IAP-25-115

PROTECTION OF SEABED CARBON IN SHELF SEDIMENT

Continental shelf sediments are recognised as long-term stores of globally significant quantities of carbon (C) and provide an important climate regulation service via the Earth’s carbon cycle. The surficial sediments within the UK EEZ are estimated to store 524 ± 68 Mt of organic carbon (OC) and 2,582 ± 168 Mt of inorganic carbon (IC), and much of this is concentrated in distinct accumulation hotspots such as fjords, estuaries, and coastal mud belts, where OC supply is enhanced by proximity to land and higher marine productivity rates (Burrows et al. 2024; Smeaton et al. 2021). Anthropogenic activities such as offshore drilling, aggregate dredging, seafloor cabling, windfarm installations and bottom fishing exert significant pressure on continental shelf sediments. Fishing with mobile bottom contacting fishing gear such as otter trawls, beam trawls and towed dredges (aka bottom fishing) is commonly recognised as the most frequent and widespread cause of disturbance on seabed sediments, primarily because bottom fishing occurs on shelf sediments (Amoroso et al. 2018; O’Hara et al. 2021). Several studies have indicated significant net loss of carbon from the seabed due to bottom fishing, that could ultimately reduce the seabed’s capacity for climate mitigation (Khedri et al. 2025; Zhang et al. 2024).

Strategies that minimize disturbances to areas with high carbon deposition, whether in previously undisturbed areas or in disturbed areas with significant recovery potential due to their high carbon accumulation rates if left intact, can effectively prevent or reduce carbon emissions. Marine Protected Areas (MPAs) have been advocated as a tool that can offer both the preservation and enhancement of natural carbon stocks. Their efficacy, however, depends on their location, the history of carbon store pressure and duration of closure. Uncertainty exists over the rate of and time to recovery of benthic carbon stocks once seabed disturbance is halted.

This PhD seeks to:
(i) Assess the vulnerability of carbon stocks in areas that are currently fished and subjected to low, medium, and high-intensity fishing,
(ii) Assess the vulnerability of carbon stocks in areas that are currently unfished. This is of importance as trawl bans in existing MPAs could lead to the displacement of fishing pressure to previously unimpacted areas,
(iii) Evaluate the extent to which the existing MPA network safeguards seabed carbon stores and identify priority locations for future MPAs to maximize carbon protection and climate benefits, including exploring of management options such as gear modifications,
(iv) Explore and predict the optimal management scenarios and recovery of C stocks in sediment after fishing.

While previous analyses have focused on the vulnerability of organic carbon (OC) in currently fished areas (Black et al. 2022; Epstein & Roberts, 2022), this study also examines the vulnerability of OC in currently unfished areas as a potential strategy for avoided emissions. Moreover, this PhD will advance on prior assessments by integrating OC reactivity maps with OC stock maps, providing a more accurate evaluation of fishing impacts, since OC reactivity governs the rate of OC remineralization when sediments are disturbed. The study area for this PhD will be the UK shelf sediments.

Significance and Impact
The management of human activities to reduce the impact on sequestered carbon is a key policy area identified under the UK Blue Carbon Evidence Partnership and identified as a priority evidence needs in the Evidence Needs statement published in June 2023. This research will develop our understanding of the extent of OC loss from bottom fishing and will provide an assessment of the efficacy of current and future MPAs in protecting carbon stocks in addition to biodiversity. The links with Cefas as Defra advisors will ensure the focus of the studentship on key policy questions of the moment as well as rapid impact and uptake of results through Defra evidence streams and mechanisms. Cefas supervisors work closely with Defra policy teams on MPAs, climate and spatial management options and the student will benefit from understanding of the policy framing and key questions of policy makers. Additionally, the new Defra carbon management programme (2026-2029) will allow the student to be embedded within a Govt R&D programme (~ £1M) which will allow additional information on OC reactivity, GIS methods and scenario testing to be explored and utilized. This programme will also support studentship placements and fieldwork opportunities.

An integrated approach will be used to understand the present condition and reactivity of organic carbon stocks within and outside the existing UK MPA network, and the extent to which the OC stocks may be at risk from bottom fishing disturbance. The workflow will include:

• A spatial modeling framework described in Diesing et al. (pre-print) will be used to estimate annual OC loss in areas subjected to fishing, and to identify OC stocks in previously unfished areas that would become vulnerable to loss if fished. Maps of OC reactivity pools together with maps of fishing footprints for the UK EEZ, will be combined with results from meta-analyses of acute fishing impacts with literature-based assessments of sediment resuspension to create predictive maps of carbon sequestration vulnerability. This integrated approach allows for the identification of OC stocks at greatest risk, supporting the development of conservation strategies aimed at mitigating emissions through the protection of high-carbon sediments.

