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https://doi.org/10.5194/bg-2019-377
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-377
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 07 Oct 2019

Submitted as: research article | 07 Oct 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Phytoplankton productivity and rapid trophic transfer to microzooplankton stimulated by turbulent nitrate flux in oligotrophic Kuroshio Current

Toru Kobari1, Taiga Honma2, Daisuke Hasegawa3, Naoki Yoshie4, Eisuke Tsutumi5, Takeshi Matsuno6, Takeyoshi Nagai7, Takeru Kanayama2, Fukutaro Karu2, Koji Suzuki8, Takahiro Tanaka3, Xinyu Guo4, Gen Kume1, Ayako Nishina1, and Hirohiko Nakamura1 Toru Kobari et al.
  • 1Aquatic Sciences, Faculty of Fisheries, Kagoshima University 4-50-20 Shimoarata, Kagoshima, Kagoshima 890-0056, Japan
  • 2Aquatic Sciences, Graduate School of Fisheries, Kagoshima University 4-50-20 Shimoarata, Kagoshima, Kagoshima 890-0056, Japan
  • 3Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency 3-27-5 Shinhama-cho, Shiogama, Miyagi 985-0001, Japan
  • 4Center for Marine Environmental Studies, Ehime University 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
  • 5Atmosphere and Ocean Research Institute, University of Tokyo 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan
  • 6Research Institute for Applied Mechanics, Kyushu University 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
  • 7Department of Ocean Sciences, Tokyo University of Marine Science and Technology 4-5-7 Konan Minato-ku, Tokyo 108-8477, Japan
  • 8Faculty of Environmental Earth Science, Hokkaido University North 10 West 5 Kita-ku, Sapporo, Hokkaido 060-0810, Japan

Abstract. The Kuroshio Current has been thought to be biologically unproductive due to oligotrophic conditions and low plankton standing stocks. Nevertheless, major foraging fishes are known to grow and recruit around the Kuroshio Current. While mixing and advection supplying nutrients to the euphotic zone are happened by eddies and meanders but limited at the Kuroshio front, there is a risk that survival of vulnerable life stages is encountered under the low food availability. Here we report that phytoplankton productivity is stimulated by turbulent nitrate flux amplified with the Kuroshio Current and rapidly transferred to microzooplankton through their grazing. Oceanographic observations demonstrate that the Kuroshio Current topographically enhances significant turbulent mixing and nitrate influx to the euphotic zone. Gradual nutrient enrichment experiments show growth rates of phytoplankton and microzooplankton communities stimulated within a range of the turbulent nitrate flux. Dilution experiments imply a significant microzooplankton grazing on phytoplankton. We propose that these rapid and systematic trophodynamics enhance invisible biological productivity in the Kuroshio.

Toru Kobari et al.
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Short summary
We report biological productivity under turbulent nitrate flux amplified with the Kuroshio. Oceanographic observations exhibit that the Kuroshio topographically enhances significant turbulent mixing and nitrate influx to the euphotic zone. Onboard experiments show phytoplankton and microzooplankton growths enhanced with the nitrate flux and a significant microzooplankton grazing on phytoplankton. These rapid and systematic trophodynamics are likely invisible productivity in the Kuroshio.
We report biological productivity under turbulent nitrate flux amplified with the Kuroshio....
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