Plastic exposure risk for costal seabirds varies seasonally with freshwater runoff

Marine litter, including Floating Marine Macro-Litter (FMML) and microplastics (MP), poses a major threat to marine ecosystems. This study investigates the seasonal distribution of FMML and MP in the Ebro Delta, a key Mediterranean wetland, and evaluates the potential risk to three tern species (family Laridae, subfamily Sterninae) that rely on the area for foraging and breeding. Seasonal surveys (October 2021-June 2022) were conducted using drone and manta trawl sampling for litter assessment, with tern densities estimated via distance sampling. Results revealed seasonal peaks in FMML and MP densities, with the highest concentrations in autumn and spring, likely influenced by river discharge and irrigation runoff. Risk assessment maps identified high-risk zones where tern densities overlapped with FMML and MP hotspots, indicating potential threats from entanglement, ingestion, and trophic transfer of pollutants. These findings underscored the relevance of seasonality and spatial risk assessment when evaluating exposure to marine litter. This study not only enhanced our understanding of marine litter impacts in the Mediterranean Sea but also provided essential data to inform evidence-based policies and management strategies aimed at protecting vulnerable species in this ecologically relevant area.