Quantitative tidal control of spatiotemporal microplastics variability in an estuary

The transport of microplastics (MPs) from rivers to the ocean remains poorly constrained, highlighting the need for quantitative evaluation of the MPs budget in estuarine environments. This study quantitatively evaluated MPs transport in an estuary using field observations and numerical simulations, focusing on vertical distribution, mass flux, and salt-wedge processes. MPs in estuarine environments were found to accumulate vertically and be re-transported by tidal forcing, resulting in seaward export over a single tidal cycle.

Field observations were conducted in the estuarine reach of the Oita River during one tidal cycle, with five sampling campaigns from high tide through low tide to the subsequent high tide. Water samples were collected at 50 cm vertical intervals from the surface to near the riverbed, and flow velocities were measured simultaneously.

The results showed that vertical MPs mass fluxes consistently followed a depth-dependent pattern of bottom > mid-depth > surface during both neap and spring tides, indicating persistent bottom-dominated transport. Spatiotemporal integration of MPs mass fluxes yielded a net seaward transport of +1.22 × 10³ mg/m over one tidal cycle, quantitatively demonstrating MPs export from land to the coastal ocean. This net export resulted from dominant land-to-sea transport during ebb tide, driven by upstream MPs fluxes and resuspension from the bottom layer, exceeding landward transport during flood tide.

Numerical simulations reproduced MPs accumulation near the halocline formed during ebb tide and subsequent seaward transport by bottom currents, consistent with the observed positive net flux. These results demonstrate that estuarine MPs transport is asymmetrically controlled by tidal flow and salinity stratification, with the halocline playing a key role in MPs accumulation and seaward export.