Not the same plastics: Comparing abundance and composition of marine macro-litter in mangroves and beaches in Southeast Asia.

Southeast Asia continues to be a global hotspot for marine litter pollution, where several countries in this region are repeatedly identified as top contributors of land-based waste leakages into the ocean. Moreover, marine pollution is a transboundary issue where the prevailing winds and currents move litter across regional water bodies. Hence, a transboundary collaboration is needed to produce comparable field data to guide actions and solutions. This study reports an integrated assessment of macro-litter in mangroves and beaches across Indonesia, Philippines, Singapore, and Viet Nam using standardised belt transect surveys and harmonised litter categorisation at 25 study sites between 2021 and 2023. Across all countries, mangroves consistently had higher litter densities than beaches (2.53× [95% CI: 1.47-4.34] and 2.66× [95% CI: 1.45-4.90] in terms of count and mass, respectively). Plastics accounted for 69.6-88.5% of total litter in terms of count densities, with foam fragments, food packaging, and cigarette butts being the most common plastic categories. There were also country-specific plastic patterns, e.g., plastic bags and fishing-related debris were most abundant in Viet Nam and the Philippines, respectively. Other notable observations included the disproportionate litter mass at several Indonesian sites due to heavier cloth-based items and the high densities of drinking straws in Singapore's mangroves. Seasonal variation was observed in Indonesia, where macro-litter densities were higher during wetter Northwest monsoon than the drier Southeast monsoon, but not in Singapore. Overall, this collaborative and harmonised study provides empirical grounding for interpreting regional patterns of macro-litter pollution and supports future modelling efforts and evidence-based policy development.