Vertical fluxes of nutrients enhanced by strong turbulence and phytoplankton bloom around the ocean ridge in the Luzon Strait

Eisuke Tsutsumi,Hiroshi Ogawa,Keunjong Lee,Daigo Yanagimoto,Ichiro Yasuda,Jing Zhang,Tomoharu Senjyu,Sachihiko Itoh,Takeshi Matsuno,Akie Sakai,Cesar Villanoy

doi:10.1038/s41598-020-74938-5

Abstract

Steep oceanic ridges and tidal currents in the Luzon Strait generate some of the world’s strongest turbulent mixing. To evaluate the impacts of the turbulence intensity on the marine ecosystem, we carried out measurements of microstructure turbulence and biogeochemical hydrography along 21°N in the Luzon Strait during the R/V Hakuho Maru cruise, KH-17-5-2, in November 2017. We found a turbulent kinetic energy dissipation rate exceeding O(10−7) W kg−1 and vertical eddy diffusivity exceeding O(10−3) m2 s−1, two orders of magnitude larger than those in the open ocean, above a shallow sub-ridge on the eastern ridge of the Luzon Strait. In addition, a clear chlorophyll a bloom was identified in the surface layer above the sub-ridge from in situ measurements and satellite observations. High values of nitrate (4.7 mmol N m−2 d−1) and phosphate (0.33 mmol P m−2 d−1) fluxes estimated near the base of the surface chlorophyll a bloom strongly suggest that enhanced turbulent mixing promotes nutrient supply to the euphotic zone and generates new production within the surface layer, contributing to the formation of a quasi-permanent local chlorophyll a bloom north of Itbayat Island on the eastern ridge.

Highlights

The Kuroshio is the western boundary current (WBC) forming part of the North Pacific sub-tropical gyre, transporting heat and material from low to high latitudes
Because primary production is likely attributed to new production by nutrient uptake in our experimental area/period[34], this study investigates a possible linkage between topographically enhanced turbulent mixing and the chlorophyll a bloom north of Itbayat Island through nutrient supply from depth
From surveys of turbulence microstructure and biogeochemical hydrography, we found elevated vertical eddy diffusivity of O(10−3) m−2 s−1 and large vertical nitrate flux of O(1) mmol N m−2 d−1 at around 100-m depth above shallow topography on the eastern ridge of the Luzon Strait in late-autumn/early-winter (Fig. 3b–d)

Summary

Introduction

The Kuroshio is the western boundary current (WBC) forming part of the North Pacific sub-tropical gyre, transporting heat and material from low to high latitudes. Within the Kuroshio, elevated turbulent kinetic energy (TKE) dissipation rate ε and vertical eddy diffusivity Kρ have been identified in the Tokara Strait where the Kuroshio enters the northwestern P acific[1,2], in the Shikoku Basin south of Japan[3] and in the Green Island area east of T aiwan[4] These studies suggested that the strong vertical mixing observed was caused by interactions between the strong Kuroshio current and the abrupt topography, such as shallow seamounts, ocean ridges, and shelf breaks. The two dominant ocean ridges in the Luzon Strait, the eastern and western ridges, generate strong internal waves and turbulent mixing associated with tidal motion, topographic resonance, and the K uroshio[6,7,8,9,10,11]. Shang et al.[32] suggested that the vertical mixing associated with the upwelling plays a role in sustaining the phytoplankton blooms

Methods

Results

Conclusion