Microplastics in the sediment of the Amazonian fluvio-estuarine system: Possible influences of sediment characteristics on microplastics composition

The distribution of microplastics in estuarine sediments is influenced by the physical characteristics of the environment, although it remains poorly understood in tropical Amazonian systems. This study investigated the occurrence and distribution of microplastics in sediments from seven sites in the Pará River fluvio-estuarine system, Brazilian Amazon, focusing on relationships with grain size and organic matter content. Samples were collected during two campaigns, at the beginning of the rainy season (December 2022) and during the dry season (September 2023). Particle morphology, size, surface area, volume, and chemical composition were analyzed in relation to sediment variables. Microplastics were detected in all analyzed sediment samples, with concentrations ranging from 26.7 to 410 items kg⁻¹ dry sediment. Fibers were the dominant morphology in all samples. Total microplastic concentration was positively correlated with the sandy fraction (Spearman's ρ = 0.605, p = 0.049). Fiber abundance was positively associated with sand and negatively associated with silt, with the strongest relationship observed between large fibers (1–5 mm) and silt (ρ = −0.72, p = 0.013). Organic matter content showed no significant association with the evaluated variables. Among the identified polymers, polyester and polyethylene terephthalate (PET) were the most frequent. The observed correlations suggest that sediment grain size may contribute to spatial patterns of microplastic distribution in the studied fluvio-estuarine system, potentially in interaction with particle physical characteristics. Further spatially and seasonally replicated studies are needed to better understand the processes underlying these associations.