Relating horizontal subsurface microplastic movement to surface fields in the North Atlantic.

Supervisors: Erik Van Sebille, Philippe Delandmeter (Univ. Utrecht).
Marine microplastics (MP) are a major concern investigated in all disciplines of marine sciences, since they are a threat to marine life, carry toxic substances and can transport invasive species through the oceans (Andrady 2011, Lei et al. 2017). Even with joint efforts from many scientists, policy makers, and citizen scientists from all over the world, there are still knowledge gaps regarding the fate of plastic entering the ocean such as the pathways and accumulation (Van Sebille et al. 2015, Hardesty et al. 2017). Hence, there is a need of instruments with the capability to increase the quality of MP quantification. The proposed ESA (European Space Agency) satellite mission SKIM will be able to measure the Stokes drift, which is needed to increase the accuracy of total surface current measurements (Ardhuin et al. 2019, Onink et al. 2019). This development can help to increase the accuracy of Lagrangian surface simulations, since flow fields from Eulerian measurements and models are used for the advection of virtual particles (Onink et al. 2019). Virtual particles can represent many kinds of marine debris such as MP. However, this only accounts for surface simulations. This thesis investigates if two-dimensional surface measurements can be used to derive three-dimensional sub-surface flows of MP in the North Atlantic. Therefore, the Lagrangian Ocean Analysis PARCELS (Probably A Really Computationally Efficient Lagrangian Simulator) (Lange and Van Sebille 2017) is used to simulate particles advected by meridional and zonal NEMO (Nucleus for European Modelling of the Ocean) flow-fields. One surface simulation and two sub-surface simulations are performed. The sub-surface MP-particle flows are simulated by applying the empirical models for vertical mixing developed by Kukulka et al. (2012) and Poulain et al. (2019). The simulation based on the model from Poulain et al. (2019) is characterised by a rise velocity of one magnitude lower than the rise velocity of the simulation based on the model from Kukulka et al. (2012). Then, the travelled particle distances and directions after 3 days are calculated, since SKIM is designed to have an orbit time of 8 hours to 4 days days. Moreover, the differences between these distances and directions of surface and sub-surface trajectories are computed. The differences in changes of direction show a less significant result than the differences between distances. Even though the differences in distances show a certain pattern, the differences in changes of direction are not spatially co-appearing with certain patterns of the North Atlantic. Specifically, the differences between distances increase along the strong ocean currents of the North Atlantic such as the Gulf Stream. And in four of five investigated subsections of the North Atlantic, the differences in distances increase with increasing particle depths of the sub-surface simulations. Overall, the surface and sub-surface MP-flows differ, but essentially in distinctive regions. And the correlations of the differences in distances and a range of NEMO variables, e.g. mixed layer depth (MLD), do not indicate a strong relationship, but the two-dimensional histograms allow some hypothesis on the relationships. The differences in distances of horizontal sub-surface and surface trajectories are not directly correlated to increasing depth due to downward mixing, but to the changes in ocean dynamics with increasing depth. And as the dynamics and their variability vary spatially, the differences vary spatially.