Predictors of sedimentary organic carbon in seagrass vegetated and unvegetated sediment in the Tamar Estuary, United Kingdom

Seagrass meadows are considered significant natural carbon sinks due to their ability to store large amounts of organic carbon (Corg) in their sediments. ‘Blue carbon strategies’ aim to utilise and enhance this ability to mitigate climate change. However, assessing the Corg content and the habitat characteristics that influence this, in both seagrass vegetated and adjacent unvegetated sediments is needed to improve ‘Blue Carbon’ assessments. Currently, adjacent unvegetated sediments are excluded from Blue Carbon assessments as investigations rarely incorporate them into study design, despite evidence suggesting that they can also store significant amounts of Corg. Mean sediment grain size and density of seagrass have been found to influence sedimentary Corg in seagrass meadows. However, site-specific evaluations are imperative as Corg exhibits strong spatial heterogeneity, especially in areas where strong spatial gradients exist, such as estuaries. Despite this, site-specific evaluations of Corg content are lacking in UK seagrass meadows. Sediment cores at three closely situated sites in an expansive patchy seagrass meadow were collected, providing the first assessment to date for this meadow. Analysis of sediment cores provided measurements of mean sediment grain size using Gradistat 8.0 and percentage of Corg using loss on ignition methods for both seagrass vegetated and unvegetated sediments. Seagrass density was assessed by calculating percentage cover using ImageJ. Results showed consistent Corg content in seagrass vegetated sediment and adjacent unvegetated sediment at two of the three studied sites. At the remaining site significantly higher sedimentary Corg (0.79±0.93 %) in seagrass vegetated sediment than unvegetated sediment was observed. Results also indicated that site-specific characteristics, such as presence of bioturbators and seagrass specific characteristics, which were not quantified in the present study, may influence Corg content to a greater extent than seagrass density and mean sediment grain size. These results further highlight the importance of site-specific evaluations, to a within meadow scale, but also the need for adjacent unvegetated sediments to be included in Blue Carbon assessments.