Distribution and Vertical Fluxes of large and small microplastics (MP) in the Calanques National Park (CNP) and Marseille Bay

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 studies^2,3 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 (<300µm) in the diagnosis of MP pollution and provide the first estimates of MP vertical fluxes in Marseille Bay.

 

References

¹Isobe, A. & Iwasaki, S. The fate of missing ocean plastics: Are they just a marine environmental problem? Sci. Total Environ. 825, 153935 (2022).

²Rowlands, E. et al. Vertical flux of microplastic, a case study in the Southern Ocean, South Georgia. Mar. Pollut. Bull. 193, 115117 (2023).

³Galgani, L. et al. Hitchhiking into the Deep: How Microplastic Particles are Exported through the Biological Carbon Pump in the North Atlantic Ocean. Environ. Sci. Technol. 56, 15638–15649 (2022).

⁴Alcaïno, A. et al. Influence of the Rhone River intrusion on microplastic distribution in the Bay of Marseille. Reg. Stud. Mar. Sci. 73, 103457 (2024).

⁵Nguyen, T. T. et al. Spatial and seasonal abundance and characteristics of microplastics along the Red River to the Gulf of Tonkin, Vietnam. Sci. Total Environ. 957, 177778 (2024).