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

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.