New Research Shows 'Neptune Balls' Help Remove Ocean Plastic
Adding to the long list of environmental services provided by seagrass is evidence that it helps to remove plastic from the world's oceans, according to a new report from scientists at the University of Barcelona.
Seagrass meadows are one of the most important coastal ecosystems. They provide nursery habitats, shelter and a source of food for a wide range of species; act as protective barriers for the shoreline and help to prevent coastal erosion; filter pollutants from the water and sequester as much as twice the amount of carbon dioxide as tropical rainforests. The value of seagrass is often overlooked, however, due to its relatively mundane appearance.
In a new study published in Scientific Reports, a team of researchers from the University of Barcelona studied fields of Posidonia oceanica, an endemic species of Mediterranean seagrass – also known as 'Neptune's grass' – at four beaches around the western Mediterranean island of Mallorca. As part of its natural lifecycle, the foliage shed by P. oceanica breaks down and is compacted into fibrous balls known as aegagropilae – or Neptune balls – by wave action, and eventually washed ashore.
The scientists found that, as the aegagropilae are formed, large amounts of small plastic particles are trapped within their fronds, and effectively removed from the water when the balls are washed onto the beach during storms. Half of the samples which the team collected between 2018 and 2019 were found to contain plastic particles, with up to 613 plastic items found in each kilogram of loose leaves.
Although only 17 per cent of the 198 Neptune balls collected were found to contain plastic particles, the amount of plastic within them was of a much greater density, with as many 1,470 plastic items per kilogram of plant material found entwined within the foliage. The size of the plastic pieces ranged from just over 1mm to 59mm in length, with an average of around 9.5mm.
While the percentage of aegagropilae found to contain plastic was quite low, the amount washed ashore combined with the material contained within the loose leaves led the researchers to estimate that up to 867 million pieces of plastic could be removed each year by seagrass from the Mediterranean alone.
At least 8 million tonnes of plastic find their way into the world's oceans each year, causing immeasurable damage to marine wildlife as they ingest plastic particles or become entangled in plastic bags or discarded fishing gear. Plastic has been discovered at even the deepest depths of the Mariana Trench and has even found its way into the human food chain through seafood, particularly shellfish.
Worldwide, seagrass meadows are in decline, with as much as 30 per cent being lost over the last 100 years. Around the UK, a 92 per cent reduction in seagrass extent has prompted a collaborative seagrass restoration project, part of which has led to the development of a 'Seagrass Spotter' app.
The new research highlights the importance of seagrass to the global environment, but more work needs to be done to determine how to safely dispose of the plastic that has been removed from the water by the seagrass. The aegagropilae could be collected and disposed of, but this may have negative consequences for the preservation of the beach and its resident wildlife.
'The fate of the [Neptune balls] is an open question,' writes lead author Anna Sanchez-Vidal, a marine biologist at the University of Barcelona. 'One possibility would be to remove the balls to eliminate their associated plastic debris. However, it would be difficult to remove the balls without removing the stranded leaf litter, which is known to protect beaches against erosion, provide nutrients for dune plants and feed beach arthropod communities.
'What happens to the plastic debris in the Neptune balls once ashore deserves further investigation.'
The complete report by 'Seagrasses provide a novel ecosystem service by trapping marine plastics.' by Anna Sanchez-Vidal, Miquel Canals, William P. de Haan, Javier Romero & Marta Veny, Sci Rep 11, 254 (2021), can be found at https://doi.org/10.1038/s41598-020-79370-3
If you are an avid diver or beach lover, you can help preserve the world's seagrass meadows by reporting your sightings using the Seagrass Spotter app, which will help to provide information about the global seagrass extent to be shared throughout the scientific community. Visit www.seagrassspotter.org for more