Observations and Simulations of Microplastic Debris in a Tide, Wind, and Freshwater-Driven Estuarine Environment: the Delaware Bay.

Microplastic (MP) in estuarine and coastal environments remains poorly characterized, despite the importance of these physically dynamic regions as a buffer between land, freshwater environments, and the open ocean where plastic debris accumulates. We sampled MP particles to determine concentration, size, and type in Delaware Bay and numerically simulated transport and distribution at a high spatiotemporal resolution of positively buoyant particles, representing common MP types. Baywide MP concentrations averaged between 0.19 and 1.24 pieces m-3 depending on size fraction (300-1000 and 1000-5000 μm) and sampling month (April and June 2017). Upper bay stations, which are located in or near the estuarine turbidity maximum, had higher MP concentrations than lower bay and New Jersey shore stations. Fragments were predominately polyethylene, and filaments predominately polypropylene. Model results suggest that buoyant particles quickly (i.e., within hours) organize in patchy, highly inhomogeneous distributions, creating "hot spots" of MP. In the presence of variable currents driven by buoyancy, wind, and tides, we predict high spatial and temporal variability of MP distributions in Delaware Bay; MP concentrations could vary by a factor of 1000 within a tidal cycle at our sample locations. Collectively, these observations and simulations provide a baseline of MP concentrations in Delaware Bay along with broader, contextual understanding for how measurements reflect MP concentrations in a dynamic estuarine system.