Factors Controlling Transport Dynamics of Microplastics in Streams

Streams are the primary conduits through which microplastics are transported from land to sea. Attributes of the plastic particles and of the streams are both likely to influence how microplastic moves, but there are few empirical studies of microplastic transport dynamics in real systems. We adopted the spiralling technique commonly used to measure nutrient cycling in streams to quantify transport distances and deposition velocities of microplastics in streams with varying geomorphological structure and level of human modification. We conducted pulse releases of trace amounts of three size classes of five different polymers spanning a density gradient in 15 streams. The streams were typical of the range of human modification in urban environments, from seminatural to highly modified. Transport distances of microplastic ranged from <1 to 111 m, with distances declining with particle size. Neutrally buoyant polymers had the longest transport distances and lowest deposition velocities. Streams that had been modified into concrete channels were the most effective in transporting microplastics downstream, as indicated by relatively low deposition velocities and long transport distances of microplastics. Our results suggest that the movement of microplastic pollution in streams depends on the physical characteristics of the stream more than on plastic properties.