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'It's a way of the sea returning the trash to us': Why plastic-filled 'Neptune balls' are washing up on beaches

As tiny pieces of plastic clog our oceans,poker view natural meadows of seagrass are bundling up microplastics and spitting them back out onto beaches in the form of "Neptune balls".

Neptune balls, or Posidonia oceanica, are round, compact bundles of seagrass mostly found in the Mediterranean Sea. For centuries, Posidonia has been used for packaging, bedding and even insulation for houses.

But researchers from the University of Barcelona have found these spongy balls are spontaneously performing another function: mopping up ocean plastic from the seafloor.

In the ocean, microplastics – particles under 5mm in size – often originate from items such as plastic bags, bottles and fishing nets. These plastic fragments can harm our health, affecting everything from bone and brain function to hormones (read more about how microplastics affect our health).

Although most plastic pollution originates on land, the ocean – including seagrass meadows – acts as a sink.

Posidonia leaves slow water down as it flows past, says Anna Sanchez-Vidal, lead author of the Barcelona study. "There are fewer currents in seagrass meadows, so they trap carbon and sediment, and act as a refuge for biodiversity."

But these swaying underwater meadows also accumulate higher concentrations of plastic. Each year, between 1.15 and 2.41 million tonnes of plastic flows from rivers to the sea. If a river enters the sea where Posidonia is growing, some of that plastic becomes caught, and accumulates. But not all of this plastic stays trapped in swaying Posidonia meadows.

Each autumn, Posidonia sheds its leaves. These fibrous strands, rich in the tough organic polymer lignin, tangle together in dense balls. "As they move, they transport plastic intertwined within the fibres," says Sanchez-Vidal. The researchers estimated that seagrass meadows may catch nearly 900 million plastic fragments in the Mediterranean every year.

In 2018 and 2019, Sanchez-Vidal's team examined seagrass balls washed up on four beaches on the island of Mallorca, Spain. On the shores of Sa Marina, Son Serra de Marina, Costa dels Pins and Es Peregons Petits, they found plastic debris in half of the loose seagrass leaf samples, up to 600 fragments per kilogram (2.2lb) of leaves.

Only 17% of Neptune balls contained plastic, but where it was found it was densely packed – nearly 1,500 pieces per kilogram. Tighter bundled balls were more effective at trapping plastic.

"After our paper was published, a lot of people started sending me [pictures of] monster Neptune balls," says Sanchez-Vidal. These are balls that capture larger and more visible pieces of plastic.

"Sometimes they had sanitary towels, tampons, wet wipes – things with a lot of cellulose, so they sink," she explains: "No, I didn't really want to receive those pictures from everybody," she jokes.

Turbulent seas, particularly during storms and tidal patterns, can dislodge Neptune balls from the seafloor, says Sanchez-Vidal. Some Neptune balls drift into deeper waters, while others wash up on the shore.

"We say it's a way of the sea returning the trash to us that was never meant to be on the seafloor," says Sanchez-Vidal.

But Sanchez-Vidal stresses that Neptune balls are not a solution to the ocean's plastic problem. "We've never seen them as a remediation, or as a way to clean the trash from the sea," she says.

She urges anyone who finds Neptune balls to leave them where they are – on the beach or in the ocean. "The balls bring humidity and nutrients to the beach. If we throw them away, we're destroying this emergent beach ecosystem," she warns.

This is at a time when seagrasses are in global decline. One study found that the global area covered by seagrass had declined by 29% since the late 19th Century. Seagrasses are at risk from poor water quality, coastline development, invasive species, rising ocean temperatures, and rising sea levels. In the eastern Mediterranean, Posidonia oceanica is facing a persistent and increasing threat from heatwaves and industrial pollution. A related species in Australia, Posidonia australis, is also in decline despite conservation efforts.

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But in the Mediterranean, local efforts are still racing to restore seagrass, such as Red Eléctrica's Marine Forest in the Bay of Pollen?a, Spain, and the citizen scientist project Posidonia Gardeners in Sicily and Malta.

Seagrass meadows provide vital ecosystem services – improving water quality, absorbing carbon dioxide, protecting coastlines, and serving as nurseries and refuges for marine species. "But it's also not a solution to plant seagrass meadows everywhere to act as filters for plastic," Sanchez-Vidal adds.

Instead, she says, the real solution lies at the source. "We just need to prevent plastic from entering the sea – and that means reducing plastic production."

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