This PhD thesis presents a first overview of floating marine litter pollution in the south-east Bay of Biscay through a combination of harmonized observations, sampling methods, and numerical modelling techniques. Abundance and composition of floating marine litter (FML) were assessed combining net tows and visual observations in coastal and open waters of the Bay of Biscay. Floating riverine litter was also collected to explore the floating fraction of marine litter transported via rivers to the south-east Bay of Biscay. Simulations performed at regional (Bay of Biscay) and sub-regional scale (south-east Bay of Biscay) provided insights into the seasonal distribution patterns and fate of fishing-related and riverine litter items according to their observed buoyancy. The model was previously calibrated with data obtained from drifters released in the south-east Bay of Biscay and forced with hourly estimated and measured winds and currents. Data collection in the coastal waters of the south-east Bay of Biscay highlights the occurrence of submesoscale convergence zones for FML (¿litter windrows¿) during Spring and Summer. Fishing, shipping, and aquaculture sectors were the main source of macrolitter (size>2.5 cm) for litter windrows. Abundances derived from sampling the south-east Bay of Biscay revealed that the area is a hotspot for microplastics (size<5 mm). Most modelled particles released both in coastal andopen waters did not abandon the Bay of Biscay, reinforcing that the basin acts as accumulation region for FML. Results also demonstrated the impact of buoyancy and wind effect on FML behaviour, mainly in summer, when highly buoyant items strongly affected French Marine Protected Areas and Gipuzkoa and Pyrénées-Atlantiques regions. This thesis represents a milestone for supporting future science and policy actions in the south-east Bay of Biscay oriented to prevent and mitigate FML at local, sub-regional and regional scale.

151 p.