Long-term variations in size and polymer type of meso- and microplastics in seabirds and on beaches since the 1980s.

Marine plastic pollution is a global issue that requires innovative ways of monitoring and mitigation. Information on how the size, mass and polymer type of floating plastic items are changing over time may improve our understanding of the complex dynamics governing fragmentation rates, dispersal, longevity, input rates and abundance at the sea surface. Procellariiform seabirds directly ingest floating meso- and microplastics, which they retain in their gizzards. As a result, petrels can be used as biomonitors to document trends in the abundance and characteristics of marine plastics. We compare the characteristics of plastics collected from regurgitated Brown Skua Catharacta antarctica pellets containing the remains and plastics ingested by four petrel taxa breeding at Inaccessible Island, South Atlantic Ocean, at roughly decadal intervals from 1987─2024. To assess if trends persist across biotic (ingested) and abiotic (beaches) compartments, we compare this to the characteristics of meso- and microplastics (2-25 mm) sieved from South African beaches from 1984─2023. Plastics were collected from beaches far from local urban source areas in an attempt to track changes in plastic floating at sea rather than local, land-based sources. Overall, there was little evidence of trends in the size and mass of ingested or beached plastics. The average mass of industrial pellets from beaches decreased up to 2015, suggesting an old, gradually eroding cohort of legacy pellets, but increased in 2023 after two major pellet spills off the South African coast. Nearly all ingested and beached plastics were polyethylene (PE) or polypropylene (PP), but the ratio of PP to PE in hard fragments increased over time, while recent increases in PE:PP ratios in industrial pellets match recent pellet spills at sea. Identifying polymer types in ingested and beached plastics is valuable for future studies, as it may be useful for marine pollution management.