From source to sink: Review and prospects of microplastics in wetland ecosystems

The source, distribution, migration, and fate of microplastics (MPs) in aquatic and terrestrial ecosystems have received much attention. However, the relevant reports in wetland ecosystems, the boundary area between water and land, are still rare. Where are the sources and sinks of MPs in the wetland? The latest researches have shown that the sources of MPs in wetlands include sewage discharge, surface runoff, and plastic wastes from aquaculture. Fibers and fragments are the most common shapes, and PE, PP, PS can be detected in water or sediment matrices, and biota of wetlands. The distribution is affected by hydrodynamic conditions, sediment properties, and vegetation coverage. Factors affecting the vertical migration of MPs include their own physical and chemical properties, the combination of substances that accelerate deposition (mineral adsorption and biological flocculation), and resuspension. Minerals tend to adsorb negatively charged MPs while algae aggregates have a preference for positively charged MPs. The wetlands vegetation can trap MPs and affect their migration. In water matrices, MPs are ingested by organisms and integrated into sediments, which makes them seem undetectable in the wetland ecosystem. Photodegradation and microbial degradation can further reduce the MPs in size. Although recent research has increased, we are still searching for a methodological harmonization of the detection practices and exploring the migration rules and fate patterns of MPs. Our work is the first comprehensive review of the source, distribution, migration, and fate of MPs in wetland ecosystems. It reveals the uniqueness of wetland habitat in the research of MPs and indicates the potential of wetlands acting as sources or sinks for MPs.