[A-OS12] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS12-09] Assessment of turbulent mixing impact on productivity and higher trophic levels along the Kuroshio in the Tokara Strait

*Gloria Silvana Duran Gomez¹, Takeyoshi Nagai², Toru Kobari³, Hirohiko Nakamura³, Ingibjorg Bjorgvinsdottir¹, Kristinn Þröstur Sigurdarson^4,1 (1.Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, 2.Department of Ocean Sciences, Tokyo University of Marine Science and Technology, 3.Faculty of Fisheries, Kagoshima University, 4.University of Iceland)

Keywords:Kuroshio, Nutrient, Turbulent Mixing, The Tokara Strait

While surface layers of the Kuroshio Current have been known for being nutrient poor, previous studies reported that the Kuroshio carries a large amount of nutrients in dark subsurface layer. Bringing these subsurface nutrients upward can be the reason for the high biodiversity that has been largely unexplained. Since the Kuroshio frequently flows over rough topography, strong vertical mixing over the topographic features, seems to be the key process to enrich the Japanese coast. Previous observations have revealed that turning the sign of potential vorticity on the steep slope of seamounts, followed by submesoscale instability, provides 100-km scale 100-1000-fold enhancement of turbulence along the Kuroshio in the Tokara Strait. Observed strong turbulence could inject nutrients to the euphotic zone and, ultimately, stimulate phytoplankton standing stocks, thus, supporting the high productivity in the region. However, a series of these physical-biological processes have not been observed directly along the Kuroshio. In this study, direct microstructure observations in the Tokara Strait were conducted using the R.T.V. Kagoshima-Maru during June 13-20, and November 14-21, 2021, by using a state-of-the-art twin tow-yo profiling system, which consists of a tow-yo microstructure profiler (Underway-VMP), and an additional tow-yo profiler for biogeochemical parameters (SUNADAYODACAM). Using these profilers, we show how chlorophyll-a and zooplankton distribute in response to the upward diffused nitrate caused by the mixing. Furthermore, we determine until which scale this nitrate injection happens across the Kuroshio.