Cross compartment microplastic contamination in the Gembong River estuary: Evidence from brackish water, sediment, and mullet (Mugil cephalus)

Microplastic contamination in tropical estuaries has become an important issue linking environmental degradation, fisheries, and dietary exposure. This study examined the cross-compartmental occurrence of microplastics in the Gembong River Estuary, Indonesia, by analyzing brackish water, sediment, and mullet, including gills, digestive tract, and edible flesh. Microplastics were identified morphologically, quantified using matrix-specific units, statistically compared across stations and organs, and supported by FTIR analysis of selected representative particles. Fourier transform infrared spectroscopy revealed recurring polymer-associated signals across water, sediment, and mullet, suggesting comparable polymer-associated signals among matrices rather than direct evidence of particle transfer. Fragments dominated all matrices, accounting for 94.45% of particles in brackish water, 93.30% in sediment, and 68.00% in mullet. Sediment showed consistently higher abundance than brackish water, with the highest values recorded at residential and industrial stations. In mullet, gills contained the highest microplastic burden, followed by the digestive tract and flesh, and organ-level differences were significant. Preliminary, screening-level, scenario-based annual intake estimates ranged from 27,102 particles for one-year-old children to 203,268 particles for the general population under the stated consumption assumptions. These findings indicate the occurrence of cross-compartmental microplastics in the Gembong River Estuary and highlight the relevance of organ-specific reporting and preliminary dietary exposure estimates for estuarine pollution and seafood-safety assessment.