Seagrasses provide a novel ecosystem service by trapping marine plastics

Abstract

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.

Figure 1

Plastic debris in seagrass remains. (A) Pie chart illustrating size of plastic debris (microplastics < 5 mm, mesoplastics 5–25 mm, and macroplastics > 25 mm), and type of plastic (filaments and fibers, fragments, films, foams, pellets and sticks) found in beached aegagropilae (EG) and loose leaves (wracks). (B) Relative abundance of each plastic polymer in plastic waste generation, floating plastic debris, plastic debris in loose leaves and aegagropilae (EG) vs. polymer density. Plastic waste generation is from Geyer et al., and plastic polymers floating at the sea surface are from Suaria et al.. Only plastic polymers with percentages > 1% in at least two matrices (floating, leaves, EG) are taken into account. PA polyamide, PE polyethylene, PET polyethylene terephthalate, PP polypropylene, PS polyestyrene, PU polyurethane, PVC polyvinyl chloride.

Figure 2

Trapping of plastic debris by seagrasses. Representation of the processes involved in the accumulation and intertwining of plastic items and sheath fibers to form plastic-rich aegagropilae (EG) found stranded in beaches.

Figure 3

Plastic debris in aegagropilae (EG) and loose leaves found in beaches. (A) Polyamide filaments trapped in aegagropilae (EG), and (B) beached EG and loose leaves along with plastic debris.