Modelling the surface transport and spatio-temporal extent of microplastic pollution in the Southern Ocean and Antarctic waters

The Southern Ocean has long been considered a pristine marine environment, protected from pollutants such as microplastics by its remote location and limited human presence (Waller et al., 2017). However, microplastics are now a ubiquitous marine pollutant and research has highlighted the transport to, and presence of, surface-floating microplastics in the Southern Ocean. Yet large gaps exist in our knowledge of the transport pathways and accumulation zones of floating microplastic in the region. To increase understanding of the surface transport of floating microplastics in the Southern Ocean, this thesis used the OceanParcels Lagrangian ocean analysis framework, forced with 1/12° resolution velocity fields from the Ocean Regulation of Climate by Heat and Carbon Sequestration and Transport project ocean-sea ice model. Assessment of the model set up that compared modelled and observed drifter trajectories revealed a generally good level of agreement, allowing confidence in the suitability of the model to identify and characterise transport pathways of floating microplastics in the Southern Ocean. Simulations of virtual microplastic transport across the Antarctic Circumpolar Current have highlighted the key role that Stokes drift plays in the southward transport of floating microplastics. The results also suggest that southward transport of floating microplastics is likely to occur in locations, such as Drake Passage, where the northern and southern boundary of the Antarctic Circumpolar Current are steered topographically northwards. Simulated trajectories of floating microplastics released from five coastal Antarctic research stations revealed that sea ice transport can have a large impact on the pathway of floating microplastics, for example by facilitating northward transport from coastal areas into the Antarctic Circumpolar Current. The spatial and temporal impact of sea ice transport was not uniform, however, highlighting the need to consider the localised oceanographic and sea ice conditions in conjunction with the source of microplastic pollution when assessing transport pathways and dispersal. The knowledge of floating microplastic transport pathways in the Southern Ocean gained by this thesis can be used alongside future development of the 3 OceanParcels-ORCHESTRA model framework to inform future research into, and management of, the impact of floating microplastic pollution and other pollutants on the ecosystem of the Southern Ocean and potentially other iceinfluenced regions.