Beyond the Surface: Vertical distribution of plastic pollution in Dutch rivers

Rivers act as the main transportation pathways for land-based plastic litter to the ocean. Recently, rivers have also been identified as potential sinks and reservoirs for plastics. A significant part of plastic remains in and around rivers for extended periods, and only travels short distances in river systems. However, knowledge of plastic transport over different depth profiles in rivers remains limited. In this study, we measured the vertical distribution of macro- and mesoplastic concentration and composition. An extensive monitoring campaign was performed in the river Rhine and its two major branches, i.e. Waal and IJssel using a larvae net and a trawl net, methodologies that allow for differentiating between sampling depths. Subsequently, in order to estimate the relationship between the surface transport of plastic items compared to the transport in deeper layers in the water column, an extrapolation factor was derived per day for the middle and bottom nets divided by those found in the surface net. The predominant recorded items among the investigated rivers and monitoring techniques were fragments of soft mesoplastic falling under the category “Plastic film plastics 0-2.5 cm (soft)". The distinction among the observed macro- and mesoplastic OSPAR categories collected in different layers in the water column was limited between techniques. At the sampling sites in the river Waal, river Rhine, and river IJssel, during larvae net monitoring, for both macroplastic and mesoplastics, hard plastics were more frequently found on the river surface, while soft plastics were more frequently detected near the river bottom. The average of the calculated extrapolation factor ranged between 0.45 - 3.51 and 0.70 – 1.72 for macroplastic and mesoplastic, respectively during larvae net monitoring. During trawl net monitoring, the average of the calculated extrapolation factor of macroplastic ranged from 0.82 – 1.30, and for mesoplastic transport ranged from 0.52 – 1.40. Additionally, during larvae net monitoring, extrapolation factor values indicated that mesoplastics showed varying abundances, with the greatest concentration at the bottom of the water column. Followed by high concentrations on the water surface, and with the lowest concentration located in the middle of the river. The trawl net method exhibited subtler differences in macro- and mesoplastic distribution across depths. Vertical mixing was intensified during higher discharge events as a result of turbulent flow. Overall, the findings of this study show that estimates of plastic concentrations solely based on surface transport could result in an underestimation of riverine plastic transport.