Microplastic 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 in this complex estuarine region, 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. During higher elevations of the sea surface, there is a greater accumulation of microplastics
on the estuary margins, 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.