Hydroclimatic-driven pulse enrichment amplifies riverine microplastic export in developing countries.

Rivers are key conduits in the global microplastic cycle, but how much they export to ` ocean and what controls this export remain unresolved. Here, we present a global daily dataset of riverine microplastic concentrations and exports and find that rivers delivered far more microplastics to the ocean than most previous estimates suggest. Ninety-six percent of this export originated in the Global South, with Southeast and East Asia contributing >50% of the total flux. Furthermore, ~70% of global freshwater discharge exhibits "pulse enrichment," whereby rainfall-driven high flows trigger spikes in microplastic concentrations and fluxes; 80% of these cases occur in developing countries. Without stronger upstream controls on plastic production and waste management, climate-driven increases in extreme rainfall may intensify these pulses, worsening marine microplastic pollution.