To better understand the plastic bundling capabilities of seagrass, Sanchez-Vidal and her team studied a species found only in the Mediterranean sea, Posidonia oceanica. In 2018 and 2019, they counted the number of plastic particles found in seaballs that had washed up on four beaches in Mallorca, Spain, which has large seagrass meadows offshore. There was plastic debris in half of the loose seagrass leaf samples, up to 600 bits per kilogram of leaves.
Only 17% of the tighter bundled seagrass fibre known as Neptune balls contained plastic, but at a much higher density — nearly 1,500 pieces per kilogram of seaball. Using estimates of seagrass fibre production in the Mediterranean, the researchers worked up an estimate of how much plastic might be filtered in the entire basin. The oval orbs — the shape of a rugby ball — form from the base of leaves that have been shredded by the action of ocean currents but remain attached to stems, called rhizomes. As they are slowly buried by sedimentation, the damaged leaf sheaths form stiff fibres that intertwine into a ball, collecting plastic in the process.
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