title: Understanding the drivers of carbon sequestration in Scottish seagrass
Creators: name: Whitlock, Danielle Sherrie
Publisher: name: Edinburgh Napier University
Year: 2022

description: Seagrass meadows are found in coastal environments worldwide and over the last decade they have been celebrated as powerful natural carbon sinks. Whilst the global importance of seagrasses to carbon sequestration and storage is accepted in general, there is wide variability in the data between sites and species, and the drivers of this variation are poorly understood. In the UK, seagrass meadows occur in estuaries, sea lochs and lagoons in settings with a diverse range of environmental and physical conditions. Few published studies have examined seagrass carbon in the UK; those that have, show large differences in carbon stocks between sites, which are not simply explained by the presence or absence of seagrass. Hence, there is a need to understand the reasons for variation in carbon stocks between seagrass sites to inform fundamental science and policy. This thesis explores three potential drivers of variation in carbon storage: decomposition, sediment characteristics and rates of burial. Effects of seagrass on decomposition were experimentally examined using a standardised material, tea litter, and the relationships between decomposition rate, sediment microbial communities and physical properties, such as particle size, were examined. Surprisingly, there was no evidence for slowed decomposition under seagrass compared with bare mudflat areas, rather seagrass habitats showed statistically significant enhanced rates of decomposition, at least for shallow (15 cm) depths. This may be linked to changes in the microbial fauna associated with seagrass.

Sediments within seagrass meadows were compared to adjacent bare mudflat areas to investigate the influence of seagrasses on sediment characteristics. Overall, sediment stability, explored through the use of a cohesive strength meter and shear vane, is lower in mudflat areas than in seagrass meadows. Sediment grain size analysis found no statistically significant evidence to support the hypothesis that seagrass meadows enhanced finer grained particle deposition where it is present, however the influence of Z.noltii on associated sediment grain size was more variable. Differences in carbon storage may be explained by long term enhancement of carbon burial within seagrass beds due to enhanced rates of sedimentation. The utility of radionuclide dating (using anthropogenic radionuclide markers 137Cs and 241Am) as a method to detect sedimentation rates within intertidal seagrass beds was tested. However due to sediment mixing within the intertidal zone it was not a reliable method for sediment dating at the field site.

The most widely adopted method used to measure carbon stocks in marine sediments is Loss on Ignition (LOI). However, this method as applied in marine and coastal sediments has not been standardised, which has led to large discrepancies in the combustion temperatures used across studies (375-550oC), and consequent difficulty in comparing between studies. Therefore, this thesis also conducted an experimental validation of LOI to explore the effects of combustion at a range of temperatures that have been used regularly in marine and coastal sediment studies. The use of a stepped combustion approach helped differentiate between organic and inorganic losses. Although 550oC is the most commonly used temperature across sediment LOI research, the main findings identified that its use may lead to an overestimate of organic matter (OM) within samples, whilst the use of 450oC is likely to lead to an underestimation, as the largest volume of CO2 is combusted at 500oC.

Overall, this thesis explored eleven sites (ten intertidal meadows and one subtidal) across Scotland. Carbon stocks were quantified for all eleven sites, including nine where no previous records are available. West coast intertidal seagrass organic carbon (OC) stocks ranged between 29.86±0.77-120.03±13.72 Mg C/ha, whilst the subtidal Z.marina OC stock was 36.96±0.84 Mg C/ha. These values were within a similar range of previous OC stocks data for Scottish seagrass meadows. However, the additional sites did not support previous research findings and seagrasses did not appear to significantly enhance the mudflat’s carbon sequestration. Nonetheless 50 cm profile depths indicated higher carbon stocks within seagrass meadows when compared to adjacent bare mudflat areas in Firth of Clyde meadows. The expected correlation between seagrass meadows and increased carbon stocks was not consistent across sites. In this study higher OC stocks were found in mudflat areas in five of the eleven sites investigated. The study provides insight into potential drivers of carbon sequestration and the role seagrasses play in carbon storage.