Estuaries as Filters for Riverine Microplastics: Simulations in a Large, Coastal-Plain Estuary

Abstract

Public awareness of microplastics and their widespread presence throughout most bodies of water are increasingly documented. The accumulation of microplastics in the ocean, however, appears to be far less than their riverine inputs, suggesting that there is a “missing sink” of plastics in the ocean. Estuaries have long been recognized as filters for riverine material in marine biogeochemical budgets. Here we use a model of estuarine microplastic transport to test the hypothesis that the Chesapeake Bay, a large coastal-plain estuary in eastern North America, is a potentially large filter, or “sink,” of riverine microplastics. The 1-year composite simulation, which tracks an equal number of buoyant and sinking 5-mm diameter particles, shows that 94% of riverine microplastics are beached, with only 5% exported from the Bay, and 1% remaining in the water column. We evaluate the robustness of this finding by conducting additional simulations in a tributary of the Bay for different years, particle densities, particle sizes, turbulent dissipation rates, and shoreline characteristics. The resulting microplastic transport and fate were sensitive to interannual variability over a decadal (2010–2019) analysis, with greater export out of the Bay during high streamflow years. Particle size was found to be unimportant while particle density – specifically if a particle was buoyant or not – was found to significantly influence overall fate and mean duration in the water column. Positively buoyant microplastics are more mobile due to being in the seaward branch of the residual estuarine circulation while negatively buoyant microplastics are transported a lesser distance due to being in the landward branch, and therefore tend to deposit on coastlines close to their river sources, which may help guide sampling campaigns. Half of all riverine microplastics that beach do so within 7–13 days, while those that leave the bay do so within 26 days. Despite microplastic distributions being sensitive to some modeling choices (e.g., particle density and shoreline hardening), in all scenarios most of riverine plastics do not make it to the ocean, suggesting that estuaries may serve as a filter for riverine microplastics.

Hydrology as a Driver of Floating River Plastic Transport

“…Partly this is caused by the lack of observations, as river mouths are often difficult to monitor. The available data do suggest that the majority of plastics do not leave the estuary (López et al., 2021). Future work may focus on collecting more observations within the complex tidal areas with bidirectional flow dynamics.…”

“…Another likely reason for the increased downstream values is the (temporary) accumulation in the river mouth. Due to the tidal dynamics, the river flow alternates direction diurnally (Blondel & Buschman, 2022; López et al., 2021; Okuku et al., 2022). The floating plastic within the tidal zone therefore also flows back and forth, increasing the likelihood of accumulation on riverbanks, or deposition on the riverbed (Acha et al., 2003; Tramoy et al., 2020).…”

Hydrology as driver of floating river plastic transport

“…Partly this is caused by the lack of observations, as river mouths are often difficult to monitor. The available data do suggest that the majority of plastics do not leave the estuary (López et al, 2021). Future work may focus on collecting more observations within the complex tidal areas with bidirectional flow dynamics.…”

“…Another likely reason for the increased downstream values is the (temporary) accumulation in the river mouth. Due to the tidal dynamics, the river flow alternates direction diurnally (Blondel & Buschman, 2022;López et al, 2021;Okuku et al, 2022). The floating plastic within the tidal zone therefore also flows back and forth, increasing the likelihood of accumulation on riverbanks, or deposition on the riverbed (Acha et al, 2003;Tramoy et al, 2020).…”

Accumulation, transformation and transport of microplastics in estuarine fronts

“…In addition to the extensive applied Lagrangian particle-tracking oceanic models summarized in ref. 127 , some other particle-tracking models have also been applied to microplastic transport simulations in coastal and estuarine regions, for instance the three-dimensional hydrodynamic and suspended sediment transport model (HYDROTAM-3D) 129 , Track Marine Plastic Debris (TrackMPD) 130 , the Delft3D-Water Quality Particle tracking module (D-WAQ PART) 131 and the ichthyoplankton dynamics tracker (Ichthyop) 132 . Unlike the Lagrangian approach, Eulerian models simulate microplastics as passive tracers in terms of their mass or volume concentrations, which are advected by the velocity fields and diffused by the parameterized turbulence 133 .…”