Surface Biogeochemical Regimes Shape Microplastic Abundance in the Eastern Indian Ocean

Abstract Open-ocean microplastic (MP) abundance is commonly reported as a contamination metric, but surface concentrations may reflect environmental redistribution, dilution, and retention in addition to source exposure. Here, we investigated surface MPs across 43 stations in the eastern Indian Ocean (EIO) during the late northeast monsoon period using in situ pumped sampling, μ-FTIR identification, and 22 environmental indicators spanning productivity, particle optical properties, hydrography, surface dynamics, mixed-layer structure, and maritime exposure. MPs were detected at all stations, with abundances ranging from 6.67 to 160.00 particles m–3 and a mean of 29.37 ± 26.72 particles m–3. This pronounced spatial heterogeneity is consistent with the intrinsic patchiness of open-ocean surface waters, where hydrodynamic transport, convergence, dispersion, and vertical redistribution can generate strong local variability in particle abundance. The regional MP assemblage was dominated by polyethylene terephthalate (PET), fibers, light-colored particles, and small MPs, with particles <200 μm accounting for 52.1% of the total. Elevated abundance was not explained by maritime exposure alone, but was associated with lower primary production, weaker particulate backscattering, warmer surface waters, and weaker surface-dynamic forcing. Using environmental variables alone to define surface-water regimes, we found that MP abundance was significantly higher in warm, low-productivity waters with weaker surface dynamics than in cooler, more productive waters with stronger surface dynamics (mean: 34.64 vs 17.18 particles m–3; Kruskal–Wallis H = 8.57, p = 0.0034). Particle composition, including polymer/material type, morphology, color, and size class, remained broadly similar across regimes. These findings suggest that environmental conditions modulate the surface abundance of an existing regional MP pool.