Edinburgh Napier University

  Seagrasses are marine foundation species that form ecologically important habitats in coastal areas around the world. They provide a range of ecosystem services, including coastal protection and the recently recognised large contribution to global carbon sequestration and storage. To date, the majority of published studies on the aforementioned ecosystem services is limited to specific geographic regions and seagrass species. This PhD study attempted to explore and provide the first evidence, to the best of our knowledge, on the role of Scottishseagrasses as carbon sinks and sediment stabilisers. In 2013, shoot dynamics of Zostera noltii plots were monitored biweekly and seasonally in the Forth estuary and digital images of the surveyed plots were taken for the development of a remote sensing technique which would accurately estimate the vegetation cover. In 2014, sediment samples from vegetated and unvegetated plots within beds of Z. marina and Z. noltii were collected from all the major estuaries along the east coast of Scotland, from the Firth of Forth in the south to Dornoch Firth in the north. Samples were analysed for organic matter, organic carbon, radionuclides 210Pb, 137Cs and 241Am, and δ13C in order to determine the organic matter and organic carbon density, longevity and sources of carbon respectively. To explore the role of seagrass in sediment deposition and stability, surface elevation was measured monthly in seagrass plots and bare sediment in the Forth estuary over two years. The results and main mechanisms underlying these findings are reported and discussed in detail in each chapter. In short, the proposed method based on digital images provided estimates of seagrass coverage that are more accurate than observers’ estimates, with some constraints when macroalge and/or extreme light are present. Intertidal seagrass meadows in Scotland showed significantly enhanced carbon storage compared with bare sediment. Seagrass plots contained variable quantities of carbon in their sediments with species composition having a significant effect on carbon stocks, whereas depth and seagrass abundance had no effect on carbon stores. Despite their small above-ground biomass Scottish seagrass plots had a strong influence on sediment deposition and prevented erosion. Further research is needed to understand what factors drive large carbon sequestration and storage at some sites, thus contributing policy-relevant information on the prediction of the seagrass carbon hot-spots. Also, long-term datasets on surface elevation change are important in order to understand the effect of all the processes involved on sediment deposition in seagrass beds.