Plankton-mediated coupling of phthalate esters across seawater, biota, and sediments in the northern Persian Gulf: Industrial vs urban coasts.

Phthalate esters (PAEs) are widely used plastic additives continuously released into marine environments, yet biologically mediated partitioning and sediment-biota coupling in semi-enclosed industrial basins remain insufficiently quantified. This study presents an integrated multi-matrix assessment of six priority PAEs (DMP, DEP, DBP, BBP, DEHP, DnOP) in seawater, plankton, and surface sediments from the northern Persian Gulf, comparing a petrochemical-dominated coastline (Assaluyeh) with an urban shoreline (Bushehr) across cold and warm seasons. Total PAE concentrations (ΣPAEs) followed a consistent matrix-dependent hierarchy: sediment > plankton > > seawater (Kruskal-Wallis, p < 0.001; ε2 ≈ 0.88). Sediment concentrations reached 59.00 ng g-1 dw in warm-season Assaluyeh, while plankton ranged from 16.80 to 30.60 ng g-1 dw and seawater from 0.325 to 0.597 ng L-1. Warm-season levels were significantly elevated across matrices (p = 0.029), coinciding with increased plankton lipid content (up to 16.52%) and higher sediment TOC (up to 1.93%). Biota-water bioaccumulation factors (BAFs) were on the order of 104-105 L kg-1, confirming lipid-driven enrichment of hydrophobic congeners, particularly DEHP (~41% of ΣPAEs) and DBP (~22%). In contrast, lipid- and TOC-normalized biota-sediment accumulation factors (BSAFs) were close to unity, indicating near-equilibrium partitioning rather than trophic magnification. Strong plankton-sediment correlations (ρ = 0.84, p < 0.001) support biologically mediated vertical transfer. These findings highlight the critical role of plankton in regulating contaminant redistribution in semi-enclosed industrial seas, with implications for bioavailability, benthic exposure, and sedimentary accumulation of PAEs. The results provide a framework for improving coastal monitoring in rapidly industrializing marine regions.