[A-CG48] Water and sediment dynamics from land to coastal zones

[ACG48-P02] Redefining the regions of Freshwater Influence of coastal seas based on the nitrogen cycle
using a hybrid Eulerian-Lagrangian NPZD model

*Shinichiro Kida¹, Yasuhiro Hoshiba², Yoshimasa Matsumura³, Takao Kawasaki³ (1.Research Institute for Applied Mechanics, Kyushu University, 2.JAMSTEC, 3.Atmosphere and Ocean Research Institute, the University of Tokyo)

Keywords:River plume, Nitrogen Cycle, Coastal Seas

ROFI (Regions Of Freshwater Influence) systems are typically defined based on salinity, as rivers introduce significant freshwater into coastal areas. However, salinity signals for small rivers often disappear within a few days due to tides, wind-induced mixing, and coastal currents. This salinity-based definition of ROFI may not be well-suited for explaining phytoplankton growth near the coast, as biological responses to riverine input occur over longer time scales. Therefore, a different definition of ROFI is likely more appropriate for understanding biological responses to riverine water. In this study, we demonstrate that a nutrient-based definition of ROFI can be clarified using a newly developed Eulerian-Lagrangian NPZD model, KARMA. This model tracks the fate of nutrients using the Lagrangian Framework while simulating phytoplankton blooms driven by river inflows and oceanic upwelling. By identifying the origin of nutrients utilized for phytoplankton growth, the model effectively clarifies the spatial extent of where riverine nutrients play a dominant role in the coastal nutrient cycle.