Seasonal transport, retention, and river-to-ocean transfer dynamics of floating macro-debris in the Umgeni River and estuarine lagoon, South Africa

Abstract

Abstract Riverine floating macroplastics are considered to contribute a substantial portion of marine plastic pollution, yet the spatiotemporal, geomorphological and hydrological factors that influence river debris passage are poorly understood. To address this, we conducted three field campaigns in the Umgeni River, South Africa, during hydrologically distinct periods (dry season, wet season and recessive season). We deployed GPS drifters ( n  = 66 + 4 pilot), each with a density of 0.94 g/cm 3 to mimic floating macroplastics (>5 cm) across five tributaries and the main river. We analysed the drifter trajectories during each campaign to investigate seasonal daily transport rates, retention zones and episodes, and river-to-ocean emission rates. Observed drifter trajectories showed modest seasonal differences in mean daily transport and retention counts. Our pilot drifters captured the 1:50–100 year return period flood occurring in April 2022 showing substantial flushing downstream (0.58 km day − 1 ) to the Indian Ocean. Mean retention durations showed a notable decrease from the dry period (mean = 508 h) towards both the wet and recessive periods (mean = 230 h and mean = 40 h; respectively), indicating increased debris mobility during the wet season. Drifters were retained frequently in upstream river sections along meanders and vegetated banks, which accounted for 48% of all retention events but the estuary emerged as the dominant sink with long-term retention (with mean retention >10 days) thus limiting drifter exports into the ocean. Our study presents quantitative insights of floating macroplastic transport across interconnected riverine domains over three seasons. The results provide novel empirical data on seasonal plastic mobility, retention zones and highlight episodic nature of debris flushing during floods. These findings support river plastic transport modelling and targeted debris cleanup and mitigation policy frameworks.