Plastic Does Not Simply Flow into the Sea: River Transport Dynamics Affected by Tides and Floating Plants

Abstract

Plastic pollution is ubiquitous in aquatic environments worldwide. Rivers connect terrestrial and marine ecosystems, and therefore play a key role in the transport of land-based plastic waste towards the sea. Emerging research suggests that in estuaries and tidal rivers, the transition zone between rivers and the ocean, tidal dynamics play a significant role in plastic transport and retention dynamics. To date, observations and analyses in these systems have been limited, and the plastic transport dynamics during single tidal cycles remain poorly understood. Here, we investigated plastic transport, trapping, and re-mobilization of macroplastics (> 0.5 cm) in a tropical tidal river, focusing on short-term dynamics of individual tidal cycles. We used GPS trackers, released at different stages of the tidal cycle (ebb, flood, neap, spring). The studied plastic items demonstrated highly dynamic and intermittent transport behavior. Items spent almost half of the time (49%) temporarily stopped, mainly due to their entrapment in water hyacinths or at infrastructure. Items were almost always re-mobilized within 10 hours (85%), leading to intermittent behaviour with successive phases of stopping and transport. The tidal dynamics also resulted in bidirectional transport of items, with the median daily total transport distance within the 40 km study reach (8.9 km day−1) being over four times larger than the median daily net distance (2.0 km day−1). The median retention time of plastic items within the reach was 21 days (mean = 202 days). In total, 81% of the retrieved items were trapped within water hyacinths, emphasizing the important role of floating vegetation on river plastic transport dynamics. With this paper, we aim to provide data-driven insights in the macroplastic transport and retention dynamics in a tropical tidal river, which are crucial in the design of effective intervention and monitoring strategies, and estimating net plastic emission from rivers into the sea.

Keywords: macroplastic, water quality, Plastic pollution, Pollution transport, Marine debris, Hydrology