10:45 AM - 12:15 PM
[MIS09-P01] Towards quantitative evaluation of "blue carbon" sequestration in pelagic ocean sediment
Keywords:Vegetated coastal ecosystems, Carbon sequestration, Offshore sediment, Blue carbon, Environmental DNA
Ocean floor is the largest permanent sink of organic carbon (OC) in the global biogeochemical carbon cycle. Annual rate of OC burial in global ocean sediment has been considered to be between 0.05 and 0.13 Pg C/yr. Although most of the OC buried in pelagic marine sediment is believed to be originated from primary production of phytoplankton living in the euphotic layer of the water column, detrital OC derived from coastal and terrestrial vegetated habitats can be laterally transported offshore and contribute significantly to the oceanic sedimentary OC stock, especially in continental margin areas. The export flux of detrital and dissolved OC from shallow-water coastal habitats, including seagrass meadows, macroalgal beds, and mangroves (collectively called "Blue Carbon") to open ocean is quite large and may exert a fundamental control on both OC burial in pelagic sediment and carbon and energy budgets of pelagic and benthic food webs. However, pathways of Blue Carbon (BC) through pelagic waters to deep-sea floor have not been resolved quantitatively. The lack of such knowledge hampers comprehensive understanding of ecosystem services provided by the BC-producing coastal habitats, including climate change mitigation through carbon sequestration.
Among several potential pathways of BC sequestration in pelagic regions, the authors have mainly investigated to date the fate of BC transported and deposited in offshore sediments surrounding the original habitats. Two years ago, we reported that BC derived from individual plant species such as mangroves, seagrasses, and macroalgae could be successfully detected and quantified from deep-sea sediments around Okinawa Trough by environmental DNA (eDNA) detection using the quantitative PCR technique (JpGU 2021 C001103). Our results evidenced the dominant control by Kuroshio Current on spatial distribution of BC-eDNA over the deep-sea floor, in such a way that eDNA derived from subtropical vegetated habitats such as mangroves and tropical seagrasses was found only in the south-to-east region of the Kuroshio path, while pelagic sediments in the north-to-west side of the path often contained a large number of eDNA of temperate macroalgae that were putatively transported from coastal regions of Continental China (e.g. the Chang Jiang River mouth and Bohai Bay) as drifting biomass.
Although such findings are significant and interesting, there are at least two major hurdles to overcome towards quantitative estimation of BC sequestration in pelagic sediment. First, we should devise and verify a reliable method to estimate the amount of OC derived from individual plant species or groups from the copy number of DNA fragments of the same plants remaining in sediment. Second, to estimate the annual accumulation rate of BC in pelagic sediment, sediment accumulation rate at the survey sites should be measured or provided by a reliable model. In this presentation, we introduce our current attempt to tackle these problems, and show the first preliminary estimate of sequestration rate of BC derived from individual plant species in some pelagic sediments. We also discuss several difficulties emerging from our recent efforts in order to include offshore BC burial quantitatively in the carbon sink inventory of the individual coastal habitats.
Among several potential pathways of BC sequestration in pelagic regions, the authors have mainly investigated to date the fate of BC transported and deposited in offshore sediments surrounding the original habitats. Two years ago, we reported that BC derived from individual plant species such as mangroves, seagrasses, and macroalgae could be successfully detected and quantified from deep-sea sediments around Okinawa Trough by environmental DNA (eDNA) detection using the quantitative PCR technique (JpGU 2021 C001103). Our results evidenced the dominant control by Kuroshio Current on spatial distribution of BC-eDNA over the deep-sea floor, in such a way that eDNA derived from subtropical vegetated habitats such as mangroves and tropical seagrasses was found only in the south-to-east region of the Kuroshio path, while pelagic sediments in the north-to-west side of the path often contained a large number of eDNA of temperate macroalgae that were putatively transported from coastal regions of Continental China (e.g. the Chang Jiang River mouth and Bohai Bay) as drifting biomass.
Although such findings are significant and interesting, there are at least two major hurdles to overcome towards quantitative estimation of BC sequestration in pelagic sediment. First, we should devise and verify a reliable method to estimate the amount of OC derived from individual plant species or groups from the copy number of DNA fragments of the same plants remaining in sediment. Second, to estimate the annual accumulation rate of BC in pelagic sediment, sediment accumulation rate at the survey sites should be measured or provided by a reliable model. In this presentation, we introduce our current attempt to tackle these problems, and show the first preliminary estimate of sequestration rate of BC derived from individual plant species in some pelagic sediments. We also discuss several difficulties emerging from our recent efforts in order to include offshore BC burial quantitatively in the carbon sink inventory of the individual coastal habitats.