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Seafloor sediments as microplastic sinks in the northern Baltic Sea – Negligible upward transport of buried microplastics by bioturbation
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A mesocosm experiment with Baltic Sea benthic invertebrates found that bioturbation caused negligible upward transport of buried microplastics from 2 cm and 5 cm depth, with <2% of particles returning to the surface. The results indicate that once microplastics are deposited in marine sediments, bioturbation is unlikely to re-introduce them to the water column, supporting sediments as effective long-term sinks.
Microplastics (MPs) are ubiquitous in the marine environment. High concentrations of MPs are found from seafloor sediments, which have been proposed to act as their final sinks. Because bioturbation is an important process affecting the burial of MPs, a mesocosm experiment was established to study whether sediment infauna may also promote MP return to the sediment surface. Thin layers of frozen sediment containing an environmentally realistic concentration (<1300 MPs per kg of dry sediment) of MP fragments in two size classes (>500 μm and 100-300 μm) were added to depths of 2 cm and 5 cm in the experimental cylinders filled with sediment. The displacement of these MPs, made of acrylonitrile butadiene styrene (ABS), by a community of common benthic invertebrates in the northern Baltic Sea (clam Limecola balthica, polychaete Marenzelleria spp., gammarid Monoporeia affinis) was studied in a 10-week experiment. After the experiment, the MPs were extracted from each sediment layer and the animals were examined for MP ingestion. The results indicated that the transportation of MPs to the sediment surface by bioturbation was negligible. Thus, in the Baltic Sea, the seafloor may act as a sink for once sedimented MPs, reducing simultaneously the MP exposure of the macrofauna feeding on the sediment surface.
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