Effects of Plastic Pollution on Blue Carbon Ecosystems

Blue carbon ecosystems (BCEs), including mangroves, salt marshes and seagrasses, play an important role in climate regulation by sequestering and storing organic carbon in coastal sediments. However, their position at the land–ocean interface also makes them effective traps for pollutants transported from land to sea. Among these, plastic waste has increased dramatically in recent decades as global plastic production and consumption have expanded, with much of this material eventually entering coastal and marine environments. As a result, BCEs are increasingly recognised as hotspots of plastic accumulation. In addition to plastics themselves, these ecosystems can also accumulate a range of associated contaminants, either introduced directly from surrounding catchments or transported on plastic surfaces. Despite the growing presence of plastics and related pollutants in BCEs, their potential effects on the biogeochemical processes that underpin carbon storage in these ecosystems remain poorly understood.This thesis aimed to evaluate the impact of plastic pollution, including co-occurring pollutants, on BCEs. First, to identify key knowledge gaps, a scientometric analysis was conducted to assess research trends and identify overlooked areas (Chapter 2). Subsequently, a series of controlled microcosm experiments was performed using mangrove and seagrass sediments exposed to different types and extents of pollutants (Chapters 3-5). A multidisciplinary, unique integration of traditional methods (scanning electron microscopy, Fourier transform infrared spectroscopy, and gas chromatography) and advanced, high-precision analytical techniques, i.e., 16S amplicon sequencing, Ultra-Performance Liquid Chromatography-Mass Spectrometry (UPLC-MS), and Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) was employed to measure biogeochemical responses. Key response variables included greenhouse gas emissions, dissolved organic carbon, organic matter composition, sediment metabolites, microbial communities, and signals of plastic degradation.Chapter 2 employed a scientometric overview, using bibliometric, altmetric, and literature synthesis methods to examine current research on plastic pollution in BCEs. Findings showed a growing breadth of research on plastic pollution within BCEs, mainly centred on mangroves. Four key research themes emerged from the literature: (a) plastic trapping by vegetated coastal ecosystems, (b) microbial plastic degradation, (c) ingestion of plastic by benthic organisms, and (d) impacts of plastic on blue carbon biogeochemistry. Highlighting the need for integrated research on plastic pollution and blue carbon science, this chapter emphasised opportunities to improve understanding of plastic reservoirs in BCEs and their effects on blue carbon biogeochemistry, with particular attention to carbon sequestration and mineralisation. Specifically, the potential for plastic degradation and organic carbon leaching into the sediment’s dissolved pool to promote mineralisation, and the interaction effects of other contaminants with plastic on carbon mineralisation, remain largely unexplored (Chapters 3-5).In Chapter 3, a 90-day microcosm experiment was conducted using conventional plastics (polyethylene terephthalate: PET and polypropylene: PP) and biodegradable plastics (polylactic acid: PLA) with mangrove sediment to assess their potential for organic matter turnover related to plastic degradability. After 90 days, low-molecular-weight PP showed visible degradation, whereas high-molecular-weight PET and PLA did not. The degradation of PP and the granular structure of PET increased the concentrations of dissolved organic carbon (DOC) and dissolved organic matter (DOM) in the sediment compared to PLA. Initially, PET contributed more bio-labile compounds, but after 90 days, recalcitrant compounds became predominant. Despite lower DOC and DOM levels, sediment containing PLA emitted more CO2, indicating that PLA accelerated organic matter degradation while PP reduced emissions. Both plastics affect sediment biogeochemistry by altering DOC and DOM, offering new insights into the role of plastics in organic carbon turnover.In Chapters 4 and 5, multi-omics techniques, including 16S amplicon, FT-ICR MS, and UPLC MS, were used to investigate how traditional (PET) and biodegradable (PLA) microplastics, along with nitrogen enrichment (Chapter 4) and polyfluoroalkyl substance (PFAS) addition (Chapter 5), affect the biogeochemistry of blue carbon sediments. Results indicate that sediments with PFAS emitted four times more methane (CH4) than controls, primarily due to methanogens, such as Methanolobus and Methanococcoides. PLA hydrolysis released carboxylic acids and derivatives, which served as substrates for sulphate-reducing and fermentative bacteria, thereby enhancing microbial metabolites linked to growth. In scenarios combining PLA and PFAS, there was a notable increase in sulphate-reducing bacteria and methanogens, associated with elevated CO2 and CH4 emissions resulting from the transformation of dissolved organic matter and the formation of microbial-derived protein-like compounds. Furthermore, PLA with nitrogen caused a 106% rise in CO2 emissions compared to nitrogen alone and a 195% increase compared to controls, with PET exerting minimal influence. Nitrogen promoted organic matter-degrading microbes, particularly from the classes Clostridia and Bacteroidia, thereby bolstering microbial metabolic potential across multiple pathways. The combined PLA and nitrogen treatments yielded the lowest residual dissolved organic carbon (–37% relative to the control), indicating accelerated carbon loss.In addition, this thesis argues that despite many countries relying on BCEs to achieve net-zero emissions, plastic-related carbon emissions and potential mitigation measures should be included in their nationally determined contributions (NDC). Employing a perspective lens, this thesis also highlighted the potential positive and negative impacts of plastic accumulation in BCEs compared with their alternative fates in the ocean or terrestrial environments. Overall, this thesis mechanistically demonstrates that plastic in blue carbon ecosystems is not just an additional carbon pool but also contributes to sediment carbon loss. Conventional plastics mainly reshape DOM composition, whereas biodegradable plastics can trigger short, intense pulses of remineralisation, especially when combined with catchment-derived nitrogen or other persistent pollutants. This has several implications for climate mitigation efforts through blue carbon. Firstly, the findings introduce “plastic carbon” as a distinct component of coastal carbon budgets, whose climate significance depends on polymer type and pollution context rather than solely on its mass. Secondly, it suggests that blue carbon research should move beyond static, stock-based estimates towards process-based, temporal approaches that explicitly link pollution status with carbon data and consider recent input and disturbance history. Lastly, this thesis highlights priorities for future research, including long-term field-scale studies tracking plastic-derived carbon through microbial pathways, quantifying priming events, and integrating pollution considerations into blue carbon ecosystems management.