Modeling the accumulation of floating microplastic in the subtropical ocean gyres

Abstract

Plastic, in particular microplastic, presents a significant threat to marine ecosystems. Floating microplastic in the oceans tends to accumulate in the subtropical gyres, forming accumulation regions commonly referred to as garbage patches in each ocean basin. The location of the garbage patches is determined by the ocean surface currents. This thesis examines the contributions of the Ekman and geostrophic surface current and Stokes drift components on the accumulation, along with the role of the eddy kinetic energy (EKE), which is seen as a proxy for mesoscale eddy activity, the mixing layer depth, as a proxy for vertical microplastic mixing, and the vertical Ekman pumping velocity, as an indicator of depth integrated current convergence. The microplastic distribution was modeled globally using both linear regression and Lagrangian modeling approaches, with emphasis on the North Pacific and North Atlantic basins.

Global Lagrangian simulations show garbage patch formation in each of the subtropical ocean gyres. The simulated North Pacific garbage patch matches the location from observations. The simulated North Atlantic garbage patch matches the garbage patch latitude from observations but is too far west. Wind-driven surface Ekman currents account for the location and variability of the garbage patch. On basin-wide scales, the depth integrated Ekman transport is less crucial than the surface Ekman transport. The geostrophic currents are not found to contribute to accumulation, instead counteracting the Ekman current induced accumulation and spreading the microplastic over a larger surfacearea. The simulations show that Stokes drift has the effect to disperse microplastic from the garbage patch in the North Pacific, while in the North Atlantic it leads to more concentrated accumulation within the garbage patch. Stokes drift also leads with increased microplastic transport to the polar regions. The average position of the garbage patches in the North Atlantic and the North Pacific has seasonal variability. Microplastic tends to accumulate in regions of minimal EKE in the North Pacific, but shows no such behavior in the North Atlantic, indicating a less prominent role for mesoscale eddies in microplastic accumulation in this basin. The mixing layer depth is not found to be a contributor to microplastic accumulation. Finally, the accumulation pattern of microplastic in the North Atlantic is more sensitive to the temporal resolution of the flow field data used to advect the microplastic than the North Pacific.

The locations of the garbage patches are sensitive to numerous different current components and other processes and this requires them to be consistently incorporated in future modeling efforts. Particularly Stokes drift is currently not always considered for microplastic modeling efforts despite having a significant influence on the global microplastic distribution. Improved microplastic transport modeling can be applied in numerous follow-up studies, including the efficacy of open-ocean plastic clean-up efforts and ecological impacts of plastic pollution in vulnerable ecosystems.