Contributions of the direct supply of belowground seagrass detritus and trapping of suspended organic matter to the sedimentary organic carbon stock in seagrass meadows

Tanaya, Toko; Watanabe, Kenta; Yamamoto, Shoji; Hongo, Chuki; Kayanne, Hajime; Kuwae, Tomohiro

Carbon captured by marine living organisms is called “blue carbon”, and seagrass meadows are a dominant blue carbon sink. However, our knowledge of how seagrass increases sedimentary organic carbon (OC) stocks is limited. We investigated two pathways of OC accumulation: trapping of organic matter in the water column and the direct supply of belowground seagrass detritus. We developed a new type of box corer to facilitate the retrieval of intact cores that preserve the structures of both sediments (including coarse sediments and dead plant structures) and live seagrasses. We measured seagrass density, total OC mass (OCinline-formulatotal) (live seagrass OC biomass (OCinline-formulabio) inline-formula+ sedimentary OC mass (OCinline-formulased)), and the stable carbon isotope ratio (inline-formulaδ13C) of OCinline-formulased and its potential OC sources at Thalassia hemprichii dominated back-reef and Enhalus acoroides dominated estuarine sites in the tropical Indo-Pacific region. At points with vegetation, OCinline-formulabio accounted for 25 % and OCinline-formulased for 75 % of OCinline-formulatotal; this contribution of OCinline-formulabio to OCinline-formulatotal is higher than in globally compiled data. Belowground detritus accounted for inline-formula∼ 90 % of the OC mass of dead plant structures (> 2 mm in size) (OCinline-formuladead). At the back-reef site, belowground seagrass biomass, OCinline-formuladead, and inline-formulaδ13C of OCinline-formulased (inline-formulaδ13Cinline-formulased) were positively correlated with OCinline-formulased, indicating that the direct supply of belowground seagrass detritus is a major mechanism of OCinline-formulased accumulation. At the estuarine site, aboveground seagrass biomass was positively correlated with OCinline-formulased but inline-formulaδ13Cinline-formulased did not correlate with OCinline-formulased, indicating that trapping of suspended OC by seagrass leaves is a major mechanism of OCinline-formulased accumulation there. We inferred that the relative importance of these two pathways may depend on the supply (productivity) of belowground biomass. Our results indicate that belowground biomass productivity of seagrass meadows, in addition to their aboveground morphological complexity, is an important factor controlling their OC stock. Consideration of this factor will improve global blue carbon estimates.

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Tanaya, Toko / Watanabe, Kenta / Yamamoto, Shoji / et al: Contributions of the direct supply of belowground seagrass detritus and trapping of suspended organic matter to the sedimentary organic carbon stock in seagrass meadows. 2018. Copernicus Publications.

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