Is macroalgal carbon sequestration the 'elephant in the Blue Carbon room' ?

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Marine plants draw down CO2, and in a world of rising atmospheric CO2 levels carbon sinks in vegetated coastal ecosystems can sequester CO2 on geological time scales and are now referred to as 'Blue Carbon'. Marine macroalgae (MA) are highly productive macrophytes that currently cover approximately 3.5 million km2 of sublittoral seabed and provide 1521 Tg C yr-1 of net primary production globally. Nevertheless, they have largely been excluded from estimates of blue carbon sequestration because they predominantly grow on hard substrates, which prevent the accumulation of detritus-rich sediments. But while MA carbon (CMA) cannot accumulate within the source ecosystem, it has been estimated that up to 82% of CMA is exported from the source ecosystem to seabed habitats at greater depths, providing a significant carbon subsidy to marine seabed ecosystems beyond the coastal zone and/or contributing to long-term carbon burial in the ocean seabed. Shelf seas are known for their significant stocks of carbon, and marine fjords have recently been proposed as major C sinks of global significance, despite their low area coverage accounting for 11 % of global annual carbon sequestration. Per unit area, fjord organic carbon burial rates are one hundred times as large as the global ocean average, and fjord sediments contain twice as much organic carbon as biogeneous sediments underlying the upwelling regions of the ocean. Studies in Arctic fjords suggest that CMA contributes up to 60% to C sequestration which would render macroalgae a major contributor to blue carbon sequestration, and global biogeochemical cycles in general. Both remineralisation and sequestration of CMA are poorly constrained, however, and the actual importance of MA detritus, and hence CMA, as a major ecosystem service to deep allochthonous (sink) benthic biota and sediments still needs to be addressed in a quantitative, geographically well-constrained investigation. This project will quantify the contribution of MA carbon to C sequestration and as food subsidy to benthic fauna in deep Kongsfjorden, one of the best-studied Arctic fjord systems. The Kongsfjord digital landscape model and already available sedimentary C data will allow to build an inventory of MA- derived blue carbon based on methodology on Smeaton et al (2017). Field work will take place alongside other planned work on board the Polish research vessel RV Oceania. Shiptime for 2021 has already been secured. While this project will gather data outside the UK, its results are of immediate relevance for the UK and presentation of results for example at the Scottish Blue Carbon Forum will facilitate quick translation into national climate mitigation and adaptation policy. Climate change is a global issue, and improved understanding and management of Blue Carbon ecosystems is central to several of the themes of Earth systems science and sustainable economies, and directly relevant to UN SDGs 3, 13 and 14.