A major continental–marine interface: Spatial patterns of microplastic distribution across South America's largest coastal lagoon

Coastal lagoons act as important interfaces for the transport and redistribution of contaminants, yet the spatial patterns of microplastics in these systems remain poorly understood. Patos Lagoon (southern Brazil), the largest choked coastal lagoon in the world, is influenced by complex hydrodynamics and multiple anthropogenic pressures, but lagoon-wide assessments of microplastics remain scarce. This study provides the first comprehensive spatial evaluation of microplastics in surface waters across Patos Lagoon, spanning freshwater and estuarine environments. Surface water samples were collected at 27 stations across the lagoon. Microplastic concentrations ranged from 0.11 to 4.63 particles L⁻¹, with the highest values in the southern estuarine region. Fibers were the dominant particle shape throughout the lagoon, followed by fragments. Most particles belonged to the 125–1000 µm size class. Blue and transparent particles were the predominant color classes, and Fourier-transform infrared spectroscopy identified polyethylene and polypropylene as the most common polymers. Spearman correlation analyses revealed a moderate positive relationship between microplastic concentration and the Global Human Modification metric (ρ = 0.55, p = 0.04), indicating the influence of human disturbance. The observed spatial patterns are consistent with the combined influence of hydrodynamic processes and anthropogenic inputs, with the northern sector acting as a source region influenced by river discharge, the central sector functioning as a mixing zone, and the southern estuarine sector representing an accumulation area with potential export to the adjacent South Atlantic Ocean. These results highlight the role of large coastal lagoons as key links connecting continental plastic sources to marine environments.