The wonders of seagrasses

Marine ecosystems are incredible. Although they’ve been besieged by plastic – among many other human-generated challenges – they’re putting up a good fight.

Previously, Cosmos reported that mangroves help clean up microplastics. Now, Spanish researchers have found that seagrass meadows could purge hundreds of millions of plastic items from the seafloor to the shore each year in the Mediterranean Sea alone.

The grasses’ natural fibres aggregate and trap the plastic in balls, “which are then ejected and escape the coastal ocean,” write Anna Sànchez-Vidal, from the University of Barcelona, and co-authors in a paper in the Nature journal Scientific Reports.

This throws a whole new light on their value, says Sànchez-Vidal.

Seagrasses, which proliferate in shallow coastal waters, are one of Earth’s key ecosystems, she explains. They provide vital services such as improving water quality, mitigating climate change, stabilising the seafloor and protecting shorelines, providing nursery habitats and refuge for marine species and supporting fisheries and food security.

To investigate their role in capturing plastic, Sànchez-Vidal and colleagues sampled Posidonia oceanica, an endemic Mediterranean seagrass that forms rich meadows in seawater at depths of 50 centimetres to 40 metres, offshore from four Mallorca Island beaches between 2018 and 2019.

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Plastic-rich balls of seagrass fibers beached in Mallorca Island. Credit: Marta Veny

When the grasses shed their leaves in autumn, the sheaths erode to release lignocellulose-rich fibres. Swirling waters cause these fibres to aggregate and form aegagropilae, or “Neptune balls”, which are washed ashore during storms.

Sampling the balls, the researchers found up to 1470 plastic items per kilogram of plant material – mostly high-density plastic that had been trapped in seagrass beds on the ocean’s floor. The size of the plastics ranged in size from 1.05 to 59 millimetres, averaging 9.48.

They also found up to 613 plastic items per kilogram of dead leaves, representing half of loose seagrass leaf samples, ranging from 0.55 to 287 millimetres.

From their discovery and estimates of seagrass fibre production they calculate that Neptune balls alone could trap up to 867 million plastic items each year in the Mediterranean Sea, although they say it’s not clear how many are washed ashore.

The study may help fill some gaps in our understanding of plastic fluxes in the ocean.

“Research on microplastic pollution has long focused on sea surface and deep-sea accumulations,” says Sànchez-Vidal, “but there is a growing body of evidence that coastal sea ecosystems may play an important role by stranding and capturing plastics on their way to offshore.”

Yet seagrass meadows are regressing in many areas from human activities such as dredging, dumping and trawling as well as pollution and climate change. Noting that seagrass meadows have decreased in area by more than a third in the Mediterranean over the past five decades, the researchers say their finding underscores the importance of their conservation.

“Given the ever-increasing plastic load reaching our oceans, seagrass ecosystems such as P. oceanica meadows will play a crucial role,” they write.

“Therefore, in addition to the key and extensively documented ecosystem services provided by seagrass beds, P. oceanica may provide a valuable added plastic buffering and trapping service.”

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