Heavy rains control the floating macroplastic inputs into the sea from coastal Mediterranean rivers: A case study on the Tet River (NW Mediterranean Sea).

This study focuses on the relevance of small watersheds in the macroplastic pollution of coastal environments. It aims to identify and quantify in terms of composition, number and mass, current riverine flows of floating macroplastics (>2.5 cm). Estimates are based on 66 visual monitoring of total litter over a 4-year-period (2016-2019) in a small coastal Mediterranean river, the Têt River (NW Mediterranean Sea). The plastic fraction represented 97 % of the observed litter, mainly cigarette butts (20.5 %), polystyrene fragments (18.8 %) and light packaging (16.3 %). The Tet River is characterized by frequent flash-flood events caused by heavy rain, that can induce a sudden rise of the water discharge. Such hydroclimatic forcing greatly influence macroplastic flows, both in terms of their average compositions and loads. We have estimated that 354,000 macroplastic items, corresponding to 0.65 tons, are discharged annually from the Tet River into the sea, and that 73 % of them are released during rain events (~6 % of the year). The short observation distance from the water surface allowed to exhibit the great abundance of small litter (80 % of them were < 10 cm) and to evaluate to 1.8 g the average mass of floating plastics. Our results suggest that remediation actions must be taken on rainy days and target small litter in order to significantly limit macroplastic inputs from rivers to the sea. Moreover, the large share of cigarette butts in macrolitter inputs demonstrates that reducing ocean pollution cannot be achieved solely by improving waste management, but that changes in social behavior are also needed to stem waste production at the source.