Microplastic distribution and potential ecological risk in the Amazon River estuary: No detectable differences in composition among environmental compartments in a mesotidal, high-discharge system

The Amazon River estuary is a highly dynamic system characterized by the world’s largest freshwater discharge (~200,000 m³ s⁻¹) and a strong mesotidal regime, with a tidal range of 2–4 m. Together, these hydrodynamic conditions strongly influence microplastic transport, redistribution, and fate. This study characterized microplastics in surface water, mid-depth water, and bottom sediments collected from five sampling stations in the northern sector of the Amazon River mouth, totaling 15 independent field samples. Particle morphology, color, size distribution (0.063–5 mm), polymer composition, and screening-level ecological hazard were assessed. Fibers (49.4–51.8%) and fragments (47.4–50.6%) predominated across all environmental compartments, whereas films were rare or absent. Blue particles were the most frequent (49.6–53.0%), and the 0.1–0.3 mm size class was dominant (32.3–33.7%). Microplastic composition showed no significant differences among compartments or stations, although the limited number of samples per compartment warrants caution in interpreting the absence of a compartment effect. The observed overlap among samples is consistent with similar particle composition within the sampled northern sector, although it does not establish estuary-wide compositional homogeneity or particle redistribution. The observed cross-compartment pattern may reflect the dynamic hydrodynamic setting of the Amazon River mouth, although the relevant hydrodynamic processes were not directly measured. Screening-level indices indicated a high polymer-hazard score (PHI = 575) and a PERI score of 3,552, corresponding to the formal extreme-danger screening category, despite the comparatively low pollution load indicated by the PLI (4.17). These indices represent conservative screening metrics and should not be interpreted as evidence of observed ecological effects. Overall, microplastics were detected across all sampled compartments in the northern sector, with no detectable compartment-level compositional differences under the present design and a contrasting screening profile characterized by low pollution load but elevated polymer-hazard and PERI scores during the 2023 rainy season.