<p>The processes controlling the transport in tidal rivers and estuaries, the interface between fluvial and marine systems, remain largely unresolved. For this reason, current estimates of riverine plastic pollution and export into the ocean remain highly uncertain. Hydrodynamics in tidal rivers and estuaries are influenced by tides and freshwater discharge. As a consequence, flow velocity direction and magnitude can change diurnally. In turn, this impacts the transport dynamics of solutes and pollutants, including plastics. </p> <p>Plastic transport dynamics in tidal rivers and estuaries remain understudied, yet the few available observations suggest that plastics can be retained here for long time periods, especially during periods of low net discharge. Additional factors such as riparian vegetation and riverbank characteristics, in combination with bidirectional flows and varying water levels, can lead to even higher likelihood of long-term retention.</p> <p>Here, we provide a first observation-based estimation of net plastic transport on daily time scales in tidal rivers. For this purpose, we developed a simple Eulerian approach using sub-hourly observations of floating plastic transport and discharge during full tidal cycles. We applied our method to the Saigon river, Vietnam, throughout six full tidal cycles in May 2022.</p> <p>We show that the net plastic transport is about 27-32% of the total plastic transport. We found that plastic transport and river discharge are positively and significantly correlated (Pearson's r = 0.87, R<sup>2</sup>= 0.75). The net plastic transport is higher than the net discharge (27-32% and 18%, respectively), suggesting that plastic transport is governed by other factors than water flow. Such factors include wind, plastic concentrations in the water, and entrapment of plastics downstream of the measurement site. The net plastic transport rates per tidal cycle alternate between positive (seaward) net transport and negative (landward) net transport, as a result of the diurnal inequality in the tidal cycles. We found that soft and neutrally buoyant items had considerably lower net transport rates than rigid and highly buoyant items (11-17% vs 31-39%), suggesting the retention time strongly depends on item characteristics.</p> <p>Our results demonstrate the crucial role of tidal dynamics and bidirectional flows in net plastic transport. We emphasize the importance of understanding fundamental transport dynamics in tidal rivers and estuaries to ultimately reduce the uncertainties of plastic emission estimates into the ocean.</p>

Understanding plastic mobility in rivers is crucial in estimating plastic emissions into the oceans. Most studies have so far considered fluvial plastic transport as a uniform process, with stream discharge and plastic concentrations as the main variables necessary to quantify plastic transport. Decelerating (e.g.: trapping effects) and accelerating effects (e.g.: increased water flows) on plastic transport are poorly understood, despite growing evidence that such mechanisms affect riverine plastic mobility. In this observation-based study, we explored the roles of an invasive floating plant species (i.e. water hyacinths) as a major disruptor of plastic transport. The different functions of aquatic vegetation in trapping and transporting plastics play a key part in our evolving understanding of how plastic moves in rivers. We collected a one-year dataset on plastic transport, densities and hyacinth abundance in the Saigon river, Vietnam, using both a visual counting method and UAV imagery analysis. We found that hyacinths trap the majority of floating plastic observed (~60%), and plastic densities within patches are ten times higher than otherwise found at the river surface. At a monthly and seasonal scale, high hyacinth coverage coincides with peaks in both plastic transport and densities over the dry season (Dec-May) in the Saigon river. We also investigated the large-scale mechanisms governing plant-plastic-water interactions through a conceptual model based on our observations and available literature. Distinguishing total and net plastic transport is crucial to consider fluctuations in freshwater discharge, tidal dynamics and trapping effects caused by the interactions with aquatic vegetation and/or other sinks.

Abstract. Plastic is an emerging pollutant, and the quantities in rivers and oceans are expected to increase. Rivers are assumed to transport land-based plastic into the ocean, and the fluvial and marine transport processes have been relatively well studied to date. However, the processes controlling the transport in tidal rivers and estuaries, the interface between fluvial and marine systems, remain largely unresolved. For this reason, current estimates of riverine plastic pollution and export into the ocean remain highly uncertain. Hydrodynamics in tidal rivers and estuaries are influenced by tides and freshwater discharge. As a consequence, flow velocity direction and magnitude can change diurnally. In turn, this impacts the transport dynamics of solutes and pollutants, including plastics. Plastic transport dynamics in tidal rivers and estuaries remain understudied, yet the available observations suggest that plastics can be retained here for long time periods, especially during periods of low net discharge. Additional factors such as riparian vegetation and riverbank characteristics, in combination with bidirectional flows and varying water levels, can lead to even higher likelihood of long-term retention. Here, we provide a first observation-based estimate of net plastic transport on a daily time scale in tidal rivers. For this purpose, we developed a simple Eulerian approach using sub-hourly observations of plastic transport and discharge during full tidal cycles. We applied our method to the highly polluted Saigon river, Vietnam, throughout six full tidal cycles in May 2022. We show that the net plastic transport is about 27–32 % of the total plastic transport. We found that plastic transport and river discharge are positively and significantly correlated (Pearson's r = 0.87, R2 = 0.75). The net transport of plastic is higher than the net discharge (27–32 % and 18 %, respectively), suggesting that plastic transport is governed by other factors than water flow. Such factors include wind, varying plastic concentrations in the water, and entrapment of plastics downstream of the measurement site. The plastic net transport rates alternate between positive (seaward) net transport and negative (landward) net transport, as a result of the diurnal inequality in the tidal cycles. We found that soft and neutrally buoyant items had considerably lower net transport rates than rigid and highly buoyant items (11–17 % vs 31–39 %), suggesting the retention time strongly depends on item characteristics. Our results demonstrate the crucial role of tidal dynamics and bidirectional flows in net plastic transport. With this paper we emphasize the importance of understanding fundamental transport dynamics in tidal rivers and estuaries to ultimately reduce the uncertainties of plastic emission estimates into the ocean.