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

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.