Reply on RC2

Although rivers contribute to the flux of litter to the coastal and marine environment, estimates of riverine litter amounts are scarce and the behaviour of riverine litter at river mouths and coastal waters is highly uncertain. This paper provides a comprehensive overview of the seasonal trends of floating riverine litter transport and fate in the south-eastern Bay of Biscay based on riverine litter characterization, drifters and high-frequency radars observations and Lagrangian simulations. Virtual particles were released close to the river mouths as a proxy of litter entering the ocean from rivers and were parameterized with a wind drag coefficient (Cd) to represent their trajectories and fate according to the buoyancy of the litter items. They were forced with numerical winds and measured currents provided by high-frequency radars covering selected seasonal week-long periods between 2009 and 2021. To gain a better insight on the type and buoyancy of the items, samples collected from a barrier placed at Deba river (Spain) were characterized at laboratory. Items were grouped into two categories: low buoyant items (objects not exposed to wind forcing e.g., plastic bags) and highly buoyant items (objects highly exposed to wind forcing, e.g., bottles). Overall, low buoyant items encompassed almost 90 % by number and 68 % by weight. Low buoyant items were parametrized with Cd=0 %, and highly buoyant items with Cd=4 %, this later one as a result of the joint analysis of modelled and observed trajectories of four satellite drifting buoys released at Adour (France), Deba (Spain) and Oria (Spain) river mouths. Results show that all regions in the study area are highly affected by rivers within or nearby the region itself. Simulations of riverine litter parametrized with Cd=4 % showed that particles drifted faster towards the coast by the wind, notably during the first 24 hours. In summer, over the 97 % of particles beached after one week of simulation. In autumn this value fell to 54 %. In contrast, the low buoyant litter items take longer to arrive to the coastline, particularly during Spring with fewer than 25 % of particles beached by the end of the simulations. When comparing coastline concentrations, the highest concentrations of particles (>200 particles/km) were recorded during summer in the French region of Pyrénées-Atlantiques for Cd=4 %. These results coupled observations and a river-by-river modelling approach and can assist policy and decision makers on setting emergency responses to high fluxes of riverine litter arrivals and on defining future monitoring strategies for heavy polluted regions within the study area.