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Distribution and Vertical Fluxes of large and small microplastics (MP) in the Calanques National Park (CNP) and Marseille Bay
Summary
Scientists studied tiny plastic particles in the Mediterranean Sea near Marseille, France, and found that these microplastics are settling to the ocean floor at measurable rates. The research shows that smaller plastic pieces are more common than larger ones, and they stick to organic matter as they sink through the water. This matters because microplastics that settle on the seafloor can enter the food chain through bottom-dwelling sea creatures that humans eventually eat.
Since the 1950s, the increase in plastic production has led to widespread mismanagement and substantial discharges of plastic waste into the marine environment. Once released, (micro-)plastics can drift, remain buoyant, sink, strand on coastlines, be ingested, or fragment into smaller particles. Understanding the marine life cycle of large (L-MP, 5mm–300µm) and small (S-MP, 300-1µm) microplastics is therefore a major challenge, particularly regarding the “missing plastic” paradox, highlighting the imbalance between plastic inputs and surface observations in the ocean1. Recent studies2,3 show that a proportion of MP is distributed throughout the water column, where it can aggregate with organic matter, marine snow, and microbial and phytoplanktonic organisms, ultimately settling to the sea floor. This project investigates MP fate in the marine environment, and the roles of MP type, shape, and environmental conditions in their sedimentation in the Calanques National Park (CNP) and Marseille Bay, a highly urbanized coastal area under strong anthropogenic pressure.Two field campaigns were conducted in March and September 2025 in Marseille Bay and the CNP. Subsurface waters were sampled using in situ pumping, while surface waters were collected using manta trawls. In addition, anchored particle interceptor traps (PIT) were deployed for one month in Marseille Bay. Microplastics from water and PIT samples were extracted following adapted protocols for L-MP4, with adjustments to recover S-MP (>32µm) and to characterize them using micro-infrared spectroscopy5. Complementary geochemical (POC, PON) and biological (chlorophyll a, TEP) analyses were performed on PIT samples.The campaigns provide robust estimates of subsurface L- and S-MP concentrations, showing a size-dependent shape and abundance distributions, with MP concentrations decreasing and fiber abundances increasing as size increases. Samples of manta and pump collected from the same location reveal differences in the abundance and polymer composition of L-MPs in surface and subsurface waters. PIT deployment provides the first MP sedimentation fluxes for this area, which are put in perspective with hydrodynamic, geochemical, and biological parameters to elucidate the role of aggregation in MP vertical transport. Overall, these findings highlight the importance of S-MP contribution (
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