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

Toko Tanaya, Kenta Watanabe, Shoji Yamamoto, Chuki Hongo, Hajime Kayanne, Tomohiro Kuwae
2017 Biogeosciences Discussions  
Carbon captured by marine living organisms is called <q>blue carbon</q>, 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
more » ... ents (including coarse sediments and dead plant structures) and live seagrass bodies. We measured seagrass density, total OC mass (OC<sub>total</sub>) [=&amp;thinsp;live seagrass OC biomass (OC<sub>bio</sub>)&amp;thinsp;+&amp;thinsp;sedimentary OC mass (OC<sub>sed</sub>)], and the stable carbon isotope ratio (δ<sup>13</sup>C) of OC<sub>sed</sub> at back-reef and estuarine sites in the tropical Indo-Pacific region. OC<sub>bio</sub> accounted for 19&amp;thinsp;% and OC<sub>sed</sub> for 81&amp;thinsp;% of OC<sub>total</sub>; this contribution of OC<sub>bio</sub> to OC<sub>total</sub> is the highest in globally compiled data. Belowground detritus accounted for ~&amp;thinsp;90&amp;thinsp;% of the OC mass of dead plant structures (>&amp;thinsp;2&amp;thinsp;mm in size) (OC<sub>dead</sub>). At the back-reef site, belowground seagrass biomass, OC<sub>dead</sub>, and δ<sup>13</sup>C of OC<sub>sed</sub> (δ<sup>13</sup>C<sub>sed</sub>) were positively correlated with OC<sub>sed</sub>, indicating that the direct supply of belowground seagrass detritus is a major mechanism of OC<sub>sed</sub> enrichment. At the estuarine site, aboveground seagrass biomass was positively correlated with OC<sub>sed</sub> but δ<sup>13</sup>C<sub>sed</sub> did not correlate with OC<sub>sed</sub>, indicating that trapping of suspended OC by seagrass leaves is a major mechanism of OC<sub>sed</sub> 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 controlling their OC stock. Consideration of this factor will improve global blue-carbon estimates.
doi:10.5194/bg-2017-522 fatcat:mnfqdwe2t5dotcxxsmrhmve6qa