Observations and idealized modelling of microplastic transport in estuaries: The exemplary case of an upwelling system (Ria de Vigo, NW Spain)

Microplastics (MPs) pollution in marine environments has received considerable attention over the past two decades due to the increased awareness of its potential risks to ecosystems. Numerical simulations can be used to provide estimates of MPs fate and distribution, but so far this approach has been largely applied to the open ocean. In this work, the distribution patterns of MPs in a prototype coastal upwelling environment (Ilia de Vigo estuary, Spain) is investigated using a combined approach of field data and modelling. Water and sediment samples were collected at different locations along the Ilia during both upwelling and downwelling conditions. Experiments using an idealized 2D-vertical model were conducted to explain the observed MP distribution and elucidate the relative importance of river discharge, wind-driven and density-driven circulation. Microplastics were found at all stations in all samples. The largest observed fraction of MPs corresponded to fibres followed by plastic paint sheets and fragments. The trapping or flushing of MPs was mainly controlled by the wind forcing, whose effects on the circulation normally exceeds those of the density-driven and river flows. During upwelling conditions, more MP items were collected near the surface at the outer half of the estuary than during downwelling. The seaward near-surface circulation induced by the wind and density gradient jointly contributed to flushing out floating MPs. Near the bottom, the landward wind-induced and gravitational circulation formed estuarine MP maxima (EMPM) inside the estuary. Pellets, fragments, and films were more efficiently trapped than fibres, as their EMPM were located more upstream. The results suggested that downwelling-favourable winds caused an overall landward (seaward) displacement of the distribution of floating (sinking) MPs. Modelling results indicate that winds dominate the circulation in the outer part of the estuary, whereas near the head the gravitational circulation takes over the control of the net flow. The particular location of the EMPM appears to be controlled by the competition of density-driven and wind-driven flows.