Title: Buoyant river outflows as a control on microplastic fate and transport: sources, transformations, dispersion and sinks

Funding agency:  NOAA

Project Lead: Bob Chant, Co-PI Grace Saba


Period of Performance:  05/11/20-06/10/22

Total budget:  $639,552

Project Summary

Plastics are frequently observed marine debris and there is growing concern about microplastic ecotoxicity and growth of biofilm on microplastic surfaces. Rivers are considered a major source of plastic marine debris. However, the fate and transport of microplastics from land based sources (e.g., storm water runoff, wastewater effluents, and remobilization of shoreline stranded debris) to the coastal and deep ocean is poorly understood thus limiting our ability to develop best management practices. We hypothesize that physical and biogeochemical processes in estuarine and river plume systems act to trap microplastics and that this trapping is strongly controlled by frontal systems. We expect that away from frontal systems microplastic abundances are low and plastic distributions are well dispersed. In contrast, in frontal systems we expect elevated concentrations of positively buoyant microplastics that coincide with elevated biological activity. Consequently, we hypothesize that river plume fronts represent a vector whereby microplastics are assimilated into the food chain via zooplankton ingestion and are predominately repackaged into zooplankton fecal pellets with rapid setting velocities. This would radically alter their transport pathways. An example of material accumulating in a river plume front is depicted in Figure 1. The frontal system is visually evident in the top photograph with particulate laden river water appearing brown in contrast to blue ocean water. The schematic below it shows the converging flows accumulating plastics and zooplankton at the front, which is consistent with acoustic images of a front passing a moored current meter (lower panel). Frontal systems, due to their tendency to concentrate material (i.e. food), are often associated with elevated biological activity. Thus, as frontal systems accumulate plastics they may be locations where microplastics both enter the food chain through zooplankton consumption and are repackaged into sinking material that would tend to be transported back into the estuary by gravitationally driven landward flows at depth. Thus, we suggest that estuaries are a sink for microplastics, and the strength of this sink varies seasonally with winds, river discharge and zooplankton phenology. Our work will also allow us to understand how these processes may vary by microplastic particles characteristics (i.e., polymer, morphology, size). An integrated field and modeling approach is proposed to:

1: Compare plastic particle distribution, composition, morphology, and abundance across a range of salinity gradients at the mouth of the Delaware Bay during high and low flow conditions.

2: Determine the role of frontal systems as an entry point for microplastics into the food chain

3: Determine the impact that packaging microplastics in zooplankton fecal pellets has on effective settling velocity of microplastics.

4: Determine the impact that variable settling velocities of microplastics has on transport and trapping of the various microplastics in estuarine and coastal systems.

To achieve these aims an interdisciplinary team has been assembled including members with expertise in the physics of estuarine and coastal systems, zooplankton ecology, water quality engineering, and polymer chemistry. The proposed study will be performed in the Delaware River plume, which includes discharge from the highly urbanized Philadelphia metropolitan area. Our team has experience studying transport pathways in urban river plumes and in the abundance and distribution of microplastics in such systems. This work will provide critical insight into the spatial controls on microplastic uptake into the food chain by marine biota and in turn how that impacts the fate and transport of microplastics. These results can inform management practices that focus on mitigation strategies targeting plastic sources reaching frontal areas and cleanup efforts focused on these areas of increased uptake.