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

Abstract

Abstract. 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 enrichment: 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 seagrass bodies. We measured 5 seagrass density, total OC mass (OCtotal) [= live seagrass OC biomass (OCbio) + sedimentary OC mass (OCsed)], and the stable carbon isotope ratio (δ 13 C) of OCsed at back-reef and estuarine sites in the tropical Indo-Pacific region. OCbio accounted for 19% and OCsed for 81% of OCtotal; this contribution of OCbio to OCtotal is the highest in globally compiled data. Belowground detritus accounted for ~90% of the OC mass of dead plant structures (>2 mm in size) (OCdead). At the back-reef site, belowground seagrass biomass, OCdead, and δ 13 C of OCsed (δ 13 Csed) were positively correlated with OCsed, indicating that the 10 direct supply of belowground seagrass detritus is a major mechanism of OCsed enrichment. At the estuarine site, aboveground seagrass biomass was positively correlated with OCsed but δ 13 Csed did not correlate with OCsed, indicating that trapping of suspended OC by seagrass leaves is a major mechanism of OCsed enrichment 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 15 controlling their OC stock. Consideration of this factor will improve global blue-carbon estimates.Biogeosciences Discuss., https://doi

Seagrass contribution to blue carbon in a shallow karstic coastal area of the Gulf of Mexico

“…The structural complexity of seagrasses is related to both the above- and belowground biomass and contributes to sediment retention processes by trapping particles accumulating in sediments for millennia in the form of organic carbon ( Hendriks et al, 2008 ; McLeod et al, 2011 ; Duarte & Krause-Jensen, 2017 ). Carbon stored in seagrass sediments comes from both the detritus produced by the degradation of its biomass (leaves, rhizomes, and roots), considered autochthonous carbon ( Agawin & Duarte, 2002 ; Hendriks et al, 2008 ; Tanaya et al, 2018 ), and from allochthonous sources such as the contributions of river sediments ( Signa et al, 2017 ; Macklin et al., 2019 ). The origin of the carbon in seagrass ecosystems differs at the regional level.…”