3:00 PM - 3:15 PM
[ACG41-12] Two-way nested blue carbon tracing simulation for elucidating fates of exported organic carbon from coastal ecosystems
Keywords:blue carbon, exported organic carbon, numerical simulation
A considerable amount of the organic carbon produced by coastal organisms such as seagrasses, mangroves, corals, is flushed out offshore, and may be potentially important for the global carbon cycles. However, the fates of the effused organic carbon of coastal origin are not well-known due to limited quantitative observation. In this study, a numerical simulation model for tracing the exported organic carbon was developed for elucidating the fates of the exported organic carbon.
The model composed of an ocean circulation model based on the Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System (Warner et al., 2010) and a newly developed low-trophic ecosystem model (modified from Nakamura et al., 2018). The low-trophic ecosystem model has the following compartments: dissolved inorganic carbon (DIC), total alkalinity (TA), dissolved oxygen (DO), ammonium (NH4), nitrate (NO3), phosphate (PO4), labile dissolved organic carbon (LDOC), nitrogen (LDON), and phosphorus (LDOP), refractory organic carbon (RDOC), nitrogen (RDON), and phosphorus (RDOP), coarse particulate organic carbon (CPOC), nitrogen (CPON), and phosphorus (CPOP), detritus type particulate organic carbon (DPOC), nitrogen (DPON), and phosphorus (DPOP), three functional groups of phytoplankton (dinoflagellates, diatoms, coccolithophorids), one functional group of zooplankton, and particulate inorganic carbon (PIC; dead CaCO3 shell of coccolithophorids). An additional carbon isotope module along with its compartments, such as DI13C, RDO13C, LDO13C, CPO13C, DPO13C, and 13C of the aforementioned phytoplankton, 13C of zooplankton, and PI13C, was incorporated into the model to trace the fates of the carbon. Isotope fractionations by all physical and biological processes were currently deactivated as the module was used for carbon tracing.
This model was applied to the Yaeyama Islands region, focusing on Nagura Bay, Ishigaki Island, for tracing organic carbon exported from the mangrove ecosystem around Nagura River. A two-grid nested configuration consisting of a coarser domain YAEYAMA1 (1.5 km grid resolution) covering the East-coast of Taiwan to Miyako Island and downscaled YAEYAMA2 (300 m grid resolution) covering the entire Ishigaki - Iriomote Island was used for the simulation. The dispersion process of the organic carbon from finer-scale to coarser-scale domain was tracked by applying two-way online nesting in the simulation.
In this simulation, completely 0% of carbon isotope is included in all compartments, and 100% of the CPOC as mangrove leaf carbon is marked as CPO13C and released from the Nagura river mouth. Then the CPOC (CPO13C) is dispersed by advection and diffusion. The carbon tracer model is able to capture the fates of the released CPOC including decomposition from CPOC to detritus to DIC, CPOC grazing and assimilation by zooplankton, detritus re-formation through organism mortality, and detritus deposition in ocean bottom sediment. This kind of carbon cycle was able to be chased and visualized by the carbon tracer. Simulation results also indicated a fraction of detritus that originated from mangrove leaves deposited into deeper water areas such as the Okinawa Trough after being dispersed from the Nagura River mouth. The amount of these mangrove-generated detritus, however, was much smaller than pelagic organism-generated detritus. Therefore, this modeling study suggests a challenge that might occur in detecting mangrove organic carbon from pelagic sediment core analysis by conventional techniques.
The model composed of an ocean circulation model based on the Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System (Warner et al., 2010) and a newly developed low-trophic ecosystem model (modified from Nakamura et al., 2018). The low-trophic ecosystem model has the following compartments: dissolved inorganic carbon (DIC), total alkalinity (TA), dissolved oxygen (DO), ammonium (NH4), nitrate (NO3), phosphate (PO4), labile dissolved organic carbon (LDOC), nitrogen (LDON), and phosphorus (LDOP), refractory organic carbon (RDOC), nitrogen (RDON), and phosphorus (RDOP), coarse particulate organic carbon (CPOC), nitrogen (CPON), and phosphorus (CPOP), detritus type particulate organic carbon (DPOC), nitrogen (DPON), and phosphorus (DPOP), three functional groups of phytoplankton (dinoflagellates, diatoms, coccolithophorids), one functional group of zooplankton, and particulate inorganic carbon (PIC; dead CaCO3 shell of coccolithophorids). An additional carbon isotope module along with its compartments, such as DI13C, RDO13C, LDO13C, CPO13C, DPO13C, and 13C of the aforementioned phytoplankton, 13C of zooplankton, and PI13C, was incorporated into the model to trace the fates of the carbon. Isotope fractionations by all physical and biological processes were currently deactivated as the module was used for carbon tracing.
This model was applied to the Yaeyama Islands region, focusing on Nagura Bay, Ishigaki Island, for tracing organic carbon exported from the mangrove ecosystem around Nagura River. A two-grid nested configuration consisting of a coarser domain YAEYAMA1 (1.5 km grid resolution) covering the East-coast of Taiwan to Miyako Island and downscaled YAEYAMA2 (300 m grid resolution) covering the entire Ishigaki - Iriomote Island was used for the simulation. The dispersion process of the organic carbon from finer-scale to coarser-scale domain was tracked by applying two-way online nesting in the simulation.
In this simulation, completely 0% of carbon isotope is included in all compartments, and 100% of the CPOC as mangrove leaf carbon is marked as CPO13C and released from the Nagura river mouth. Then the CPOC (CPO13C) is dispersed by advection and diffusion. The carbon tracer model is able to capture the fates of the released CPOC including decomposition from CPOC to detritus to DIC, CPOC grazing and assimilation by zooplankton, detritus re-formation through organism mortality, and detritus deposition in ocean bottom sediment. This kind of carbon cycle was able to be chased and visualized by the carbon tracer. Simulation results also indicated a fraction of detritus that originated from mangrove leaves deposited into deeper water areas such as the Okinawa Trough after being dispersed from the Nagura River mouth. The amount of these mangrove-generated detritus, however, was much smaller than pelagic organism-generated detritus. Therefore, this modeling study suggests a challenge that might occur in detecting mangrove organic carbon from pelagic sediment core analysis by conventional techniques.