Microplastic abundance, distribution and composition in the Atlantic and Arctic oceans
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Microplastics are ubiquitous, persistent particles that are capable of posing a threat to organisms that inhabit or depend upon marine ecosystems. Understanding the origin, transport pathways and fate of these particles in the ocean is fundamentally important when evaluating the risks associated with such particles to marine organisms. In the Atlantic Ocean, the Canary and Benguela Upwelling Ecosystems are regions of high primary productivity which sustain large commercial fisheries. The influence of oceanic phenomena such as upwellings on microplastic abundance, distribution and composition remains unknown. Any microplastics that are present within such upwelling ecosystems can potentially interact with the associated biota of such systems. The Arctic Ocean, one of the most remote oceanic basins in the world, is particularly distinct due to its abiotic features and the highly specialised ecosystem that it supports. Thus far, a few studies have reported the presence of microplastics in various environmental compartments of this polar ecosystem. To date, however, there has been a particular paucity of information regarding the Arctic Central Basin (ACB).
The overall goal of the research presented in this dissertation was to target specific knowledge gaps regarding microplastics in the Atlantic and Arctic Ocean. This dissertation is divided into 6 chapters which include an introductory chapter, four core chapters which detail specific components of the research and a discussion chapter that contextualizes the research findings and indicates prospects for future research.
The first core chapter (Chapter 2) of the present dissertation details the sampling of sub-surface waters at a single depth (11 m) between the Bay of Biscay, France and Cape Town, South Africa. This component of the research presented information regarding microplastic abundance, distribution and composition in the Benguela Upwelling Ecosystem (BUE). This specific sampling technique (sub-surface waters at a single depth, 11 m) revealed that there were no significant differences between microplastic abundance at upwelled and non-upwelled sites in the Atlantic Ocean. The provision of information about the environmentally relevant concentrations and composition of microplastics at the BUE is particularly important for laboratory experiments which seek to assess the potential threats posed by microplastics to organisms that inhabit or depend upon such productive regions.
The second core chapter (Chapter 3) of the present dissertation details the sampling of sub-surface waters in the ACB by two independent methods i.e. bow water sampling at a single depth (8.5 m) and sampling using a CTD rosette sampler at multiple depths (8 – 4400 m). Both methods provided data on microplastic abundance, distribution and composition within the ACB and emphasised the pervasiveness of these particles throughout the water column in this oceanic basin. Such findings suggested that there were mechanisms operating within this oceanic basin that were responsible for the vertical transport of these particles through the water column.
The third core chapter (Chapter 4) of the present dissertation presented preliminary information regarding the presence of microplastics in surficial sediments in the ACB. Opportunistic sampling that involved the retrieval of surficial sediments from 11 sampling locations in the ACB, indicated that microplastics were potentially making their way to the deep-sea realm of this oceanic basin and that the sediment environmental compartment was potentially functioning as a sink.
The fourth and final core chapter (Chapter 5) of the present dissertation detailed the sampling of surface waters underlying ice floes as well as sea ice at 25 ice stations in the ACB. Microplastic concentrations in sea ice from the ACB were several orders of magnitude higher than those recorded in surface waters underlying the ice floes. Backward drift trajectories for the sampled sea ice indicated that they possibly originated from the Siberian shelves, the western Arctic and the central Arctic basin. The present study found that there was no apparent pattern in the vertical distribution of microplastics in the sampled ice cores. These findings suggest that sea ice in the Arctic Ocean is functioning as a temporary sink, transport medium and a secondary source of microplastics.
While the research presented in this dissertation does provide some headway in addressing some of the knowledge gaps regarding microplastics in the Atlantic and Arctic Ocean, there is still much that remains unknown and thus there is much scope for future research.