In the Mediterranean Sea, the Posidonia oceanica meadows are serving an unexpected function: capturing microplastics from the seabed and returning them to the beaches in the form of Neptune balls, compact vegetal structures that form naturally every autumn.
This phenomenon was documented by a team from the University of Barcelona, led by the oceanographer Anna Sánchez-Vidal, who warns that, although surprising, it should not be considered a solution to the plastic pollution problem.
What are Neptune balls?
Vegetal formations emerging from the sea with plastic fragments trapped in their fibers.
Neptune balls are round clusters of fibrous leaves of Posidonia, a marine plant endemic to the Mediterranean. These leaves, rich in lignin, detach every autumn and intertwine forming balls that, as they move along the seabed, trap plastic fragments.
In studies conducted between 2018 and 2019 on the beaches of Mallorca, up to 1,500 pieces of plastic per kilogram were found in the densest balls.
A vegetal sink that filters plastic from the ocean
Marine meadows slow down currents and accumulate sediments, carbon, and waste.
The Posidonia leaves reduce water velocity, allowing sediments, carbon, and microplastics to accumulate. Every year, between 1.15 and 2.41 million tons of plastic reach the sea from rivers.
If these flow into areas with marine meadows, part of the plastic gets trapped. Sánchez-Vidal’s team estimates that the meadows could retain up to 900 million plastic fragments per year in the Mediterranean.
The importance of Neptune balls on the beaches
Restoration and conservation: local initiatives against global decline
Community projects aim to recover the ecological role of Posidonia in the Mediterranean.
Despite their importance, marine meadows are in decline. Since the 19th century, the global surface covered by seagrasses has decreased by 29%, affected by pollution, coastal development, invasive species, and climate change.
In the eastern Mediterranean, Posidonia faces heatwaves and industrial pressure. However, projects like the Marine Forest of Red Eléctrica in Mallorca and Posidonia Gardeners in Sicily and Malta are working on their restoration.
Ecosystem services and scientific warnings
Seagrass meadows improve water quality, protect coasts, and are a biodiversity refuge.
Posidonia absorbs carbon dioxide, protects coasts from erosion, and serves as a breeding ground for marine species.
Although Neptune balls help bring plastic back to solid ground, Sánchez-Vidal warns that they should not be removed from the beaches, as they provide moisture and nutrients. Furthermore, she insists that the solution is not in planting meadows as filters, but in reducing plastic production at its source.
