5:15 PM - 7:15 PM
[AOS21-P12] Dynamics of Karenia mikimotoi red tide in Imari Bay: Factors behind the formation of patchy-like water masses with high cell density
Keywords:Karenia mikimotoi, red tide, Imari Bay, Particle tracking method
Imari Bay is an enclosed bay located in northern Kyushu, with a complex topography of large and small islands. Aquaculture of tiger pufferfish and other fish is popular here, but red tides by the dinoflagellate Karenia mikimotoi (hereafter referred to as K. mikimotoi) have caused mass mortality of the farmed fish, resulting in significant damage to the fishing industry (Yamatogi et al., 2006). Recently, a large-scale red tide occurred in summer 2022. At this time, initial proliferation was observed in the eastern sea area of Fukushima, located at the inner bay, which then turned into a red tide, later spreading to the southern area of Takashima at the mouth of the bay. Based on the specific proliferation rate using observed cell count, this spread of the red tide was thought to be due to an advection-type red tide. In addition, patchy-colored areas were confirmed here. In this study, we analyze the dynamics of K. mikimotoi red tide in 2022 using a high-resolution coastal ocean model and a particle tracking model that incorporates active vertical migration and biological growth and death.
The numerical model used was FVCOM (Finite Volume Community Ocean Model, Chen et al., 2006), with Dreams_D (Hirose et al., 2005) for open boundary conditions, the rainfall runoff inundation model RRI (Rainfall Runoff Inundation model, Sayama and Iwami, 2014) for freshwater inflow assessment, and the mesoscale numerical forecast model MSM/GPV for meteorological conditions. FVCOM uses a triangular unstructured grid in the horizontal direction. In this study, resolution of 50m at the finest was used, faithfully representing the coastal and seafloor topography. This allowed us to reproduce diffusion and advection field with high accuracy, and by introducing virtual lagrangian particles that incorporated active vertical migration and biological growth and death (biological growth and death (Moum and Lueck, 1985; Loyer et al., 2001; Gentien et al., 2007), we analyzed the dynamics of the red tide. Active diurnal vertical migration was based on field observation in summer, 2022.
The virtual particles were released just above the seabed in eastern Fukushima at midnight on August 17 when a rapid increase in cell count was confirmed. They were then transported to the south of Takashima, creating patchy areas with high cell denisty. The red tide that proliferated in eastern Fukushima repeatedly migrated vertically and passed through a narrow channel to north with decrease in the sea surface salinity. Here, a strong inward current consisted of tidal current and mean current from Hibi straight in surface to middle layer flowed intermittently to the south of Takashima. Red tide particles which moved from eastern Fukushima to the north rose from the surface to the middle layer during the day due to diurnal vertical migration, and some were taken up by this current and transported in patches to the south of Takashima. We also confirmed that the biological processes maintained higher cell denisty in the patches.
The numerical model used was FVCOM (Finite Volume Community Ocean Model, Chen et al., 2006), with Dreams_D (Hirose et al., 2005) for open boundary conditions, the rainfall runoff inundation model RRI (Rainfall Runoff Inundation model, Sayama and Iwami, 2014) for freshwater inflow assessment, and the mesoscale numerical forecast model MSM/GPV for meteorological conditions. FVCOM uses a triangular unstructured grid in the horizontal direction. In this study, resolution of 50m at the finest was used, faithfully representing the coastal and seafloor topography. This allowed us to reproduce diffusion and advection field with high accuracy, and by introducing virtual lagrangian particles that incorporated active vertical migration and biological growth and death (biological growth and death (Moum and Lueck, 1985; Loyer et al., 2001; Gentien et al., 2007), we analyzed the dynamics of the red tide. Active diurnal vertical migration was based on field observation in summer, 2022.
The virtual particles were released just above the seabed in eastern Fukushima at midnight on August 17 when a rapid increase in cell count was confirmed. They were then transported to the south of Takashima, creating patchy areas with high cell denisty. The red tide that proliferated in eastern Fukushima repeatedly migrated vertically and passed through a narrow channel to north with decrease in the sea surface salinity. Here, a strong inward current consisted of tidal current and mean current from Hibi straight in surface to middle layer flowed intermittently to the south of Takashima. Red tide particles which moved from eastern Fukushima to the north rose from the surface to the middle layer during the day due to diurnal vertical migration, and some were taken up by this current and transported in patches to the south of Takashima. We also confirmed that the biological processes maintained higher cell denisty in the patches.