Episodic fluvial inputs from a small Mediterranean intermittent catchment drive persistent beach litter accumulation: Evidence from multi-temporal UAV surveys

Rivers are widely recognised as the primary pathway through which terrestrial litter reaches the coastal zone, yet the contribution of small intermittent Mediterranean catchments remains poorly quantified. This study investigates beach litter dynamics at the mouth of the Mazzarrà Stream (northern Sicily, Italy) through six UAV-based surveys conducted between October 2024 and October 2025. A single extreme rainfall event (return period: 3.6 years) increased beach litter concentrations from 115 to 180 items per 100 m of coastline, exceeding the MSFD Good Environmental Status threshold by nearly one order of magnitude and maintaining elevated litter levels throughout the monitoring year despite repeated cleanup operations. Hotspot analysis identified four persistent accumulation areas with comparable litter abundances but distinct compositional signatures. The combined interpretation of hotspot persistence, litter composition and UAV orthomosaics indicates that hotspots adjacent to the river mouth are primarily sustained by flood-driven fluvial inputs, whereas similarly contaminated sectors farther from the river are mainly associated with direct local litter releases. These findings demonstrate that integrating repeated UAV surveys with hotspot and compositional analyses provides a robust framework for identifying litter sources and supports more effective, source-oriented coastal litter management.