• The co-benefits of the existing UK MPA network for both biodiversity conservation and organic carbon storage through protection from bottom fishing will be evaluated using spatially explicit assessment using Geographic Information System (GIS) models. The expected outcomes include spatial maps highlighting areas where carbon sequestration potential is most at risk (i.e. risk maps), and where protection measures such as ‘carbon protection zones’, ‘fisheries closures’ and marine protected areas should be established.

• To understand the rate of recovery of C stocks in the sediment after bottom fishing, and therefore the effectiveness of area closures, this PhD will collate all existing studies that document the recovery of particulate organic carbon (POC) and particulate inorganic carbon (PIC) in the seabed in a systematic review and meta-analysis by extracting data on POC and PIC density and time since closure, for closed area and a control site from published studies. One possible outcome of this analysis is that some seabed C is irrecoverable (Goldstein et al. 2020). It might be possible to supplement this with additional targeted sampling in closed areas through ongoing Cefas programmes, which will provide the PhD student with fieldwork opportunity at sea.

• The new understanding on seabed carbon vulnerability and recovery from the methods above will be drawn together within the Cefas SMMART (Spatially Managed Marine Area Tool) – GIS approach to explore management scenario options and trade-offs. This will include the existing MPA network but also explore the benefits of protection of differing carbon areas and hotspots and will additionally include fisheries gear modifications and displacement effects.

Year 1

1.1. General literature review and discussions with the supervisory team and collaborators to develop scope and thesis chapters outline (approx. 50% of Year 1).
1.2. Systematic review and meta-analysis of carbon recovery trajectories in closed areas (approx. 30% of Year 1).
1.3. Specialist research training depending on student’s needs (approx. 5% of Year 1), e.g. training in evidence synthesis techniques and meta-analysis.
1.4. Generalist training on scientific writing, time and project management, effective communication (approx. 5% of Year 1).
1.5. Develop ethics assessments and data management plan (approx. 5% of Year 1).
1.6. Collate spatial data layers required for Year 2 (approx. 4% of Year 1).
1.7. Presentation of research scope, objectives and thesis outline at Cefas’s annual student conference (approx. 1% of Year 1).

Year 2

2.1. Completion of systematic review and meta-analysis of carbon recovery trajectories in closed areas (approx. 10% of Year 2).
2.2 Spatial modelling of benthic carbon vulnerability by integrating OC reactivity, fishing footprints, and fishing impact data (approx. 80% of Year 2).
2.3 Presentation of results at national conference, Cefas’s annual student conference and additional specialist training if needed (approx. 10% of Year 2).

Year 3

3.1. Spatial assessment of UK MPAs to map biodiversity–carbon co-benefits and examine influence of management scenarios (other than the current MPA network) for carbon protection using SMMART tool (approx. 90% of Year 2).
3.2. Presentation of results at international conference, Marine Alliance for Science and Technology for Scotland (MASTS) fora, Cefas’s annual student conference, University seminar series (approx. 10% of Year 2).

Year 3.5

Writing-up of results and completion of thesis, submission of papers for publication.

The candidate will be jointly supervised by Dr Marija Sciberras (Heriot-Watt University) and Dr Craig Smeaton (University of St Andrews) and will be primarily based at the Lyell Centre. Research stays at Cefas under the supervision of Ruth Parker and Richard Heal will be organised as co-supervisors
This research project will equip the student with a range of skills including:
• Evidence synthesis tools including systematic review and meta-analysis,
• Sediment carbon analysis such as thermogravimetric analysis,
• Spatial ecological modelling,
• Big-data analysis.
The student will be provided with an opportunity to attend transferable skills training such as data management, scientific writing and science communication. These are provided for free within Heriot-Watt University’s Research Futures Academy. Further training in key transferable skills is available through the International Education and Lifelong Learning Institute (IELLI) at the University of St Andrews, which provides learning and teaching support through combining educational development, pedagogical workshops, courses in oral and written communication, and opportunities for media and public engagement.

PhD student support
The Lyell Centre/ILES and EGIS have a large research student cohort that will provide peer-support throughout the studentship, including participation in the annual postgraduate research conference and monthly seminars. All supervisors are all based in research-active departments that span a broad range of ecological, environmental and biogeoscience research, exposing the scholar to a range of other research areas. Active participation in these research groups will provide the opportunity to discuss cutting-edge topics in the field, review recent papers and to present current research plans to academics with a common research interest in an informal and supportive atmosphere.
The PhD student will also have the opportunity to undertake a 3-month placement with Cefas, providing work experience within a non-academic organisation to improve understanding of the science-policy interface and context of their work. This studentship is aligned with a 3-year Defra funded programme to begin in 2026 and so the student will benefit with Government scientists working on this topic. Furthermore, the student will benefit from collaboration with ongoing UKRI funded project SeaSTORE, which the entire supervisory team is involved in. This project explores how bottom-trawl fishing affects the ocean’s ability to store carbon and regulate climate and will identify management solutions that protect carbon stores, reduce emissions, and support both net-zero goals and sustainable seafood production.