Underwater plants in a shallow sea

Seagrass meadows could help remove plastic from the oceans

Image credit: John Mark Arnold | Unsplash

Underwater seagrass meadows may trap, extract and carry marine plastic debris to shore, thereby helping to remove plastic litter from the sea, according to a study published in Scientific Reports.

Previous research had suggested that most plastics end up in the seafloor and that some are washed back to shore; however, how this occurs was unclear.

Seagrass meadows are widespread in shallow coastal waters and are involved in trapping and binding sediment particles that form the seabed. These seagrass meadows also provide important ecosystem services and benefits, such as water quality improvement; CO2 absorption; climate change mitigation; sediment production for seafloor and beach stabilisation; coastal protection; nursery and refuge areas for many species, and support in fisheries production.

To assess the role that seagrass may have in trapping and removing marine plastic, Anna Sanchez-Vidal and colleagues from the University of Barcelona, Spain, measured the amount of plastic debris collected from seagrass litter from four beaches in Mallorca, Spain, between 2018 and 2019.

Mallorca has extensive seagrass meadows and high levels of plastic near the shore. The authors found plastic debris among 50 per cent of 42 loose seagrass leaf samples and intertwined in 17 per cent of 198 balls of seagrass fibres, known as aegagropilae or Neptune balls. Up to 613 and 1,470 plastic items were found per kilogram of loose leaves and Neptune balls, respectively.

Using this data and estimates of seagrass fibre production in the Mediterranean, the authors propose that Mediterranean seagrass meadows may trap up to 867 million plastic items in Neptune balls alone each year, although the number of these carried to shore and the fate of plastic once washed ashore is unknown.

seagrass meadows tackling microplastics illustration

Image credit: Anna Sanchez-Vidal

Microplastics - plastic particles smaller than 5mm in size - derive from fragmentation and degradation of large plastic items, and also from direct manufacturing of microscopic particles such as virgin plastic pellets, cosmetic microbeads and clothing microfibres.

Research on microplastic pollution has previously focused on sea surface accumulations. However, there is a growing body of evidence that floating plastic debris account for less than 1 per cent of the global ocean plastic inventory, whereas the vast majority sinks to the seafloor.

Microplastics have been found in all marine environments, shallow and deep, close to shore and amidst ocean basins. Recent studies have also shown that bottom currents control the distribution of microplastics on the seafloor, transporting them from shallow to deep waters where they accumulate.

The University of Barcelona study has shown evidence of the entrapment of plastic debris from the shallow marine environment by seagrasses, representing a continuous purge of plastic debris out of the sea that has been omitted in surface (nearshore to offshore) and bottom (shallow to deep) simulations of microplastics transport.

The research team's findings suggest that seagrass meadows may help counteract marine plastic pollution. As previous research found that seagrass areas in the Mediterranean Sea have decreased by up to 50 per cent since 1960, seagrass meadow conservation should remain a priority, according to the authors.

The full study was published in Nature, 'Seagrasses provide a novel ecosystem service by trapping marine plastics'.

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