Abstract
Abstract. The Kuroshio Current has been thought to be biologically unproductive because of its oligotrophic conditions and low plankton standing stocks. Even though vulnerable life stages of major foraging fishes risk being entrapped by frontal eddies and meanders and encountering low food availability, they have life cycle strategies that include growing and recruiting around the Kuroshio Current. Here we report that phytoplankton growth and consumption by microzooplankton are stimulated by turbulent nitrate flux amplified by the Kuroshio Current. Oceanographic observations demonstrate that the Kuroshio Current topographically enhances significant turbulent mixing and nitrate influx to the euphotic zone. Graduated nutrient enrichment experiments show that growth rates of phytoplankton and microheterotroph communities were stimulated within the range of the turbulent nitrate flux. Results of dilution experiments imply significant microzooplankton grazing on phytoplankton. We propose that these rapid and systematic trophodynamics enhance biological productivity in the Kuroshio.
Highlights
The Kuroshio Current is the western boundary current of the North Pacific Subtropical Gyre (Qiu, 2001; Hu et al, 2015)
Turbulent diffusivity and nitrate concentrations were measured in order to estimate the vertical turbulent nitrate flux along the transects across the Kuroshio Current in the Tokara Strait, where a shallow ridge lies in the path of the Kuroshio
Compared with the turbulent nitrate fluxes reported in previous studies, the fluxes observed in the Tokara Strait were 1 order of magnitude higher than those reported in the Kuroshio Extension front (Kaneko et al, 2012, 2013; Nagai et al, 2017), which is greater than those at other oceanic sites, and equivalent to those at coastal sites (Cyr et al, 2015)
Summary
The Kuroshio Current is the western boundary current of the North Pacific Subtropical Gyre (Qiu, 2001; Hu et al, 2015). The Kuroshio has been thought to be biologically unproductive because ambient nutrient concentrations and plankton standing stocks in its waters are low (Guo, 1991; Hirota, 1995) In spite of such seemingly unproductive conditions, the Kuroshio in the East China Sea (ECS-Kuroshio) is adjacent to major spawning and nursery. Good fishing grounds have been found for various fishes and squid near the Kuroshio, and the catches from those grounds account for more than half of the total catch in Japanese waters (Saito, 2019) It is risky, for highly vulnerable early life stages of many foraging species to grow and recruit in the oligotrophic and unproductive waters of the ECS-Kuroshio (hereafter called the “Kuroshio Paradox”; Saito, 2019), even if the warm temperatures of the Kuroshio Current can enhance cellular metabolic processes and thereby stimulate growth. Use of waters in the vicinity of the oligotrophic Kuroshio as a nursery and feeding ground would appear to be a risky strategy, unless there is a mechanism that enhances biological production in the Kuroshio
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