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 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 (OCtotal) (live seagrass OC biomass (OCbio) + sedimentary OC mass (OCsed)), and the stable carbon isotope ratio (δ13C) of OCsed 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, OCbio accounted for 25 % and OCsed for 75 % of OCtotal; this contribution of OCbio to OCtotal is higher than 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 δ13C of OCsed (δ13Csed) were positively correlated with OCsed, indicating that the direct supply of belowground seagrass detritus is a major mechanism of OCsed accumulation. At the estuarine site, aboveground seagrass biomass was positively correlated with OCsed but δ13Csed did not correlate with OCsed, indicating that trapping of suspended OC by seagrass leaves is a major mechanism of OCsed 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.
Highlights
The carbon captured by marine living organisms has been termed “blue carbon” (Nelleman et al, 2009)
Live seagrasses have air-filled lacunae so that they float; we considered buoyant seagrasses captured by the sieve to be OC biomass (OCbio) (Borum et al, 2006)
Our results showed that the sedimentary OC mass (OCsed) was the main component of the total organic carbon mass (OCtotal; i.e., all stock components: live and dead aboveground and belowground biomass and sediments) at our study sites
Summary
The carbon captured by marine living organisms has been termed “blue carbon” (Nelleman et al, 2009). The organic carbon (OC) accumulation rate of vegetated coastal systems such as seagrass meadows, mangrove forests, and salt marshes is estimated to be higher than that of terrestrial forests (Mcleod et al, 2011). The global total OC stock contained in the top 1 m of sediment and in the plant biomass in these vegetated ecosystems is estimated to be 0.63–8.54 Pg C (Pendleton et al, 2012). Vegetated ecosystems are expected to contribute greatly to the mitigation of global warming. In this regard, seagrass meadows have attracted particular attention because they are one of the most dominant blue carbon sinks (Kennedy et al, 2010; Fourqurean et al, 2012). The OC stock of a seagrass meadow is highly variable, depending on geographical region (Miyajima et al, 2015), seagrass species
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