Lush meadows of underwater seagrass are removing plastic from the sea

Underwater seagrass may be naturally trapping millions of pieces of marine plastic and removing them from the sea.

Posidonia oceanica, a seagrass endemic to the Mediterranean, forms lush meadows on the sea floor in coastal waters up to 40 metres deep.

When P. oceanica sheds leaves, fibres in the leaf sheaths intertwine, forming tangles known as Neptune balls. Anna Sanchez-Vidal at the University of Barcelona in Spain and her colleagues have found that these balls trap plastic items.

“When there’s a storm, and these balls are ejected from the sea to land, the plastic also is ejected back to shore,” says Sanchez-Vidal.

Her team estimates that these Neptune balls may trap up to 867 million plastic items in the Mediterranean Sea every year.

Between 2018 and 2019, the team measured the amount of plastic collected from seagrass litter from four beaches in Mallorca, Spain, which has high levels of plastic near the shore, as well as widespread seagrass meadows.

The team found plastic debris in half of the 42 loose seagrass leaf samples they took, with up to 613 plastic items per kilogram of loose leaves.

Of the 198 Neptune balls of P. oceanica fibres that the team collected, 17 per cent had intertwined plastic items.

The finding points to the need for better conservation of seagrass meadows, says Sanchez-Vidal. “Strict measures should be taken to protect these systems,” she says.

In addition to their newly discovered role in trapping and removing plastic, seagrass meadows are also an important reservoir of carbon dioxide and sediment, and a nursery area for many marine animals.

P. oceanica is found only in the Mediterranean Sea, but other related seagrass species are found in shallow waters off the coast of Australia. It is unclear whether other species are able to form Neptune balls and function similarly in removing plastic.

Journal reference: Scientific Reports, DOI: 10.1038/s41598-020-79370-3

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