Microplastic accumulation and vertical distribution in the Delaware Estuary estuarine turbidity maximum

Marine plastic debris is a pervasive pollutant of global concern. Highly urbanized estuaries and coastal regions serve as a potential sink for microplastic debris. Due to dynamic tidal influences in an estuarine system, features such as the Estuarine Turbidity Maximum (ETM) trap particulate debris including microplastics. Thus, accumulation and aggregation of particulate material can occur at higher rates with an estuarine system than the open ocean. In this study, we characterize the morphology, size, and polymer identities of microplastic material throughout the water column within the Delaware Estuary ETM region as well as infer estuarine particle accumulation mechanisms through modeling of particulate debris. Field sampling found microplastic (MP) material at all stations throughout the Delaware Estuary ETM, with average concentrations of 1.4 MP pieces m^-3, 0.5 MP pieces m^-3, and 0.8 MP pieces m^-3 found at the surface, mid-water, and bottom depths, respectively. Fragments were the most abundant plastic type, followed by fibers, across all depths, whereas beads were found more commonly at depth. Polymer μFTIR analysis suggested fragments were predominately polyethylene, fibers were predominantly polyester and a semi-synthetic material rayon, and beads were predominantly polystyrene. Model results suggest particle distributions along the ETM and with depth are driven by the complex three-dimensional estuarine circulation due to density gradients, tides, and bathymetry. These analyses establish that microplastics are not simply a surface water feature and Estuarine Turbidity Maxima can influence the spatial distribution of material and microplastic morphology in a salt wedge estuary.