Microplastics hotspots on a tropical estuarine-bay system

ABSTRACT The accumulation and export of microplastics are associated with growing negative socio-environmental impacts, which are intensified in coastal and estuarine areas. The Santos Estuarine System is the most populous estuarine region and the largest seaport in Brazil, where the movement and accumulation of microplastics is an integrated effect of diverse hydrodynamic conditions. To identify the vulnerability of pollution by microplastics in extreme conditions of sea surface elevation, the coupling between Lagrangian and Eulerian modelling was employed, an approach that has become popular in the study of ocean movements. The trajectories of microplastics from potential sources were modelled using Opendrift, forced by currents from the Eulerian model Estuarine, Coastal and Ocean Model (ECOM), and waves from Wavewatch III. Particle concentrations and three-dimensional stochastic probability of contact with the continent were evaluated, demonstrating that, regardless of the sea surface elevation condition, microplastics are more exported to the South Brazilian Bight than accumulated on the margins and/or bottom of the system. There is a greater accumulation of microplastics on the estuary margins during higher elevations of the sea surface, with spring tide promoting faster accumulation. Although the entire coastline of Santos and adjacent beaches are prone to the largest accumulations, portions further to the Northeast of the estuary have not shown the propensity to be contaminated with microplastics released into estuarine channels. The highest concentrations are actively drifting in the system’s water column.