What’s happening? Underwater seagrass meadows may be naturally trapping millions of fragments of marine plastic and removing them from the oceans, according to a study from the University of Barcelona.
Fibres in the leaves shed by the Posidonia oceanica seagrass, endemic in the Mediterranean and found in coastal waters up to 40 metres deep, create tangles known as Neptune balls that can trap the plastic, the study reveals.
When the balls are ejected from the sea during storms, they also return the plastic to the shore. Plastic items were also found in 50% of the loose leaves sampled. Why does this matter? This novel ecosystem service provided by seagrass meadows could aid efforts to combat plastic pollution in oceans, which not only affects the growth and survival of marine species, but also food webs and human health. The world’s oceans contain around 150 million mt of plastic, with an additional eight million mt added each year. The scale of microplastic pollution has in the past been significantly underestimated due to measurement methods missing smaller fragments – last year it was found there is at least twice as many plastic particles in the oceans than previously reported.
Although high-level commitments to reduce the use of some plastic items are gaining traction from countries such as China and the UK, removing plastics already in global waters on a large scale has not yet been addressed.
The University of Barcelona study found high concentrations of plastic debris present in 17% of the Neptune balls sampled, reaching up to 1,470 plastic items per kg of seagrass. Neptune balls are formed when lignocellulosic fibres from seagrass leaf sheaths are released, which can interlace with plastics when the ball-shaped clusters are shaped.
Natural wave action aids the accumulation and compaction of the balls, also enabling an outer, hard shell to form from repeated impact with the seabed and sudden sea motion changes. Researchers estimate that Neptune balls could trap up to 867 million items of plastic debris in the Mediterranean annually. This newly discovered plastic-trapping ability of seagrass, however, could be hindered by the shrinkage of these ecosystems. At least 22 of the world’s 72 seagrass species are in decline from threats including climate change, unregulated fishing practices and pollution. Benefiting from this nature-based solution on a larger scale requires efforts to conserve seagrass habitats. A seagrass restoration project in the UK, for instance, aims to restore eight hectares of lost seagrass around UK waters by initially cultivating the plants in a laboratory. The cultivation of such meadows can also provide other important ecosystem services, such as carbon sequestration, coastal protection and nurseries for marine species.
The potential for seagrass meadows in other locations – such as related species near the coast of Australia – to provide similar functions gathering and trapping plastic remains unclear.
Elsewhere, a study by the Plymouth Marine Laboratory is underway to explore if the natural feeding process of mussels can offer a similar natural solution. The project, funded by Waitrose’s Plan Plastic: The Million Pound Challenge, will investigate whether ‘bioreefs’ of mussels could be harnessed to remove microplastics from marine environments.