Cross-compartment differentiation of microplastics at the water-sediment interface in the Changjiang Estuary under an enhanced high-density flotation approach.

The vertical distribution and fate of microplastics at the water-sediment interface remain insufficiently understood in energetic estuarine environments. In this study, surface water, bottom water, and sediments were investigated in the Changjiang Estuary during May and August 2020 to compare the composition, size distribution, and environmental associations of microplastics across environmental compartments. To improve the recovery of high-density polymers in sediments, a saturated potassium formate solution with a density of 1.5 g/cm3 was used for sediment flotation. Clear compositional differences were observed among surface water, bottom water, and sediments. Polyethylene terephthalate (PET) was abundant in the detected particle assemblages across the three environmental compartments, while fibers were more enriched in sediments. In contrast, differences in microplastic length distribution among the three environmental compartments were not significant in either May or August. These results suggest that particle size alone did not explain the observed cross-compartment differentiation at the scale of this study. Spatial patterns, redundancy analysis, and Mantel tests further indicated that turbidity, water depth, and other hydrological gradients were associated with variation in microplastic composition across compartments. Taken together, the results suggest that the Changjiang Estuary may not function solely as a passive transport pathway for microplastics. Instead, microplastic characteristics differed among surface water, bottom water, and sediment compartments at the water-sediment interface, and these differences were associated with local environmental gradients. The enhanced high-density flotation strategy improved the detection of high-density polymers and provides methodological support for evaluating the transport and fate of microplastics in estuarine environments. These findings highlight the importance of considering bottom water and sediments, in addition to surface water, when assessing estuarine microplastic budgets and environmental fate.