Where do the microplastics in Marseille's bay come from?

5 to 12 million tonnes of plastic in the ocean every year

Annual global plastic production has reached 390.7 million tonnes by 2021, of which 4.8 to 12.7 million tonnes will end up in the ocean every year. These eloquent figures illustrate the ubiquitous contamination of the environment by microplastics (MP), which has become a major societal issue.

While macro-waste, visible to the naked eye, accounts for only 10% of the plastic in the sea, the vast majority of this plastic pollution remains invisible, because it is so small. The plastics studied here refer to particles smaller than five millimetres in diameter. Despite a growing number of studies, their dispersion and accumulation dynamics remain largely undocumented. The need for quantitative data, with recurrent monitoring, remains particularly crucial in the marine environment in order to better estimate stocks and flows, as well as the impact of currents on MP dispersion.

Microplastics in surface waters and sediments in the bay of Marseille

In this work, microplastic contamination was measured in the surface waters and sediments of Marseille bay twice a year, in spring and autumn, from 2020 to 2022. Average concentrations of 5.79±12.71 MP.m^-3 (9.13 10⁵ MP.km^-2) were observed at the surface. No spatiotemporal trends were statistically demonstrated over the three years of monitoring, except in February 2020 with an abnormally high MP concentration (22.47±8.85 MP.m^-3 on average). These data indicate an average level of contamination comparable to those observed in surface waters in the bay of Marseille over the past decade. In addition, this study provides the first set of data on MP concentration in bay sediments, estimated to average 865±63 MP.kg^-1 of dry sediment.

The particles sampled in water and sediment are small (75% between 250µm and 1mm) and mainly fibrous (over 65%). The most abundant polymers are polypropylene (PP) and polyethylene (PE), two of the most widely used plastics today.

The Rhône impacts the hydrodynamics of the bay

The mouth of the Rhône, located 35 km from the bay of Marseille, is the site of one of the largest inputs of freshwater and nutrients into the Gulf of Lion (mean discharge 1710m³.s^-1). Although the river does not flow directly into the bay, the Rhône does have a significant impact on the hydrodynamics of the bay during specific episodes when its plume extends eastwards. Indeed, while the Rhône plume generally disperses westwards, it extends southwards under a north/north-westerly wind regime, and eastwards along the north coast in the bay of Marseille under a south/south-easterly wind regime.

In the study, surface water data were combined with modeling results (MARS3D-ROHMA model and Ichthyop module) to investigate Lagrangian transport of MP as a function of hydrodynamic conditions in the area. The influence of multiple Rhone intrusions into Marseille's bay, linked to specific factors such as wind, water flow and salinity levels, and their impact on the particularly high MP concentration levels in February 2020 were thus highlighted.

The need for regular monitoring of the study site

In conclusion, this study captured the impact of an exceptional hydrodynamic event characterized by a succession of Rhône plume intrusions impacting particle distribution and MP contamination in the bay of Marseille. The results highlight the need for regular field monitoring combined with high-resolution modeling to capture fine-scale events in complex hydrodynamic coastal zones.