Microplastics research at the interface of aquatic and terrestrial systems – Opinion

Rivers are often pictured as transport routes for plastics and microplastics from land to sea. Indeed, from a geological and geomorphological perspective, the main system function of rivers is the transport of water and sediments. An increasing amount of studies has detected microplastics not only in river waters but also in river sediments, banks and floodplains. The occurrence of microplastics in both aquatic and terrestrial systems raises the question of processes related to microplastics at the two system’s interface. However, in microplastics’ research, aquatic and terrestrial compartments are usually investigated separately from each other. Such a restricted perspective cannot explain reality adequately: Rivers are highly dynamic and complex systems, they are framed by and interact with terrestrial systems on different spatial and temporal scales. This interaction is known with regard to sediment deposition and erosion as well as pollutant or nutrient enrichment. In microplastics’ research, to date, little is known on interface processes such as (potentially bidirectional) horizontal and vertical plastic transport, deposition and erosion as well as remobilization. Little is known on the fate of plastics within both systems and at the interface: fragmentation, leaching and absorption of chemicals, biofilm formation, homoaggregation, heteroaggregation, intake by plant and animal organisms. However, a comprehensive understanding of sources, transport paths, as well as sinks is not only an academic problem but can support political stakeholders to manage pollution by microplastics. Therefore, we suggest to shed light not only on microplastics’ abundance in rivers or soils but also on processes at the interface of aquatic and terrestrial systems.