Effects of particle buoyancy, release location, and diel vertical migration on exposure of marine organisms to microplastics in Delaware Bay

Abstract

Plastics are ubiquitous in the global ocean and present a potential hazard to marine organisms. One challenge in assessing the impact of plastics on marine organisms is calculating their exposure to the background plastics field. Guided by observational data of marine organisms and microplastics in the Delaware Bay, this study applies a rational approach for determining exposure times based on a hydrodynamic model of the Delaware Bay paired with particle tracking. We consider two different marine organism behaviors, organisms that stay at the surface and those that exhibit diel vertical migration (DVM), as well as different release locations in the bay in order to understand the spatial variability in microplastics exposure. Microplastics are either assumed to be neutrally-buoyant or surface-trapped, so that they either move with the full three-dimensional estuarine circulation or only with the surface currents. Total exposure time is then determined from the modeled time-integrated background microplastics concentrations for each simulated marine organism path. Exposure was found to be greatest for surface-trapped organisms exposed to surface-trapped plastics as they both tend to aggregate in tidelines. DVM limits exposure to plastics by removing organisms from the surface-trapped plastics field. For neutrally-buoyant plastics, surface-trapped organismal exposure time greatly depends on the organism release location. Exposure is modified by two competing DVM influences: Lesser plastic concentrations at depth decrease the exposure, whereas limiting motion toward the mouth of the bay where the plastic concentrations are smaller increases the exposure. Marine organism exposure to microplastics in a dynamic estuarine environment greatly depends on plastic buoyancy and an organism's vertical position. Both of these factors need to be taken into account for comprehensive microplastic impact studies.