Plastic dispersion in the global ocean

Abstract

Plastic pollution is ubiquitous within the marine environment, found on coastlines, at the sea surface, within the water column, on the seafloor, in deep-sea sediments, and in sea ice at both Poles. However, the transport pathways and processes that determine the three-dimensional distribution of plastics in the global ocean are still not yet fully understood. In this thesis, the three-dimensional distribution of plastics is investigated using an ocean general circulation model, in a range of different scenarios. Firstly, the distribution of positively, neutrally, and negatively buoyant plastics in the global ocean with no processes of removal is explored. This reveals that plastics of different densities inhabit different regions of the ocean: buoyant plastics reside at the sea surface in subtropical gyres, with model results suggesting the possibility of an unreported ‘garbage patch’ in the Gulf of Guinea; neutrally buoyant plastics are present throughout the whole of the water column; negatively buoyant plastics sink and settle coastally, but are also transported to abyssal plains and trenches. Secondly, the accumulation and transport of microplastics in sea ice, in both the Arctic and Southern oceans is investigated. Arctic sea ice is most susceptible to buoyant microplastic pollution, with a predicted distribution reflecting observations, whereas Southern Ocean sea ice is more susceptible to neutrally buoyant microplastics, although observational evidence is too limited to validate these results. Finally, building upon existing modelling research, the effects of biofouling on the vertical and horizontal distribution of a range of small microplastics, both with and without the effects of strong vertical mixing are considered. The impact of density changes caused by biofouling are size-dependent and independent of vertical mixing, with 10 μm microplastics most affected by biofouling in the model. However, this is a complex problem with many variables to consider and requires further, detailed research. While this thesis has shed light on some aspects of the subject of marine microplastics, there are still significant gaps in our knowledge of the global distribution and behaviour of marine plastics.