Plastic debris budget and fluxes along the Barcelona coastline

Plastic debris densities in Barcelona waters are comparable to those in subtropical gyres, with the Barcelona coastline being among the most affected by plastic pollution in the Mediterranean Sea. A mass budget for discharged plastic debris was estimated for the year 2020-2021. The calculation used discharge volume data from the combined sewer overflow system and surrounding rivers, combined with linear regressions derived from collected samples at these sites and representative weights for each plastic size class. Fluxes were determined from Lagrangian numerical simulations, which were informed by the amounts of discharged plastic debris during discharge events. To account for unresolved coastal processes, a probabilistic beaching module was applied to the Lagrangian numerical model. Beaching amounts reached 98.4% for discharges from the combined sewer overflow system, mostly affecting urban beaches. River discharges were more strongly influenced by hydrodynamic conditions, although there was substantial retention of particles in the Llobregat River mouth. A total of 6.0 × 10¹⁰ items year^-1 of plastic debris were discharged, corresponding to a mass budget of 17.0 metric tonnes year^-1. Such high levels of plastic debris discharges from the Barcelona coastline underscore the urgent need to reduce plastic waste at its source and implement mitigation measures before it reaches the marine environment.