Lagoons as Ocean Gatekeepers? Seasonal Transport and Retention Dynamics of Floating Macro-debris in the Umgeni River, South Africa

Rivers are major transport routes for land-based ocean plastic pollution. Spatiotemporal, geomorphological and hydrological factors that influence river debris passage are poorly understood. To address such knowledge gaps, we employed GPS drifters (n = 66 + 4 pilot) mimicking macroplastics (> 5 cm) across three different rainfall phases in the Umgeni River, South Africa. We analysed the drifter trajectories to investigate seasonal transport and retention to estimate debris emission rates into the Indian Ocean. 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 to the Indian Ocean. Mean retention durations showed a notable decrease from the dry phase towards both the peak wet and the wet–dry transition phases, indicating increased debris mobility during the wet season. Drifters were retained frequently in upstream river sections along meanders and vegetated banks, but the downstream estuarine lagoon emerged as the dominant sink with long-term retention limiting drifter exports into the ocean. Our study presents spatiotemporal insights of macroplastic debris transport to inform river plastic transport modelling and effective debris cleanup and mitigation policy frameworks.