[A-CG31] Nutrient footprint of primary production in the coastal and marginal seas of East and Southeast Asia

[ACG31-08] Microplankton community succession after the inflow of the Kuroshio Current at the mouth of Kagoshima Bay

*Tomohiro Komorita¹, Mutsuo Ichinomiya¹, Toru Kobari², Gen Kume², Shin'ichro Kako², Akimasa Habano², Youichi Arita², Humihiro Makino² (1.Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto, 2.Kagoshima University)

Keywords:Kuroshio, microplankton

The inflow of the Kuroshio current into coastal areas is known to stimulate biological production, however; there are no detailed reports on its temporal changes. In the Kagoshima Bay, Japan, the date of inflow of the Kuroshio branch current can be identified a posteriori based on water temperature data. Therefore, by compiling the results of multiple observations, it is possible to track temporal change of biological productivity related to the inflow of the Kuroshio branch current. It has hypothesized that phytoplankton blooms occur in this area due to upwelling of the bottom water at the bay mouth during the inflow of the Kuroshio branch current. From this hypothesis, it is expected that microplankton will increase with the number of days elapsed from the inflow of the Kuroshio branch current. In this study, we compiled the results of ship observations conducted in the Kagoshima Bay from 2016 to 2019 to examine changes in water quality (temperature, salinity, nutrients), microphytoplankton and microzooplankton versus days elapsed the Kuroshio branch current inflow. In this study, significant relationships were obtained for microplankton in terms of both biomass and cell abundance, with specific growth rates could be calculated ranging from 0.060 to 0.079 d^-1, comparable to the growth rate of Chl-a. Over time, phytoplankton, mainly athecate dinoflagellates, and microzooplankton, mainly naked ciliates, proliferated and might be quickly preyed upon by mesozooplankton. Such events occur once every two weeks in most years, suggesting that frequent phytoplankton blooms support high biomass of mesozooplankton and larval fish.