Microplastic Hotspots on a Tropical Estuarine-Bay System

The accumulation and export of microplastics are associated with growing negative socio-environmentalimpacts, which are intensified in coastal and estuarine areas. The Santos Estuarine System is the mostpopulous estuarine region and the largest seaport in Brazil, where the movement and accumulation of microplasticsis an integrated effect of diverse hydrodynamic conditions. To identify the vulnerability of pollutionby microplastics in extreme conditions of sea surface elevation in this complex estuarine region, the couplingbetween Lagrangian and Eulerian modelling was employed, an approach that has become popular in thestudy of ocean movements. The trajectories of microplastics from potential sources were modelled usingOpendrift, forced by currents from the Eulerian model Estuarine, Coastal and Ocean Model (ECOM), andwaves from Wavewatch III. Particle concentrations and three-dimensional stochastic probability of contactwith the continent were evaluated, demonstrating that, regardless of the sea surface elevation condition, microplasticsare more exported to the South Brazilian Bight than accumulated on the margins and/or bottomof the system. During higher elevations of the sea surface, there is a greater accumulation of microplasticson the estuary margins, with spring tide promoting faster accumulation. Although the entire coastline ofSantos and adjacent beaches are prone to the largest accumulations, portions further to the Northeast ofthe estuary have not shown the propensity to be contaminated with microplastics released into estuarinechannels. The highest concentrations are actively drifting in the system’s water column.