Quantification of blue carbon in seagrass ecosystems of Southeast Asia and their potential for climate change mitigation
Milica StankovicRohani Ambo‐RappeFilipo CarlyFloredel Dangan-GalonMiguel D. FortesMohammad Shawkat HossainWawan KiswaraCao Van LuongPhan Minh-ThuAmrit Kumar MishraThidarat NoiraksarNurjannah NurdinJanmanee PanyawaiEkkalak RattanachotMohammad RozaimiU. Soe HtunAnchana Prathep
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Seagrasses store large amounts of blue carbon and mitigate climate change, but they have suffered strong regressions worldwide in recent decades. Blue carbon assessments may support their conservation. However, existing blue carbon maps are still scarce and focused on certain seagrass species, such as the iconic genus Posidonia, and intertidal and very shallow seagrasses (<10 m depth), while deep-water and opportunistic seagrasses have remained understudied. This study filled this gap by mapping and assessing blue carbon storage and sequestration by the seagrass Cymodocea nodosa in the Canarian archipelago using the local carbon storage capacity and high spatial resolution (20 m/pixel) seagrass distribution maps for the years 2000 and 2018. Particularly, we mapped and assessed the past, current and future capacity of C. nodosa to store blue carbon, according to four plausible future scenarios, and valued the economic implications of these scenarios. Our results showed that C. nodosa has suffered ca. 50 % area loss in the last two decades, and, if the current degradation rate continues, our estimations demonstrate that it could completely disappear in 2036 ("Collapse scenario"). The impact of these losses in 2050 would reach 1.43 MT of CO2 equivalent emitted with a cost of 126.3 million € (0.32 % of the current Canary GDP). If, however, this degradation is slow down, between 0.11 and 0.57 MT of CO2 equivalent would be emitted until 2050 ("Intermediate" and "Business-as-usual" scenarios, respectively), which corresponds to a social cost of 3.63 and 44.81 million €, respectively. If the current seagrass extension is maintained ("No Net Loss"), 0.75 MT of CO2 equivalent would be sequestered from now to 2050, which corresponds to a social cost saving of 73.59 million €. The reproducibility of our methodology across coastal ecosystems underpinned by marine vegetation provides a key tool for decision-making and conservation of these habitats.
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Abstract Seagrass meadows are important sinks for organic carbon and provide co-benefits. However, data on the organic carbon stock in seagrass sediments are scarce for many regions, particularly The Bahamas, which accounts for up to 40.7% of the documented global seagrass area, limiting formulation of blue carbon strategies. Here, we sampled 10 seagrass meadows across an extensive island chain in The Bahamas. We estimate that Bahamas seagrass meadows store 0.42–0.59 Pg organic carbon in the top-meter sediments with an accumulation rate of 2.1–2.9 Tg annually, representing a substantial global blue carbon hotspot. Autochthonous organic carbon in sediments decreased from ~1980 onwards, with concomitant increases in cyanobacterial and mangrove contributions, suggesting disturbance of seagrass ecosystems, likely caused by tourism and maritime traffic activities. This study provides seagrass blue carbon data from a vast, understudied region and contributes to improving climate action for The Bahamas and the Greater Caribbean region.
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Seagrass meadows are globally important blue carbon sinks, accumulating organic carbon within their beds from both seagrass and the burial of non-seagrass organic matter.  To date, substantial attention has been attracted on this nature-based carbon offsetting solution, especially for countries with large blue carbon resources (e.g., Australia, Indonesia, and USA). However, the carbon sink potential of small coastal regions characterized by high carbon densities of blue carbon ecosystems and its contribution to regional climate change mitigation efforts remain largely unclear. This study focusses on the quantification of seagrass carbon stock in an urbanized coastal area, Hong Kong. We collect 1-meter sediment cores from four sites (i.e., San Tau, Yam O, Sheung Pak Nai and Ha Pak Nai) covering two seagrass species (i.e., Halophila beccarii and Halophila ovalis). Our investigation encompasses the analysis of total organic carbon (OC%), organic carbon sources (δ13C and δ15N), labile and recalcitrant organic carbon pools, and organic carbon accumulation rates (Pb210) at various depths within the sediment cores. The results of this study will shed light on the extent to which seagrass ecosystems can contribute to Hong Kong's Nationally Determined Contributions, as assessed under the IPCC Tier-II framework.
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