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Blue Carbon in an Exposed Temperate Island Nation: Organic Carbon Variability in Coastal and Shelf Sediments in the Isle of Man / HANNAH MUIR
Swansea University Author: HANNAH MUIR
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PDF | E-Thesis – open access
Copyright: the author, Hannah Catherine Muir, 2025. Distributed under the terms of a Creative Commons Attribution 4.0 License (CC BY 4.0)
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DOI (Published version): 10.23889/SUThesis.71062
Abstract
Blue carbon ecosystems remove and store atmospheric CO₂ for centuries to millennia, offering a scalable and cost-effective solution for climate change mitigation. Their integration into national climate strategies is hindered by limited regional data on their spatial extent and long-term organic car...
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Swansea
2025
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| Institution: | Swansea University |
| Degree level: | Doctoral |
| Degree name: | Ph.D |
| Supervisor: | Evans, C.; Unsworth, R.; and Strong, J. A. |
| URI: | https://cronfa.swan.ac.uk/Record/cronfa71062 |
| Abstract: |
Blue carbon ecosystems remove and store atmospheric CO₂ for centuries to millennia, offering a scalable and cost-effective solution for climate change mitigation. Their integration into national climate strategies is hindered by limited regional data on their spatial extent and long-term organic carbon (OC) storage, especially in temperate and human-impacted regions such as trawled seabed. In this thesis, I address these gaps to inform blue carbon management on the Isle of Man.Through empirical sedimentary analyses and habitat mapping, I quantify OC stocks, accumulation, and burial rates across a range of blue carbon ecosystems, in the island’s saltmarshes, seagrass meadows, and trawled muddy shelf sea sediments in the Western Irish Sea Mud Belt (WISMB). The WISMB was found to be a major long-term OC reservoir due to its OC density and vast extent (49,212.30 ha),exceeding saltmarshes and seagrass meadows. The average long-term (≥100 years)OC burial rate for the WISMB is estimated as 23 gC m⁻² yr⁻¹, with an OC burial efficiency of 74% compared to an average short-term (≤30 years) OC accumulation rate of 31 gC m⁻² yr⁻¹. The disparity between the average OCAR and OCBR in the WISMB likely reflects the degradation of OC over the first 100 years, from 1.06 ±0.27 % OC and 10.37 ± 1.62 gC cm-3 in the top 10 cm of sediments to 0.70 ± 0.20 %OC and 7.72 ± 1.51 gC cm-3 below 30 cm sediment depth. However, areas where modern trawling pressure was highest in the WISMB were associated with the lowest measured OC stocks in subsurface sediments. Trawling pressure was significantly associated with sedimentary OC depletion in sediments deposited around 1960–1990 (10–20 cm), while gear penetration into deeper sediments from 1860–1960 (≥20 cm)had less effect, and sediment mixing by competing environmental factors potentially masked trawling effects in surface sediments deposited around 1990–2022 (≤10 cm).Saltmarsh had the highest OC density of 94.14 ± 62.51 MgC ha-1 and accumulation rate of 1.08 ± 0.76 MgC ha-1 yr-1 (107.96 ± 75.83 gC m⁻² yr⁻¹), amounting to 694.78 ± 461.34 MgC and 7.97 ± 5.60 MgC yr-1 over their full extent of 7.38 ha, which were comparable with other temperate regions. The seagrass meadow OC density of 1.33± 0.25 MgC ha-1, amounting to 261.10 ± 49.08 MgC in the top 10 cm over their full extent of 195.88 ha, was among the lowest values reported and comparable only with exposed temperate meadows. In all three ecosystems, OC content declined with sediment depth, which were found to vary within and between saltmarshes. These findings highlight the importance of quantifying sediment depth and age alongside OC content to estimate long-term OC storage for accurate blue carbon assessments.This study demonstrates the potential for temperate blue carbon ecosystems to contribute to climate change mitigation, and provides vital insights for informing robust, evidence-based national and international blue carbon strategies. |
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| Keywords: |
Blue Carbon, Seagrass, Saltmarsh, Western Irish Sea Mud Belt, Irish Sea, Organic Carbon, Fishing, Habitat Extent |
| College: |
Faculty of Science and Engineering |