11:30 〜 11:45
[AOS13-10] Occurrence of phytoplankton bloom as the Kuroshio passes an island
キーワード：生物地球化学的モデル、黒潮、島
When the Kuroshio flows over islands or seamounts, it causes irregular fluid motions in the wake, such as eddy generation and shedding (Von Karman Vortex Street), turbulence mixing, upwelling and downwelling, and even instability. Vertical perturbations such as mixing and upwelling, bring the nutrients from deep layer to the oligotrophic surface layer and therefore enhance biological activity in the euphotic layer. Biological enrichment forms in the vicinity of the island and extends downstream, which is known as ‘Island Mass Effect’ (IME).
To investigate the IME and its community variation in the Kuroshio region, we combine a hydrodynamic model (Princeton Ocean Model) and a lower-trophic-level ecosystem model (eNEMURO). The eNEMURO includes 4 nutrient compartments (NO3, NH4, Si(OH)4, PO4), 4 phytoplankton compartments (2 types of micro-phyt, nano-phyt, pic-phyt), 4 zooplankton compartments (macro-zoo, meso-zoo, micro-zoo, nano-zoo), 3 detritus compartments (POM, DOM, Opal). We apply the combined two models to a simulation of ideal baroclinic geostrophic current flowing over an island with an ambient biological condition similar to the situation at the PN section across the Kuroshio in the East China Sea.
The model produces the Von Karman Vortex Street in the case of a current with a speed of 1m/s passing over an island with a diameter of 10 km. The eddies form just behind the island and shed toward downstream with a period of 13 hours. The cold water in the cyclonic recirculation corresponds to local upwelling that supplies extra nutrient to the oligotrophic surface and induces the surface phytoplankton bloom and uplift of Subsurface Chl-a Maximum (SCM) in the downstream wake area. At the depth below SCM (50m), the concentration of DIN increases inside the cyclonic eddies, while the phytoplankton decreases in the same location in the vicinity of the island. The variation of biological components in Von Karmen Vortex Street is caused by a combination of physical processes and biochemical processes. From the island to the place 100 km behind the island, the variation is mainly controlled by physical processes. After that, with the eddies extending downstream, effects of biogeochemical process increase and gradually become important. The increase in the cyclonic eddy region caused by biogeochemical processes is mainly contributed by photosynthesis supported by extra nutrient supply. In the oligotrophic surface water, the supply of nitrate is more efficient to the photosynthesis of PS, which has the lowest half-saturation concentration of nitrate.
In the realistic ocean, the Kuroshio passes islands with a variety of diameter and with a variety of speed. The variation in current speed and island size can cause different hydrodynamic condition for the ecosystem. To understand the effect of such variation, we design a series of numerical simulations to obtain a general understanding of hydrodynamic condition and biological response at island wake in the Kuroshio region.
To investigate the IME and its community variation in the Kuroshio region, we combine a hydrodynamic model (Princeton Ocean Model) and a lower-trophic-level ecosystem model (eNEMURO). The eNEMURO includes 4 nutrient compartments (NO3, NH4, Si(OH)4, PO4), 4 phytoplankton compartments (2 types of micro-phyt, nano-phyt, pic-phyt), 4 zooplankton compartments (macro-zoo, meso-zoo, micro-zoo, nano-zoo), 3 detritus compartments (POM, DOM, Opal). We apply the combined two models to a simulation of ideal baroclinic geostrophic current flowing over an island with an ambient biological condition similar to the situation at the PN section across the Kuroshio in the East China Sea.
The model produces the Von Karman Vortex Street in the case of a current with a speed of 1m/s passing over an island with a diameter of 10 km. The eddies form just behind the island and shed toward downstream with a period of 13 hours. The cold water in the cyclonic recirculation corresponds to local upwelling that supplies extra nutrient to the oligotrophic surface and induces the surface phytoplankton bloom and uplift of Subsurface Chl-a Maximum (SCM) in the downstream wake area. At the depth below SCM (50m), the concentration of DIN increases inside the cyclonic eddies, while the phytoplankton decreases in the same location in the vicinity of the island. The variation of biological components in Von Karmen Vortex Street is caused by a combination of physical processes and biochemical processes. From the island to the place 100 km behind the island, the variation is mainly controlled by physical processes. After that, with the eddies extending downstream, effects of biogeochemical process increase and gradually become important. The increase in the cyclonic eddy region caused by biogeochemical processes is mainly contributed by photosynthesis supported by extra nutrient supply. In the oligotrophic surface water, the supply of nitrate is more efficient to the photosynthesis of PS, which has the lowest half-saturation concentration of nitrate.
In the realistic ocean, the Kuroshio passes islands with a variety of diameter and with a variety of speed. The variation in current speed and island size can cause different hydrodynamic condition for the ecosystem. To understand the effect of such variation, we design a series of numerical simulations to obtain a general understanding of hydrodynamic condition and biological response at island wake in the Kuroshio region.