Tourism, waste management and macroplastic accumulation in headwater riparian-stream ecosystems

• Macroplastics were ubiquitous in all streams, yet low abundances were registered • No consistent pattern between land uses and plastic pollution was observed • Plastics types and polymers varied across the rivers from two different regions • Geopolitical and socioeconomic context may contribute to observed plastic pollution patterns Plastic pollution is widespread across ecosystems, yet studies in fluvial environments have predominantly focused on lowland agricultural and urban areas. This study aims at expanding this perspective by investigating how tourism-related activities in mountain regions influence macroplastic (MacP; > 5 cm) pollution in stream-riparian ecosystems. We selected two mountain rivers: the Valira del Nord (Andorra) and the Ter River (Catalonia), each subject to varying degrees of anthropogenic pressure. MacP was sampled at seven sites on three occasions under different environmental conditions and characterized by abundance, use type category and polymer composition. MacP were found at all sites except the most pristine site, indicating that MacP pollution is present even in remote, protected mountain regions. Considering all sites together, MacP abundances ranged in items from null to 0.16 items/m² and in mass from null to 5.38 g/m². The dominance of plastic fragments indicates a significant contribution from legacy pollution and in situ fragmentation. The Valira del Nord River exhibited greater polymer diversity than the Ter River, which was primarily composed of polyethylene. No consistent correlation emerged between MacP abundance and indicators of human pressure or main catchment land use. Instead, regional differences suggest that local waste management practices and visitor behavior are key drivers of MacP pollution in the study streams. This emphasizes the urgent need to improve regulation, public education, and targeted monitoring to mitigate plastic pollution in mountain stream–riparian ecosystems from touristic regions.