11:15 〜 11:30
[AOS15-09] A Lagrangian Perspective on Nitrogen Cycle in the Lower-Trophic Marine Ecosystems around Japan
キーワード：海洋低次生態系モデリング、ラグランジアン粒子追跡
Japan and East Asia heavily depend on marine resources to meet their food demands, making it critical to understand the mechanisms that sustain high biological production in the northwestern Pacific. Recent advances in ocean modeling enabled simulations over wider domains and longer periods with spatial resolutionto successfully resolve the complex coastline geometry and sub-mesoscale mixing processes. The progress of terrestrial water modeling has also enabled the acquisition of precise river discharge data. In this study, we constructed a numerical model covering the marine region from the East China Sea to the Northwestern Pacific with the resolution of 1/180 degree (approximately 500 m) and drove a lower-trophic marine ecosystem model in this region. A simplified NPZD (nutrient-phytoplankton-zooplankton-detritus) framework is employed to simulate the nitrogen cycling and integration performed for three years using atmospheric and river discharge from 2017 to 2019. We have assigned dissolved inorganic nitrogen concentration of 5 mg/l to river inputs to quantify the impact of land-derived nutrient supplies on the coastal marine ecosystem.
To elucidate the origins and transport pathways of nutrients that support high biological production around Japan, we introduced an Euler-Lagrange hybrid approach. This novel modeling system simultaneously drives a Lagrangian particle-based ecosystem model, where each particle represents a fixed amount of nitrogen in one of four phases (N, P, Z, or D), alongside a conventional Eulerian NPZD model. Each particle continuously undergoes probabilistic state transitions with synchronizing the Eulerian model and hence number density of particles of the specific phase over a certain spatial-temporal window coincides with the result of the Eulerian model. By tracking of trajectories and history of the phase-transition of particles, we can analyze the origins of nutrients as well as their consumption and regeneration cycles.
The results indicate that most nutrient provided by Japanese rivers is rapidly consumed in bays and estuaries, accumulating as detritus along the coastal zone. Previously deposited detritus may act as nutrient sources through benthic processes. Although our model does not explicitly simulate benthic processes, we assume a local equilibrium in which the resuspended nitrogen equals the deposited amount, allowing us to estimate the contribution of sediment-derived nutrients. In offshore regions, while riverine nutrients constitute only a small fraction of overall biological production, significant nitrogen pulses during discharge events contribute to offshore productivity.
By extracting particles that have experienced N -> P phase transition inside the regions of interest, we can quantitatively determine the nutrient source and their pathways supporting the biological production at the region. Future work will focus on enhancing model accuracy by incorporating iron and phosphate limitations, explicit sediment processes, and dynamic riverine dissolved substance concentrations.
To elucidate the origins and transport pathways of nutrients that support high biological production around Japan, we introduced an Euler-Lagrange hybrid approach. This novel modeling system simultaneously drives a Lagrangian particle-based ecosystem model, where each particle represents a fixed amount of nitrogen in one of four phases (N, P, Z, or D), alongside a conventional Eulerian NPZD model. Each particle continuously undergoes probabilistic state transitions with synchronizing the Eulerian model and hence number density of particles of the specific phase over a certain spatial-temporal window coincides with the result of the Eulerian model. By tracking of trajectories and history of the phase-transition of particles, we can analyze the origins of nutrients as well as their consumption and regeneration cycles.
The results indicate that most nutrient provided by Japanese rivers is rapidly consumed in bays and estuaries, accumulating as detritus along the coastal zone. Previously deposited detritus may act as nutrient sources through benthic processes. Although our model does not explicitly simulate benthic processes, we assume a local equilibrium in which the resuspended nitrogen equals the deposited amount, allowing us to estimate the contribution of sediment-derived nutrients. In offshore regions, while riverine nutrients constitute only a small fraction of overall biological production, significant nitrogen pulses during discharge events contribute to offshore productivity.
By extracting particles that have experienced N -> P phase transition inside the regions of interest, we can quantitatively determine the nutrient source and their pathways supporting the biological production at the region. Future work will focus on enhancing model accuracy by incorporating iron and phosphate limitations, explicit sediment processes, and dynamic riverine dissolved substance concentrations.