Assessing the Capacity of Seagrass Meadows in the Solent (UK) for Blue Carbon Sequestration

Abstract

Seagrass has been established as a coastal blue carbon habitat for its capacity to sequester and store carbon. This has led to growing interest to utilise blue carbon habitats, such as seagrass, as a manageable resource to sequester carbon and reduce greenhouse gas concentrations. Carbon accreditation aims to utilise climate mitigation services as natural capital to provide financial revenue mechanisms for carbon capture delivered by seagrass restoration activities. Carbon accreditation focuses on delivering; (1) additionality, the additional reduction of greenhouse gas emissions, compared to the baseline scenario without the activity intervention; (2) permanence, for carbon stored within the sediment to be considered a true reduction of carbon dioxide emissions it must remain stored over long-time scales; and (3) provenance, assurance the accredited proportion of the carbon is autochthonous in origin. This study assesses if carbon accreditation represents a suitable way to finance temperate intertidal seagrass restoration, utilising UK intertidal seagrass Zostera marina and Nanozostera noltii as an archetype. This study demonstrates that care should take when interpreting organic carbon (Corg) net gain from an averaged unvegetated baseline (e.g., reference unvegetated Corg stock), as vegetated and unvegetated habitats are spatially connected by flux and transport of Corg. The seasonality of temperate intertidal seagrasses and the environments they live in, influence the provenance (e.g., high allochthonous loading in low energy environments) and permanence (e.g., reworking of sediment) of the sedimentary Corg. Finally, it demonstrated that seagrass derived carbon is not likely to be a dominant source of organic carbon within these intertidal seagrass sediments. Holistically when you consider (1) the small quantities and site-specific evidence of net Corg gain, (2) the risk of carbon impermanence due to seasonal stochasticity, and (3) high proportions of allochthonous carbon present in UK intertidal seagrass sediments, the overarching implication is that intertidal seagrass restoration should not rely solely on carbon centric funding. Thus, management of coastal habitats which looks to derive financial investment from the enhancement of ecosystem services, should consider which services may be of the most ‘value’ and importance at each site. Therefore, the ecosystem services of UK intertidal seagrass should be valued holistically, potentially through a stacked accreditation pathway to incorporate the value of co-benefits into financial frameworks to assist with the investments required for restoration and conservation.

Date of Award	20 Jun 2024
Original language	English
Awarding Institution	University of Portsmouth
Supervisor	Joanne Preston (Supervisor), Sarah Reynolds (Supervisor) & Marianna Cerasuolo (Supervisor)