Seagrass collects up to 900 million plastic items in the Mediterranean Sea each year

There is strong evidence that the seafloor constitutes a final sink for plastics from land sources. There is also evidence that part of the plastics lying on the shallow seafloor are washed up back to the shoreline. However, little is known on the natural trapping processes leading to such landwards return. Here we investigate microplastics and larger plastic debris within beached seagrass remains including balls (aegagropilae) made of natural aggregates of vegetal fibers intertwined by seawater motion. We found up to 1470 plastic items per kg of plant material, which were mainly composed of negatively buoyant polymer filaments and fibers. Our findings show that seagrass meadows promote plastic debris trapping and aggregation with natural lignocellulosic fibers, which are then ejected and escape the coastal ocean. Our results show how seagrasses, one of the key ecosystems on Earth in terms of provision of goods and services, also counteract marine plastic pollution. In view of our findings, the regression of seagrass meadows in some marine regions acquires a new dimension.

Introduction Microplastics -plastic particles smaller than 5 mm in size1-derive from fragmentation and degradation of large plastic items2,3, and also from direct manufacturing of microscopic particles such as virgin plastic pellets, cosmetic microbeads and clothing microfibres4,5. Research on microplastic pollution has long focused on sea surface accumulations6,7,8. However, there is a growing body of evidence that floating plastic debris account for less than 1% of the global ocean plastic inventory9, whereas the vast majority sinks to the seafloor1,10,11. Microplastics have indeed been found in all marine environments, shallow and deep, close to shore and amidst ocean basins11,12,13,14,15. Further, recent studies have shown that bottom currents control the distribution of microplastics on the seafloor, transporting them from shallow to deep waters where they accumulate14,16. In this study, we provide evidence of the entrapment of plastic debris from the shallow marine environment by seagrasses. This represents 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 transport3,8,17,18. Seagrass meadows are widespread in shallow coastal waters19 and provide important ecosystem services and benefits, such as water quality improvement20, CO2 absorption21, climate change mitigation22, sediment production for seafloor and beach stabilization23, coastal protection24, nursery and refuge areas for many species25, and support in fisheries production26. We have investigated microplastics and larger plastic debris washed ashore together with natural debris of the seagrass Posidonia oceanica, a Mediterranean endemic seagrass forming lush, extensive meadows from 0.5 to 40 m of water depth. According to the latest and more accurate estimate the total area covered by P. oceanica is 1.2 M Ha27. P. oceanica has long, ribbon-like leaves, with a clear differentiation in leaf blade (photosynthetic) and leaf base or leaf sheath (non pigmented and fibrous) that attaches the leaf to the stem, called rhizome28. As a temperate species, P. oceanica loses leaves in autumn, which are washed by waves and currents and accumulate on adjacent beaches as wrack beds. These vegetal deposits, besides attenuating wave energy, protecting the shoreline and preventing coastal erosion, influence also dune vegetation not only by providing it with nutrients but also by preventing substrate aridity29. In addition, in this species (as in other congenerics in the southern hemisphere) leaf sheaths remain attached to the rhizome when leaves shed, and are slowly buried by sedimentation in the so called “matte”, an accumulation of dead rhizomes and roots that can persist for millennia30. During the burial process, leaf sheaths, which are rich in lignocellulose, suffer mechanical erosion, releasing part of the constituent fibers that intertwine to form ball-shaped agglomerates known as seaballs, Neptune balls or aegagropilae (EG)31. These balls are also washed ashore. While leaf sheaths are present in almost all seagrass genera, only sheath cells in Posidonia have thin and lignified walls28, and thus fibers provide the needed stiffness to form EG32. The genus Posidonia has a unique fragmented distribution in the temperate waters of the Mediterranean Sea and southern Australia. Regrettably, it is estimated that between 13 and 50% of potential initial P. oceanica area may have been lost since 196027,33. To examine the role played by these piles of vegetal remains in trapping and extracting plastic debris from sea and carrying them to shore we have examined both EG and beach wracks accumulated on different beaches of Mallorca Island, in the Western Mediterranean Sea. This island, 3640 km2 in area and ca. 560 km in perimeter, is an optimal site to address these issues, because of both the extensive meadows of P. oceanica in its waters34 and the fact that the highest accumulations of floating plastic debris in the Mediterranean Sea occur in its nearshore35.