Microplastics in the marine environment: From top to bottom

Abstract

The first reports of small plastic debris floating at the ocean surface were recorded in the 1970s, but it is only in the last decade that scientific and media attention has soared. Microplastics (plastic 1 µm – 5 mm) have since been acknowledged as a global marine contaminant, raising concerns about the interactions between anthropogenic debris and natural biological processes. In this thesis, I explore the hypothesis that microplastics can be transported via biotic-driven mechanisms through the water column and into coastal sediments, leading to adverse impacts on the health and functioning of marine fauna and ecosystems. In Chapter 2, I demonstrate that a key pelagic species, the copepod Calanus helgolandicus, alter their prey selection dependent upon the size or shape of the plastic in their ambient surroundings, with the capacity to reduce feeding. I also establish that C. helgolandicus faecal pellets sink slower when contaminated with low density polyethylene (PE), whereas sinking rates increase when contaminated with high density polyethylene terephthalate (PET), highlighting potential impacts to marine nutrient flux. In Chapter 3, I develop a method utilising the differential density of sediment and plastic to isolate and recover microplastics from sediments; I apply this method in Chapter 4, and latterly discuss harmonisation of microplastic estimates between studies and its use across the wider international field (Chapter 5). In Chapter 4, I employ a multi-faceted study to explore the role that benthic fauna play in the uptake of microplastics by the seabed. My environmental data demonstrate that microplastics are being permanently buried in coastal sediments, and that this process is ubiquitous across sampled sites and seasons. I further identify that benthic faunal functional groups that move sediment vertically (“conveyors”) and randomly (“biodiffusers”) influence sediment plastic loading differently, affecting ultimate burial and deep sediment loading. Furthermore, experimental data indicate that a key benthic species, the brittlestar Amphiura filiformis, buries nylon fibres along its burrow structure and that burial activity deep in the burrow is impaired when plastic is consumed. Collectively, my research contributes to our understanding of the mechanisms governing microplastic transport through the water column and into the sediment matrix, highlights risks posed to marine fauna and ecosystems and provides evidence that coastal sediments are final sinks for microplastics