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The Complex Dynamics of Microplastic Fate in Saltmarshes
Summary
This doctoral thesis tested the drivers of microplastic transport and fate in saltmarshes through controlled flume experiments, optimized laboratory methods, field studies in North Norfolk, UK, and quantification of microplastics on detrital leaves in the Tamar Estuary. Results showed microplastic characteristics rather than vegetation structure governed deposition in simulated wetlands, and microplastic adherence to saltmarsh leaf litter was identified as an export mechanism potentially releasing over 22 million microplastics into the Tamar Estuary with every tide.
Coastal wetlands are characterized by unique biological and physical dynamics that enable them to provide many ecosystem benefits (e.g., coastal protection)—and, potentially, the entrapment of marine microplastics. However, the mechanisms underlying microplastics entrapment and retention are poorly understood. In this thesis, I test the underlying drivers affecting microplastic transport and fate in saltmarshes. Chapter 2 uses a controlled flume experiment to investigate the influence of vegetation structure and microplastic type on microplastic deposition. Following this, Chapter 3 takes an exploratory methods comparison approach to demonstrate the importance of method validation in microplastics research and develop methods for isolating microplastics from saltmarsh sediments and leaf surfaces. Using these optimized methods, Chapter 4 explores the influence of vegetation presence and complexity on microplastic accumulation in saltmarsh sediments in a field site (North Norfolk, UK). Finally, Chapter 5 investigates one route of microplastic export from saltmarshes by quantifying microplastics on detrital leaves of Atriplex portulacoides in the Tamar Estuary, UK. My results demonstrate that: (1) microplastic characteristics influence microplastic fate, while vegetation structure does not, in a simulated wetland (Chapter 2); (2) method validation is necessary to increase harmonisation across microplastics research, particularly when dealing with complex matrices (Chapter 3); (3) microplastic accumulation in saltmarsh sediments is not determined by vegetation presence or complexity (Chapter 4); and (4) microplastic adherence to saltmarsh leaf litter is an export mechanism, potentially releasing over 22 million microplastics into the Tamar Estuary with every tide (Chapter 5). Collectively, this thesis shows that, while saltmarshes are a reservoir for microplastics, microplastic deposition and retention are dependent on a myriad of biophysical variables. By increasing our understanding of the transport and fate of microplastics in coastal wetlands, this work contributes to a widening knowledge base on the global cycling of microplastics.