Microplastic concentrations at the water surface are reduced by decreasing flow velocities caused by a reservoir

Abstract

Microplastic particles are found globally in all kinds of water bodies posing a serious threat to aquatic organisms and ecosystems. However, plastic concentrations are not homogeneously distributed and can decrease along a river course. Thus, potential sinks need to exist. Dams and reservoirs arc proposed to be such potential sinks for microplastics. We investigated plastic concentrations at the water surface upstream and downstream of a reservoir inundated by a wastewater treatment plant effluent by filtering water with a net with 200 pm mesh size. We sampled at flow velocities ranging from 6 cm s(-1) to 35 cm s(-1). Upstream of the reservoir significantly higher concentrations of microplastics (19.9 +/- 7.3 particles per m(-3)) than downstream (3.4 +/- 2.2 particles per m(-3)) were identified. Furthermore, the plastic concentrations increased with increasing flow velocities. Hence, reservoirs can be sinks of microplastics as decreased flow velocities may lead to an increased sedimentation or decreased erosion potential. Consequently, flow velocities are critical when identifying plastic loads. Single time or single spot measurements may not well represent plastic loads if flow velocities vary considerably in time or space.