Evaluation des apports fluviaux de microplastiques et modélisation de leur dispersion en mer Méditerranée

The purpose of this work was to better understand the microplastics (MP) cycle from their sources on land to their sinks in the marine environment. It starts with an evaluation of the annual MP fluxes in rivers, which play a key role in their transfer from continental areas to the sea: they are the convergence point for both natural surface water flows and inputs from point sources such as wastewaters. We demonstrate in the following that current estimates of riverine MP inputs to the oceans are overestimated. The previously established empirical models include several methodological errors that were corrected in this thesis based on an in-depth statistical analysis of available observations at the global scale. Instead of several million tons, rivers probably only carry several thousand tons to the sea annually. In combination with the estimated oceanic stock of floating MP, the average residence time of MP in the ocean consequently raises from a few days to several years. This strongly reduces the necessity for a "missing oceanic plastic sink" and supports the idea of a slow drift of particles at the ocean surface, extending the exposure time of ecosystems to the deleterious effects of floating MP.Then, we adapted our global model of riverine inputs to the Mediterranean drainage basins in order to simulate the dispersion of MP in the Mediterranean Sea, one of the most critical polluted areas due to its semi-enclosed structure and its highly anthropized coastline. The Lagrangian particle tracking in the ocean currents was performed with the 3D hydrodynamic model SYMPHONIE, taking into account the interaction between the Stokes drift due to waves and the lower frequency circulation due to thermohaline gradients and wind. A wide range of vertical buoyant velocities, derived from size, shape and polymer type distributions observed in rivers, characterizes the particles released in our model: 65% drift in the surface layer and 35% in the water column to the seabed. We show that at surface, the Eastern Mediterranean and especially the Ionian Basin accumulate many of the MP that potentially present the most advanced state of ageing as the result of a relative long drift at the sea surface. Moreover, the North-Balearic front, the Tyrrhenian Sea and the South Adriatic cyclonic gyre efficiently trap MP temporarily. At depth, the transport of MP is very slow and the accumulation occurs mainly on the continental shelves or along the continental slopes. In our model, beaching is considered to represent a strong and rapid sink of MP. After 2 years of continuous river inputs, the floating MP stock we produce is lower than the current estimates derived from in situ observations, which means either that our model is far away from steady state conditions, or that beaching rates are overestimated. Our work therefore provides important information for further model improvements for the prediction of MP cycling in the sea.