Storyline of an event of marine litter input and windrow formation in the Northwestern Mediterranean Sea

Abstract

High-resolution satellites now enable the detection of submesoscale aggregations of floating litter, known as litter windrows, creating new opportunities for studying marine litter pollution. Here, we combine satellite observations with temporal modeling to examine how rainstorms influence the magnitude and timing of plastic release from land to sea. Windrow detections in the northwestern Mediterranean Sea during spring 2018 were used to adjust a daily-scale model driven by surface runoff and river water discharges. Plastic inputs peaked simultaneously with rainfall and the rising phase of river levels. During the 90-day study period, half of the total plastic inputs occurred in a single three-day pulse linked to an intense rainstorm in April 2018. Although high river water discharges persisted for nearly a month afterward, subsequent litter inputs were relatively small, suggesting extensive watershed wash-off during the initial surge. The maximum plastic load on the sea surface (8.4 t) occurred on the third day after peak rainfall, while windrowed plastic peaked on the ninth day (1.2 t). Overall, 15 % of floating inputs aggregated into windrows. Wind direction and speed strongly influenced both windrow formation and the persistence of litter at the sea surface. During the study period, 68 % of marine floating litter was advected back to shore and 31 % sank to the seafloor. Our results demonstrate that storm-driven pulses play a key role in shaping marine litter dynamics, a process often overlooked in studies based on low-frequency sampling, highlighting the importance of integrating such events and satellite observations into monitoring and early-warning systems.