From surface to bottom: Tracking the temporal and morphological heterogeneity of microplastics in the maximum turbidity zone of the Yangtze Estuary

The estuary channels the inland to the coastal environment, considered a significant route of marine microplastic (MP) pollution inflow. However, the water column profiles of MPs in this area remain elusive, especially at high temporal resolution scale, hindering our ability to approximate its flux and evaluate the effectiveness of mitigation efforts. Hence, we investigated the physical and chemical diversity of MPs in the water column of the Yangtze River Estuary Maximum Turbidity Zone by a time-series monitoring strategy. Utilizing an advanced Laser Direct Infrared Chemical Imaging System (LDIR) imaging method, we quantified the shift patterns of MPs in this area. Overall, the average abundance of MPs reached 35.62 ± 34.86 items/L and 27.54 ± 31.54 items/L in surface and bottom waters throughout two tide cycles, respectively. MPs smaller than 100 μm were the most prevalent, with polyethylene (PE) and polypropylene (PP) being the dominant types in both layers. Despite moderate shape variability but relatively uniform sizes of surface MPs across polymer types, MPs in bottom estuarine water showed substantial heterogeneity in both size and shape among different polymers. This work underscores the necessity of high temporal resolution monitoring and integrating vertical stratification and multidimensional morphological analyses in MP studies of ecological assessment.