Posidonia Marine Seagrass Can Remove Plastics from the Sea

Posidonia oceanica seagrass — an endemic marine phanerogam (a seed-producing plant) with an important ecological role in the marine environment — can remove plastic materials that have been left in the sea, according to a study published in the journal Scientific Reports. The study describes for the first time the outstanding role of the Posidonia as a filter and trap for plastics in coastal areas.

The article’s first author is the lecturer Anna Sànchez-Vidal, from the Research Group on Marine Geosciences of the Faculty of Earth Sciences at the University of Barcelona (UB). Other authors of the study are the experts Miquel Canals, William P. de Haan and Marta Veny, from the Research Group on Marine Geosciences of the UB, and Javier Romero, from the Faculty of Biology and the Biodiversity Research Institute (IRBio) of the UB.

The Posidonia oceanica makes dense prairies that present a habitat with great ecological value (nutrition, shelter, reproduction, etc.) for marine biodiversity. As part of the study, the team analyzed the trapping and extraction of plastic in great seagrasses of the Posidonia off the coast of Majorca, Spain. “Everything suggests that plastics are trapped in the Posidonia seagrass. In the grasslands, the plastics are incorporated to agglomerates of natural fiber with a ball shape — aegagropila or Posidonia Neptune balls — which are expulsed from the marine environment during storms,” says Dr. Sànchez-Vidal.

According to the analyses, she says, the trapped microplastics in the prairies of the Posidonia oceanica are mainly filaments, fibers and fragments of polymers such as polyethylene terephthalate (PET) which are denser than the sea water

This marine phanerogam has a vegetative structure made by a modified stem with a rhizome shape from which the roots and leaves appear. When the leaves fall, its bases (pods) are added to rhizomes and give them a feather-like appearance. “As a result of the mechanical erosion in the marine environment, those pods under the seafloors are progressively releasing lignocellulosic fibers which are slowly added and intertwined until they make agglomerates in a ball-shape, known as aegagropilae.

Aegagropilae are expulsed from prairies during periods of strong waves and a certain part ends up on the beaches,” says Professor Romero.

Although there are no studies that quantify the amount of aegagropilae expelled from the marine environment, it is estimated that about 1,470 plastics are taken per kilogram of plant fiber, amounts which are significantly higher than those captured through leaves or sand. As researcher Dr. Sànchez-Vidal says, “We cannot completely know the magnitude of this plastic export to the land. However, first estimations reveal that Posidonia balls could catch up to 867 million plastics per year.”

The polluting footprint of plastics that come from human activity is a serious environmental problem affecting coastal and ocean ecosystems worldwide. Since plastics were created massively in the 20th century fifties they have been produced on a massive scale since the 1950s and are known to accumulate in the sea — seafloors seafloors act as a sink for microplastics and they are transported by ocean currents, wind and waves. “The plastics we find floating in the sea are only a small percentage of everything we have thrown into the marine environment,” warns Dr. Sànchez-Vidal.

The new ecosystemic service of the Posidonia described in the article has a significant value in a marine area such as the Mediterranean where high quantities of plastic float above and on the seafloors. Posidonia seagrass can occupy large areas up to forty meters deep. “This is why we need to protect and preserve these vulnerable ecosystems. However, the best environmental protection strategy to keep oceans free of plastic is to reduce landfills, an action that requires limiting its use by the population,” conclude the researchers.