CONTAMINATION BY MICROPLASTICS IN THE BAY OF MARSEILLE (Gulf of Lion, France): AN INTEGRATIVE DIAGNOSIS FROM THE SURFACE TO THE SEAFLOOR

Microplastics (MPs) contaminate all marine compartments, from the surface down to the seafloor. Despite a growing number of studies, accurately estimating MP stocks in marine environments remains challenging, as the water column and the sediment remain largely understudied. Most studies focus on MP present at the surface, leading to an observational bias that could contribute to the "Missing Plastic Paradox".

Hosting 150 million inhabitants, the Mediterranean Sea faces high and diverse anthropogenic pressures. Ranked as the sixth greatest accumulation zone for marine litter, it is assumed to concentrate 7% of all global MPs, an estimation that could be reconsidered if all marine compartments were considered. Here we present the results of an integrative survey of large MP (>300μm) distribution, performed on a biannual basis between 2020 and 2023 in the Bay of Marseille, second largest city (by population) and one of the most important ports in France. We provide quantitative data on MP contamination level and spatiotemporal distribution in (1) surface waters, (2) water column, and (3) surface sediments at 15 stations. For the first time in the area, we also provide an historical record of MP contamination since the 50's based on a short undisturbed sediment core.

Mean concentrations of 5.83±1.97 MP.m^-3 and 865±63 MP.kg dry sediment^-1 were estimated in surface waters and sediments, respectively. No spatiotemporal trends were statistically demonstrated, except in February 2020 with abnormally high concentration of MP in surface water (22.47±8.85 MP.m^-3 on average). We assume that this episodic high level of contamination results from repeated intrusions of the Rhône river plume into the bay in February 2020, as revealed by the MARS3D-RHOMA model.

In the water column, MPs were found at all depths, with concentrations averaging 1.43 ± 0.38 MP.m^-3 at 5 meters below the surface, and 0.59 ± 0.38 MP.m^-3 at the bottom (approx. 40 - 70m). Overall, MPs found below the surface represent more than 99% of the total MP mass collected in the entire water column. By contrast to surface water, vertical MP distribution exhibits a clear contrast between late winter and late summer, suggesting hydroclimatic processes exert a major control on distribution and transport in the water column (e.g., wind strength vs stratification). Interestingly, fibres with light and medium density (PP, PE, Nylon) were the dominant MP types found from the surface down to the sediment, questioning vertical transport processes at work in the area. 

Historical record of MP accumulation in the sediment matches well the global increase in plastic production since the 50's. MP burial rate increased by barely less than 500% since the 90's, and remained stable since then despite local and global plastic management policies. This original data set will enable, for the first time, to provide a quantitative estimation of MP stocks accumulated in water and sediment compartments along the 57km coastline of the Bay of Marseille.