Plastic debris that mysteriously returns from the shallow ocean floor to the coastal shoreline may be hitching a ride on uniquely capable but long-deteriorating Mediterranean seagrass meadows, according to a study published Thursday in Scientific Reports.

The study's authors from the University of Barcelona say their findings add another layer to efforts to conserve these seagrasses, which also provide other important ecosystem services including coastal protection, greenhouse gas absorption and fisheries support.

Scanning seagrass remains along four of Spain’s Mallorca beaches in 2018 and 2019, the researchers found up to 1,470 plastic items per kilogram of plant material, indicating the grasses’ natural fibers help them trap plastic debris that is then carried to shore.

“The amount of plastic per kilo of natural debris is really, really very high,” said Anna Sanchez-Vidal, the study’s lead author and an associate professor of earth sciences at the University of Barcelona’s Research Group on Marine Geosciences.

Her research focused on the Posidonia oceanica seagrass found in temperate waters of the Mediterranean Sea and southern Australia. Its lush meadows range from 0.5 to 40 meters deep and span 1.2 million hectares, one of which is equal to 2.471 acres, or 10,000 square meters.

The slow-growing, long-living species has lengthy ribbon-like leaves, which when shed in the fall are ushered by storms and currents toward adjacent beaches to assemble in deposits known as wrack beds. Here, the plant remains prevent coastal erosion by absorbing wave energy, while also providing dune vegetation with nutrients and moisture.

Simultaneously, the plant’s base, or sheath, slowly erodes, releasing fibers containing a wealth of stiffening lignocellulose that then intertwines to form aegagropilae, or Neptune balls, which apparently can capture the plastic debris and are similarly washed ashore. This fibrous characteristic is unique to the species, meaning the research findings may not translate to other seagrasses or regions.

“We think those balls are sufficiently hard and stiff to remain as a ball with the plastic inside,” Sanchez-Vidal said, adding that the seagrass also protects beached plastics from degradation by ultraviolet light. “They remain on the beach until there is a storm and they enter again into the marine system.”

The University of Barcelona team found wracks containing plastic debris in half of samples, tallying as many as 613 items per kilogram of dead leaves. More than six in 10 were plastic fragments, while pellets comprised one-third of the trash and foams accounted for less than 3%.

In aegagropilae, the researchers found plastics 17% of the time, with each kilogram of dead material containing up to 1,470 plastic items. About 65% were filaments and fibers, while fragments comprised approximately one-fifth, films accounted for 8% and foams made up the rest.

In both types of seagrass remains, microplastics made up a little under half of identified plastics, while macroplastics larger than 25 millimeters accounted for under 10%. Microplastics, which pose health risks to both fish and humans, are plastic particles less than 5 millimeters in size. Mesoplastics land between the two and comprised the rest of the debris.

The findings are also novel because while evidence is building that floating debris comprises less than 1% of all plastics in the ocean, microplastic pollution research has historically focused on accumulations at the surface, and previous literature on microplastics transport has omitted shallow marine environments.

Overall, the results suggest Neptune balls could potentially entrap 867 million pieces of plastic debris every year, the study says, although, "How many of these plastic-rich aegagropilae are annually flushed ashore is unknown.”

“P. oceanica may provide a valuable added plastic buffering and trapping service. This may be particularly important in the Mediterranean Sea,” which is nearly landlocked, the study said. “Given the ever-increasing plastic load reaching our oceans, seagrass ecosystems such as P. oceanica meadows will play a crucial role.”

But scientists have found that at least 1.5% of seagrass beds disappear annually and close to one-third have vanished since 1879 because of climate change, invasive species and coastal human activities like development and aquaculture. In the last 60 years, up to half of areal extent for the species examined in the new study is thought to have been lost, and in the last half-century, its meadows are estimated to have regressed 34%.

More recent research found signs for optimism. Posidonia oceanica is one of four species found along European coasts, and seagrass meadow losses there peaked in the last four to five decades, according to a 2019 study. It found that trends reversed thanks to improvements during the 1990s and 2000s, concluding that “decline is not the generalized state among seagrasses nowadays in Europe.”

Still, the European Union has made strict protection of seagrass meadows and other carbon-rich ecosystems part of its Biodiversity Strategy for 2030. The grasses bring economic benefits as well, the EU has noted, by providing material for thatched roofs, furniture, animal feed and fertilizer. They were traditionally used to stuff mattresses and deter bedbugs.

In collaboration with the California-based Surfrider Foundation, Sanchez-Vidal is now working on a citizen science-type project in which associations and individuals are using manta trawls — essentially nets attached to paddleboards, kayaks and the like — to capture floating plastics from around a dozen very-nearshore stations along the Catalan coast. The goal is to better understand dispersal patterns in the extremely active area.

“Plastic pollution may also pose a significant threat to seagrasses around the world,” although there is not yet sufficient evidence of the clear consequences to these ecosystems, the study notes. “What is clear is that the deterioration of seagrass meadows may compromise the services they provide, so it is crucial to undertake specific actions to mitigate threats causing regression and ensure conservation.”

The study “Seagrasses provide a novel ecosystem service by trapping marine plastics,” published Jan. 14 in Scientific Reports, was authored by Anna Sanchez-Vidal, Miquel Canals, William P. de Haan, Javier Romero and Marta Veny, University of Barcelona.