged leaf sheaths form stiff fibres that intertwine into a ball, collecting plastic in the process. “We don’t know where they travel,” said Sanchez-Vidal. “We only know that some of them are beached during storms.”
