Dynamic signatures of microplastic distribution across the water column of Yangtze River Estuary: Complicated implication of tidal effects

Riverine microplastic (MP) discharge into the ocean contributes greatly to global MP contamination, yet our understanding of this process remains primitive. To deepen our interpretation of the dynamic MP variation throughout the estuarine water columns, we sampled at Xuliujing, the saltwater intrusion node of the Yangtze River Estuary, over the course of ebb and flood tides in four seasons (July and October 2017, January and May 2018 respectively). We observed that the collision of downstream and upstream currents contributed to the high MP concentration and that the mean MP abundance fluctuated with the tide. A model of microplastics residual net flux (MPRF-MODEL), taking the seasonal abundance and vertical distribution of MP along with current velocity into consideration, was developed to predict the net flux of MP throughout the full water columns. 2154 ± 359.7 t/year of MP was estimated to flow into the East China Sea via the River in 2017–2018. Our study suggests that riverine MP flux can be overestimated due to reciprocating current carried MP from the estuary. Using the tidal and seasonal variation in MP distribution, we calculated the tide impact factor index (TIFI) for the Yangtze River Estuary to be between 38.11% and 58.05%. In summary, this study provides a baseline of MP flux research in the Yangtze River for similar tidal-controlled rivers and a contextual understanding of how to appropriately sample and accurately estimate in a dynamic estuary system. The redistribution of microplastics may be impacted by complex tide processes. Although not observed in this study, it may merit investigation.