Tracking the origin of microplastics in the South Atlantic Subtropical Gyre

Plastic pollution poses one of the most urgent threats of present time to the marine environment. Global plastic production is growing exponentially and causing an unprecedented increase in the amount of mismanaged plastic waste available to enter the world’s oceans. Microplastic particles with a density lower than that of seawater float on the ocean surface and tend to accumulate in the Subtropical Gyres of all ocean basins due to geostrophic and Ekman currents and Stokes drift. This work develops a method to analyse the possible pathways followed by particles that were sampled in the South Atlantic Subtropical Gyre in order to identify their sources. To do so, particle trajectories were computed by means of Lagrangian simulations backward in time and for each particle the probability of being sourced from nine selected cities bordering the South Atlantic Ocean was calculated. Probability calculations were performed following both an exponential and a quadratic modelling approach on two different datasets. Both models were implemented in such a way that the probability decreased with an increase in distance between the particle and the potential source. The probabilities were also a function of the source uncertainty distance, a parameter representing the uncertain distance at which a particle could be identified as being sourced from a given location. The consistency between the results obtained from two modelling approaches and the different datasets allowed to pinpoint the Rio de la Plata estuary (comprising Montevideo and Buenos Aires), Cape Town and Rio de Janeiro as the most likely sources of microplastics to the South Atlantic Subtropical Gyre. Additionally, from sensitivity analysis the most meaningful source uncertainty distance was determined to range between 25 and 100 km. The methodology developed to calculate land source probabilities is fully transferable and could be used regardless of the study domain and the source locations that want to be addressed. For this reason, its simplicity and reliability might make it a powerful tool to identify the relative contribution of different sources and provide a base on which quantitative studies can be performed to assess the amount of microplastics that accumulate in a given region from selected sources.