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Uncovering microplastic surface transport pathways in the North Sea using Lagrangian coherent structures
Summary
This thesis used Lagrangian coherent structures — mathematical features of ocean flow — to identify the dominant pathways for microplastic transport across the surface of the North Sea. Understanding how ocean currents concentrate and move microplastics helps predict where marine life faces the highest exposure risk.
Microplastics in the ocean have the potential to be ingested by marine life, enter both the human and wildlife food chain, and release chemicals through plastic degradation. Due to their small size (less than 5mm), they have the ability to be transported great distances through various ocean dynamics. Often caused by global current systems and surface wind stresses, these dynamics can form transport barriers; persistent structures in the ocean where matter is unable to cross. Such features have the potential to reveal where vast amounts of microplastics collect on the surface. At the core of these attraction zones are conduits for strong downwelling through which plastics settle on the ocean floor. To study this transport, it is necessary to perform a Lagrangian analysis, which takes into account the particles full trajectory. The Finite-Time Lyapunov Exponent (FTLE) assesses the local stretching, where the highest FTLE values indicate potential transport barriers. This approach, applied to twenty-eight months of ocean forecasts, has been used to analyze surface movement on the North Sea. A seasonal analysis with and without the inclusion of wind stresses uncovered a substantial transport barrier above the Norwegian Trench; a deep canyon that runs adjacent to the West Coast of Norway. Most prominent during the winter and relatively irrespective of surface wind, this feature acts as an obstruction to particles travelling to and from the Norwegian Coast. The presence of this barrier has been further demonstrated by the release and advection of simulated particles near coastal cities. The detection of this feature, in addition to transport barriers elsewhere, will help locate surface accumulations of microplastics and ultimately lead to their extraction and disposal.
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