5:15 PM - 6:30 PM
[ACG41-P08] Detecting mangrove- and seagrass-derived eDNA from deep-sea sediment around Yaeyama Islands: Implication for offshore blue carbon sequestration
Keywords:Coastal vegetated ecosystems, Organic carbon export, Carbon sequestration, Deep-sea sediment, Environmental DNA
Coastal vegetated ecosystems (mangroves, salt marshes, seagrass beds) have been recognized as potential carbon dioxide (CO2) sink due to their capacity to capture CO2 and store organic carbon (OC) in anoxic sediment inside them. At the same time, these ecosystems are exporting OC (a.k.a. blue carbon) to outer ocean through tidal action, and the magnitude of OC export is believed to be as large as or even greater than that of OC burial within the ecosystems. Although the fate of exported OC has not been assessed due to technical difficulty, a part of exported OC may be transported across the pycnocline to deep ocean and isolated from the atmosphere for long term. For comprehensive evaluation of carbon sequestration capacity of coastal ecosystems, we need to clarify the fate of OC exported from these ecosystems, focusing on whether and to what extent OC may be sequestered in deep ocean.
In this study, we tested applicability of environmental DNA (eDNA) for detecting and estimating mangrove- and seagrass-derived OC in offshore sediments including those from the bathypelagic zone (>1000 m). As a case study, we focused OC export from mangroves and subtropical seagrass beds around Sekisei Lagoon, Okinawa, Japan, to the Lagoon and further to offshore area off Yaeyama Islands. Surface sediment samples were collected and analyzed for eDNA concentration by quantitative PCR, as well as OC, δ13COC, and specific surface area. Target species of the eDNA analysis include dominant plant species in this region, i.e., Rhizophora stylosa, Bruguiera gymnorhiza, and Sonneratia alba for mangroves, and Thalassia hemprichii, Cymodocea rotundata, and Enhalus acoroides for seagrasses.
Due to relatively high-energy conditions, shallow sediments of Sekisei Lagoon were mostly composed of coarse carbonate sand and often lacked any traces of mangrove and seagrass eDNA. However, significant concentrations of both mangrove and seagrass DNA fragments were detected from sediments of narrow nearshore area of Nagura Bay, where silty sediment supplied by the Nagura River prevailed. Offshore sediments commonly contained OC with the bulk δ13C of -23‰ to -19‰, indicating that pelagic primary producers are the predominant source for OC stored in pelagic sediments. Nevertheless, DNA fragments of mangroves, but not seagrasses, could be detected in offshore sediments outside the Lagoon. Mangrove-derived eDNA was often detected significantly in bathypelagic sediments collected from northern area of Yaeyama Island. The concentration of bulk OC was also relatively high (>500 µmol C g-1) in these sediments.
These observations suggest that: (i) organic detritus produced in coastal habitats such as mangroves and seagrass beds is exported by tidal action; (ii) exported OC is not accumulated significantly in shallow lagoon outside the habitats due to high-energy physical environment, but (iii) flushed away further to offshore water by tidal residual current, which usually flows from south to north across the lagoon; (iv) a part of OC then sinks down to bathypelagic zone and stored in deep-sea sediment. Thus, deep-sea sediment located downstream of major coastal vegetated habitats can serve as potentially important blue carbon sink.
In this study, we tested applicability of environmental DNA (eDNA) for detecting and estimating mangrove- and seagrass-derived OC in offshore sediments including those from the bathypelagic zone (>1000 m). As a case study, we focused OC export from mangroves and subtropical seagrass beds around Sekisei Lagoon, Okinawa, Japan, to the Lagoon and further to offshore area off Yaeyama Islands. Surface sediment samples were collected and analyzed for eDNA concentration by quantitative PCR, as well as OC, δ13COC, and specific surface area. Target species of the eDNA analysis include dominant plant species in this region, i.e., Rhizophora stylosa, Bruguiera gymnorhiza, and Sonneratia alba for mangroves, and Thalassia hemprichii, Cymodocea rotundata, and Enhalus acoroides for seagrasses.
Due to relatively high-energy conditions, shallow sediments of Sekisei Lagoon were mostly composed of coarse carbonate sand and often lacked any traces of mangrove and seagrass eDNA. However, significant concentrations of both mangrove and seagrass DNA fragments were detected from sediments of narrow nearshore area of Nagura Bay, where silty sediment supplied by the Nagura River prevailed. Offshore sediments commonly contained OC with the bulk δ13C of -23‰ to -19‰, indicating that pelagic primary producers are the predominant source for OC stored in pelagic sediments. Nevertheless, DNA fragments of mangroves, but not seagrasses, could be detected in offshore sediments outside the Lagoon. Mangrove-derived eDNA was often detected significantly in bathypelagic sediments collected from northern area of Yaeyama Island. The concentration of bulk OC was also relatively high (>500 µmol C g-1) in these sediments.
These observations suggest that: (i) organic detritus produced in coastal habitats such as mangroves and seagrass beds is exported by tidal action; (ii) exported OC is not accumulated significantly in shallow lagoon outside the habitats due to high-energy physical environment, but (iii) flushed away further to offshore water by tidal residual current, which usually flows from south to north across the lagoon; (iv) a part of OC then sinks down to bathypelagic zone and stored in deep-sea sediment. Thus, deep-sea sediment located downstream of major coastal vegetated habitats can serve as potentially important blue carbon sink.