Depth-stratified microplastic pollution in the Arabian Sea: Abundance, composition, and transport mechanisms

Microplastics (MPs) have emerged as pervasive contaminants in marine ecosystems; however, their vertical distribution within the Arabian Sea water column remains poorly understood. This study investigated the abundance, characteristics, polymer composition, and vertical transport of MPs across surface (0-10 m), middle (10-25 m), and bottom (25-40 m) waters at six offshore stations extending from Kerala to Gujarat along the eastern Arabian Sea. A total of 2758 MPs were identified from all sampling stations, Surface waters exhibited the highest standardized MP abundance among the investigated depth layers (2452 MPs m^-3), followed by middle (244 MPs m^-3) and bottom layers (62 MPs m^-3), indicating strong vertical stratification. Karnataka recorded the greatest MPs abundance (577 MPs m^-3), whereas Gujarat exhibited the lowest concentration (235 MPs m^-3). Fibres were the dominant morphotype (54.7%), followed by fragments, sheets, and foam particles, while particles <2 mm constituted the major size fraction across all depths. Blue and red MPs predominated throughout the study area. ATR-FTIR analysis identified LDPE (13.3%), PET (8.3%), PS (8.3%), PA (6.7%), and PP (6.7%) as the major polymers, reflecting inputs from fishing activities, packaging materials, synthetic textiles, and maritime operations. Multivariate analyses (ANOVA, PCA, HCA, correlation, and NMDS) revealed significant station-specific and depth-dependent variability, confirming selective vertical redistribution through biofouling, aggregation, and density-driven settling. These findings demonstrate that the Arabian Sea functions as an active transport and storage continuum for MPs and highlight the necessity of incorporating vertical water-column assessments into regional marine pollution monitoring and ecological risk evaluation frameworks.