Scotland’s blue carbon: the contribution from seaweed detritus
The ecological and biological importance as well as economic, and cultural value of macroalgae is becoming more prominent. Introduction of the term ‘blue carbon’ (BC) has drawn attention to natural coastal habitats and their capacity to fix CO2. The overall aim of this thesis was to place the importance of carbon sequestration within the already essential services that macroalgae provide to the biosphere. This thesis focused on: 1) Quantifying the amount of detritus produced by species in Scottish macroalgae habitats and providing annual figures of total carbon leaving kelp forests in fluxes, 2) understanding the processes of degradation of detritus from three dominant kelp species and estimating the pathways of carbon loss of the detritus, and 3) identifying the sources of sediment carbon using biomarkers and environmental DNA primers specific to the class Phaeophyceae. In Chapter 2, by combining the habitat extent of macroalgae, and the depositional rates of detritus in kelp forests, it was estimated that between 0.32-0.68 Mt C yr-1 is produced in the form of detritus, a significant amount of export compared to other BC habitats in the region. There was a constant presence of macroalgal detritus in benthic areas beyond kelp habitats suggesting benthic communities use macroalgal detritus as a source of nutrients. Detritus is a constant presence on some beaches in Scotland and high rates of carbon turnover occur in macroalgal wrack (shown as up to 23 t C yr-1 on one beach). In Chapter 3, incubations of kelp species found that during decomposition DOC is released at higher rates than DIC initially (31.6% compared to 10.6% respectively). DOC is rapidly consumed but is often ignored as a pathway of carbon release during decomposition. The mean refractory potential (Rp) of three kelp species tested increased throughout decomposition (from 0.46-0.50). In Chapter 4, macroalgae DNA was present in 87% of sediment samples taken in Scottish fjords and in areas of the North-East Atlantic shelf. The presence of macroalgae DNA also correlated with presence of polyphenols, strongly suggesting compounds from macroalgae accumulate in sediments. Macroalgae DNA was also found in sediment cores up to 60 cm deep, which indicates a historic retention of macroalgae carbon in sediments. Macroalgae in the North-East Atlantic: 1) fix significant amounts of CO2 through photosynthesis thus removing it from the atmosphere, 2) release the carbon fixed through photosynthesis as detritus which accumulates and is buried, broken down by bacteria, and contributes to food webs, and 3) contributes carbon to sediment stores in Scotland and the wider North-East Atlantic shelf. These three criteria are fundamental blue carbon habitat characteristics. It is thereby recommended that macroalgae are henceforth included in blue carbon frameworks and directives, particularly in Scotland, where the contribution to long-term carbon stores in fjord and shelf systems is potentially greater than any other BC habitat in the region. It is estimated that 0.2 Mt C yr-1 is transferred to sediments from macroalgae in Scotland, the equivalent of 0.04 g C m-2 of kelp forest.
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