Microplastic distribution in the marine and freshwater ecosystems of Cocos Island National Park, Costa Rica

Abstract The presence of microplastics (MPs) has been documented in remote sites, including Cocos Island, an oceanic island in the Costa Rican Pacific Ocean, which serves as a temporary home to park rangers, researchers, volunteers, and tourists. While previous investigations have attributed the presence of MPs in the marine environment to ocean currents, doubts have arisen about the sources of MPs in the freshwater ecosystem. This study aimed to analyze the distribution of MPs in both the marine and freshwater ecosystems by collecting water, sediment, and biota samples from both environments. A total of 288 samples were assessed, resulting in the identification of 178 MPs. Statistical analyses revealed a uniform distribution of MPs around the island in both ecosystems, suggesting that the activities carried out by the island’s inhabitants do not represent a significant source of MPs. The study also found higher abundance and larger-sized MPs in the marine ecosystem, which was expected. Drawing on findings from other studies in remote areas, sources of MPs in Cocos Island’s freshwater ecosystem are likely linked to atmospheric deposition, air-sea exchange, long-range transport, and biovectors. Oceanic insular environments, such as Cocos Island, are particularly vulnerable to plastic pollution as they can retain plastics from adjacent seas through various oceanographic mechanisms. These results underscore the importance of considering microplastics as a planetary boundary threat. Understanding the distribution and sources of MPs in remote and ecologically sensitive areas is crucial for effective conservation and pollution management efforts.