IAP-25-116

Investigating the organic matter reactivity continuum in UK Coastal Shelf Seas.

The UK’s coastal shelf seas are increasingly recognised as critical zones for carbon cycling, with marine sediments acting as both repositories and potential sources of organic carbon (OC). These environments are central to the UK’s blue carbon strategy, offering opportunities for climate mitigation, biodiversity enhancement, and nature-based solutions. However, current assessments of carbon in these systems primarily focus on static inventories of OC content. While valuable, these maps do not capture the reactivity of carbon: its potential to be mobilised, transformed, or mineralised under human pressures or changing environmental conditions.

This project addresses a key evidence gap identified by the UK Blue Carbon Evidence Partnership (UKBCEP) focussing on the need to move beyond bulk carbon quantification toward understanding the composition and reactivity of organic matter in marine sediments. Reactivity determines whether carbon remains sequestered or is re-released into the water column and atmosphere, influencing its role in climate mitigation and likely causing long-term ecological shifts. This is particularly urgent considering increasing anthropogenic disturbances in UK marine waters ranging from bottom trawling and fisheries activities, subsea cable and renewable energy infrastructure installation, such as offshore wind farms, which involve seabed preparation. These activities can notably affect the fate of OC, especially in coastal sediments where deposition of marine-derived dissolved organic matter (DOM) and particulate organic matter (POM) is ongoing. The transformation of DOM and POM during deposition, burial, and subsequent disturbance is poorly understood, yet it is central to evaluating the resilience and vulnerability of sedimentary blue carbon stores.

This project will investigate the size reactivity continuum between DOM and POM in UK shelf sea sediments, focusing on their reactivity during deposition and mobilisation following disturbance. By characterising the molecular composition, age, and transformation potential of organic matter, this research project will provide a dynamic understanding of carbon cycling in these environments. It will also assess how different sediment types and disturbance regimes influence carbon reactivity, offering insights into the risk of carbon loss and the potential for long-term sequestration. In doing so, the project moves beyond static carbon maps to develop reactivity indices that can inform policy and management. These indices will support any future plans to integrate marine sediments into the Greenhouse Gas Inventory (GHGI), guide marine spatial planning of human activities, and shape carbon-conscious infrastructure development. Ultimately, this research will contribute to a more nuanced and actionable understanding of blue carbon in UK coastal waters, aligning with national and international climate and biodiversity goals.

The project will focus on identifying key organic matter (OM) compound groups using the latest liquid chromatography organic carbon and nitrogen detection (Heriot-Watt University) and ramped/oxidation SoliTOC (St Andrews University) systems to explore OM transformations of sediments and their leachates from our North Sea archive under changing environmental conditions using the climate-controlled facilities at the Lyell Centre (Work Package 1). This will be combined with fieldwork in partnership with CEFAS to conduct in-situ reactivity experiments from the Southwest Approaches and Celtic Sea or inshore regions (Work Package 2).

CEFAS will host the student to translate the fundamental new insights from this primary research into actionable policies addressing the methodologies and evidence needs identified by the UKBCEP.

Year 1

Carry out literature review of aquatic DOM and POM interactions and processes.
• Learn key skills in DOM and POM techniques at St Andrews and Heriot-Watt Universities.
• Meet all supervisors at the CEFAS in-house student conference (Dec 2026) to finalise PhD objectives and plan fieldwork for Year 2 in addition to regular project meetings.
• Set up initial OM climate-controlled laboratory experiments.
• Submit NEIF Radiocarbon proposal for 14C sample analysis support.
• Identify training needs to complete ongoing workplan.

Year 2

Analyse the results on Year 1 climate-controlled experiments.
• Complete fieldwork in Southern Approaches /Inshore sites.
• Conduct new perturbation experiments based on field work observations.
• Complete CEFAS placement to enhance research outcomes into policy.
• Meet supervisors to assess progress and plan for Year 3.
• Present results at national conference

Year 3

Participate in additional fieldwork if necessary
• Complete outstanding experiments
• Make substantial progress towards thesis preparation and define a detailed timeline to ensure completion with 3.5 years
• Present results at international conference
• Prepare first publication

Year 3.5

• Complete and submit thesis
• Prepare additional data for
• Archive Data in NERC data repository

The supervisors will provide full training in their individual expertise areas, enabling the student to combine current best practice and develop the skills necessary to advance this science. Supervision will be provided jointly by Heriot-Watt (RP) and St Andrews (CS) universities and CEFAS (RP/CP). Directly relevant training will be provided by both institutes to equip the student with the skills necessary for all aspects of the project. IAPETUS fosters a strong sense of “community” that encourages students to organise a range of activities (e.g. annual conference) and identify additional training needs to be addressed via tailored opportunities. The student will have opportunities for additional training at the partner institution (see CASE partner contribution below). These may be desk based for policy understanding, laboratory based, or seagoing on Cefas Endeavour. The student should develop strong collaborations that will potentially result in further opportunities.