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Submesoscale eddies and their potential for buoyant microplastic accumulation
Summary
This study investigates how small ocean eddies called submesoscale eddies can trap and concentrate buoyant microplastics below the water surface, not just at the top. Using both physical oceanographic measurements and laboratory experiments, researchers found that these rotating water masses create subsurface attractors that pull floating particles downward. This matters because it helps explain why microplastics are found throughout the water column rather than only at the surface, complicating efforts to clean up or track ocean plastic pollution.
Eddies are ubiquitous throughout the world oceans, and represent a mechanism for transport and re-distribution of particles such as microplastics trapped within. While much research is devoted to plastic distribution at the ocean surface and near-surface, very little is known about their subsurface distribution. A particularly promising candidate for this mechanism is the submesoscale eddy – also ubiquitous and characterized by strong vertical velocities. However, investigating the potential of particle redistribution by submesoscale eddy interaction requires structural information about these eddies that is also quite scarce in the literature. In Chapter 2, we conduct a drifter analysis of a compact, particularly coherent submesoscale eddy discovered in the Mediterranean Sea. We identify characteristics of its flow and geometry, finding that vortex stretching dominates the vorticity equation, the elliptical eddy interior is approximately in cyclogeostrophic balance, and the eddy core is approximately in rigid body rotation. Upon characterizing this eddy, we transition to studying where and how particles might accumulate in an eddy with vertical structure. Specifically, the next chapters seek to understand potential ways in which buoyant microplastics can be re-distributed at depth by eddies characterized by an overturning circulation in addition to an azimuthal circulation. To this end, I conduct a laboratory experiment characterizing the evolution of buoyant microsphere distributions within a symmetric lid-driven eddy model in Chapter 3, in which particles demonstrate relatively fast accumulation towards a subsurface attractor. This phenomenon and its implications for the resulting particle distribution are further investigated through numerical modeling of the eddy flow and properties of corresponding fluid and microsphere trajectories. Finally, in Chapter 4 I investigate a perturbation to the above laboratory experiment in which the lid angle is tilted, resulting in buoyant microspheres attracted to a tilted subsurface attractor while transiently interacting with perimetric resonant island features.