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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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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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Published: Swansea 2025
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
first_indexed 2025-12-03T13:27:42Z
last_indexed 2025-12-05T09:33:49Z
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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&#x2019;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 (&#x2265;100 years)OC burial rate for the WISMB is estimated as 23 gC m&#x207B;&#xB2; yr&#x207B;&#xB9;, with an OC burial efficiency of 74% compared to an average short-term (&#x2264;30 years) OC accumulation rate of 31 gC m&#x207B;&#xB2; yr&#x207B;&#xB9;. 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 &#xB1;0.27 % OC and 10.37 &#xB1; 1.62 gC cm-3 in the top 10 cm of sediments to 0.70 &#xB1; 0.20 %OC and 7.72 &#xB1; 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&#x2013;1990 (10&#x2013;20 cm), while gear penetration into deeper sediments from 1860&#x2013;1960 (&#x2265;20 cm)had less effect, and sediment mixing by competing environmental factors potentially masked trawling effects in surface sediments deposited around 1990&#x2013;2022 (&#x2264;10 cm).Saltmarsh had the highest OC density of 94.14 &#xB1; 62.51 MgC ha-1 and accumulation rate of 1.08 &#xB1; 0.76 MgC ha-1 yr-1 (107.96 &#xB1; 75.83 gC m&#x207B;&#xB2; yr&#x207B;&#xB9;), amounting to 694.78 &#xB1; 461.34 MgC and 7.97 &#xB1; 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&#xB1; 0.25 MgC ha-1, amounting to 261.10 &#xB1; 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. 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Distributed under the terms of a Creative Commons Attribution 4.0 License (CC BY 4.0)</documentNotes><copyrightCorrect>true</copyrightCorrect><language>eng</language><licence>https://creativecommons.org/licenses/by/4.0/</licence></document></documents><OutputDurs/></rfc1807>
spelling 2025-12-03T13:30:50.9325801 v2 71062 2025-12-03 Blue Carbon in an Exposed Temperate Island Nation: Organic Carbon Variability in Coastal and Shelf Sediments in the Isle of Man 68aea1722c5f41c388bfcd10ec67fd9b HANNAH MUIR HANNAH MUIR true false 2025-12-03 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. E-Thesis Swansea Blue Carbon, Seagrass, Saltmarsh, Western Irish Sea Mud Belt, Irish Sea, Organic Carbon, Fishing, Habitat Extent 12 9 2025 2025-09-12 10.23889/SUThesis.71062 COLLEGE NANME COLLEGE CODE Swansea University Evans, C.; Unsworth, R.; and Strong, J. A. Doctoral Ph.D Isle of Man Government 2025-12-03T13:30:50.9325801 2025-12-03T13:24:03.1284782 Faculty of Science and Engineering School of Biosciences, Geography and Physics - Biosciences HANNAH MUIR 1 71062__35738__03bd542b03a34be68ee53b4dc076c062.pdf 2025_Muir_H.final.71062.pdf 2025-12-03T13:27:05.0764540 Output 11273454 application/pdf E-Thesis – open access true Copyright: the author, Hannah Catherine Muir, 2025. Distributed under the terms of a Creative Commons Attribution 4.0 License (CC BY 4.0) true eng https://creativecommons.org/licenses/by/4.0/
title Blue Carbon in an Exposed Temperate Island Nation: Organic Carbon Variability in Coastal and Shelf Sediments in the Isle of Man
spellingShingle Blue Carbon in an Exposed Temperate Island Nation: Organic Carbon Variability in Coastal and Shelf Sediments in the Isle of Man
HANNAH MUIR
title_short Blue Carbon in an Exposed Temperate Island Nation: Organic Carbon Variability in Coastal and Shelf Sediments in the Isle of Man
title_full Blue Carbon in an Exposed Temperate Island Nation: Organic Carbon Variability in Coastal and Shelf Sediments in the Isle of Man
title_fullStr Blue Carbon in an Exposed Temperate Island Nation: Organic Carbon Variability in Coastal and Shelf Sediments in the Isle of Man
title_full_unstemmed Blue Carbon in an Exposed Temperate Island Nation: Organic Carbon Variability in Coastal and Shelf Sediments in the Isle of Man
title_sort Blue Carbon in an Exposed Temperate Island Nation: Organic Carbon Variability in Coastal and Shelf Sediments in the Isle of Man
author_id_str_mv 68aea1722c5f41c388bfcd10ec67fd9b
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author HANNAH MUIR
author2 HANNAH MUIR
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doi_str_mv 10.23889/SUThesis.71062
college_str Faculty of Science and Engineering
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hierarchy_top_id facultyofscienceandengineering
hierarchy_top_title Faculty of Science and Engineering
hierarchy_parent_id facultyofscienceandengineering
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department_str School of Biosciences, Geography and Physics - Biosciences{{{_:::_}}}Faculty of Science and Engineering{{{_:::_}}}School of Biosciences, Geography and Physics - Biosciences
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description 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.
published_date 2025-09-12T05:27:58Z
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