Seasonal transition of the dominant nitrogen source for phytoplankton reconstructed by determining the nitrogen isotope composition for nitrate, ammonium, and individual amino acids of sinking particles in the Northwestern Pacific

Chisato Yoshikawa1, Nanako O Ogawa2, Yoshito Chikaraishi3, Akiko Makabe4, Yohei Matsui4, Yoshikazu Sasai5, Masahide Wakita6, Makio C Honda7, Yoshihisa Mino8, Maki Noguchi Aita9, Tetsuichi Fujiki10, Takuro Nunoura4, Naomi Harada11 and Naohiko Ohkouchi2, (1)JAMSTEC, Tokosuka, Japan, (2)JAMSTEC, Research Institute for Marine Resources Utilization (Biogeochemistry Program), Yokosuka, Japan, (3)Hokkaido University, Institute of Low Temperature Science, Sapporo, Japan, (4)JAMSTEC, Yokosuka, Japan, (5)Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Research Institute for Global Change (RIGC), Yokohama, Japan, (6)JAMSTEC, MIO, Mutsu, Japan, (7)JAMSTEC, Kanagawa, Japan, (8)Nagoya University, Nagoya, Japan, (9)Japan Agency for Marine-Earth Science and Technology, Research Institute for Global Change, Yokohama, Japan, (10)JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, (11)Japan Agency for Marine-Earth Science & Technology, Yokosuka, Japan
Abstract:
The availability of nitrogenous nutrients, such as nitrate and ammonium, controls marine productivity. In the high-latitude ocean, phytoplankton growth is assumed to rely on nitrate due to its convective supply in winter. However, the uptake rates reported previously were snapshots; thus, the year-round interaction of nitrate and ammonium uptake by phytoplankton remains unexplored. Here, we describe the seasonal transition of the dominant nitrogen source for phytoplankton, reconstructed by determining the nitrogen isotopic composition (δ15N) for nitrate, ammonium, and individual amino acids of sinking particles in the Northwestern Pacific. The seasonal δ15N patterns demonstrate that the dominant nitrogen source for phytoplankton switches from nitrate to ammonium in autumn. A data-constrained nitrogen isotope model reveals that ammonium supports approximately half of the annual primary production there. Our findings stress the importance of ammonium as a major nitrogen source of phytoplankton, even at high latitudes.