Plastic transport in rivers: Bridging the gap between surface and water column

Rivers act as an important transportation pathway for land-based plastic litter to the ocean. Recently, rivers have also been identified as potential sinks and reservoirs for plastics. Knowledge of plastic transport over different depth profiles in rivers remains limited. In this study, we evaluated the vertical distribution of macro- and mesoplastics, using a larvae net and a trawl net in the river Rhine and its two major branches, i.e. Waal and IJssel. Subsequently, to estimate the relationship between the surface transport of plastic items, i.e., floating items, compared to the transport in deeper layers in the water column, including suspended and bed-transported plastic, an extrapolation factor was derived per day for the middle and bottom nets divided by those found in the surface net. The observed macro- and mesoplastic OSPAR categories collected in different layers in the water column were rather consistent between different sampling techniques. Fragments of soft mesoplastic falling under the category "Plastic film plastics 0-2.5 cm (soft)" were recorded most frequently in the investigated rivers with our monitoring techniques. During larvae net monitoring, hard plastics were more frequently found at the river surface than at the middle or bottom of the river for both macroplastic and mesoplastics, while soft plastics were more frequently detected near the bottom. For larvae net monitoring, the extrapolation factor, reflecting the concentration ratio of macroplastic items transport at different depths, i.e., from the surface downwards to the middle and the bottom ranged between 0.38 to 2.2 and 0.36 to 5.7, respectively. The extrapolation factor of mesoplastic transport from the surface downwards to the middle and the bottom ranged between 0.70 to 1.84 and 0.69 to 2.57. During trawl net monitoring, the extrapolation factor, reflecting the concentration ratio, for macroplastic ranged between 0.82 and 1.30, and for mesoplastic between 0.52 and 1.40. Overall, the findings of this study show that estimates of plastic concentrations solely based on surface transport could result in an under- or overestimation of riverine plastic transport.