Abstract
Urban beaches are increasingly vulnerable to litter accumulation, especially during large-scale coastal events that create short-lived but intense pollution pulses. Despite growing interest in marine litter monitoring, traditional methods often lack the spatial and temporal sensitivity required to capture such episodic surges. This study presents a methodological adaptation of the OSPAR beach litter monitoring protocol, applying a stratified sampling framework to a high-use coastal site during the RFM SOMNII festival in Figueira da Foz, Portugal, one of Europe's largest beach music festivals. Over a 5-year period (2019-2023), including pre- and post-COVID-19 seasons, 17 seasonal surveys were conducted across three functional zones (STAGE, VIP, CHILLOUT) to assess the spatiotemporal dynamics of litter accumulation. Results indicate clear spatial heterogeneity, with litter densities peaking in high-traffic areas and artificial polymer materials, particularly single-use plastics, accounting for over 90% of all litter items. Temporal trends show sharp declines in 2020-2021 during festival cancellations, with subsequent rebounds following the event's return, and further reductions after targeted cleanup measures in 2023. The stratified sampling approach revealed patterns and hotspots that would likely be overlooked by conventional OSPAR layouts, highlighting the potential for this framework to enhance marine litter monitoring in event-prone coastal zones. Findings also inform broader sustainability strategies, reinforcing the need for adaptive cleanup planning, reusable alternatives to single-use items, and coordinated engagement between researchers, event organizers, and policymakers. The approach offers a replicable blueprint for improving beach litter assessments under dynamic, high-pressure conditions worldwide.