[A-OS12] Physical, biogeochemical, and ecological processes and variability in the Indian Ocean

[AOS12-02] Temporary phytoplankton bloom induced by physical disturbances in the Eastern Indian Ocean

*江 思宇¹、Hashihama Fuminori²、Saito Hiroaki¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo、2.Tokyo University of Marine Science and Technology)

キーワード：Indian Ocean、Phytoplankton、Microzooplankton、Dilution technique、Physical disturbances、Wyrtki Jet

Phytoplankton growth and microzooplankton grazing are critical processes which support higher trophic levels in marine ecosystems. In oligotrophic marine ecosystems, these processes are generally coupled with each other and the balance between them keeps phytoplankton biomass consistently low. However, it has been reported the coupling was easily violated by physical disturbances. The Eastern Indian Ocean is an oligotrophic ecosystem with strong seasonal and intraseasonal variabilities induced by physical activities. However, phytoplankton growth and microzooplankton grazing mortality were rarely studied in this area, and their responses to physical disturbances remain unclear. In this study, we conducted a series of dilution experiments in the surface layer (10 m) in subtropical and tropical Eastern Indian Ocean (88°E, 16.5 °N to 20 °S) to measure phytoplankton growth and microzooplankton grazing mortality rates. The study area was throughout oligotrophic (dissolved inorganic nitrogen concentration lower than 60 nM). The Chl a concentration was low (0.13 ± 0.09 μg L^-1) except the equatorial stations (0.26 ± 0.06 μg L^-1). However, the high biomass at the equator decreased to approximate 0.1 μg L^-1 after the cruise, as demonstrated by satellite Chl a observations. Further, the phytoplankton growth at the equator was low or even negative (-0.07 and 0.28 d^-1) and was limited by nutrients availability, while at other stations growth rates were higher (0.68 ± 0.33 d^-1) and the nutrient limitation was not serious. The significant response of phytoplankton growth to additional ammonium (by ~3.3 times high growth rate) and the higher phytoplankton Chl:C ratio at the equator indicated there was once relatively abundant nutrients. The equatorial Wyrtki Jet formed before the cruise possibly transported the nutrient-rich water to the Eastern Indian Ocean. The time-lag between the jet formation and the cruise might allow phytoplankton to exhaust the nutrients and accumulate the biomass. Our results indicated in the Eastern Indian Ocean, the balance between phytoplankton growth and grazing mortality could be violated by physical disturbances and the growth exceeded mortality could induce temporary phytoplankton blooms.