Nitrogen Isotopes of Sinking Particles Reveal the Seasonal Transition of the Nitrogen Source for Phytoplankton
Chisato Yoshikawa, Nanako O. Ogawa, Yoshito Chikaraishi, Akiko Makabe, Yohei Matsui, Yoshikazu Sasai, Masahide Wakita, Makio C. Honda, Yoshihisa Mino, Maki Noguchi Aita, Tetsuichi Fujiki, Takuro Nunoura, Naomi Harada, Naohiko Ohkouchi
Abstract
Abstract In the high‐productivity subarctic ocean, phytoplankton growth is assumed to rely mainly on nitrate, based on snapshot observations conducted in spring or summer. However, the year‐round utilization of nitrate and ammonium by phytoplankton remains unexplored. Here, we show the seasonal transition of the dominant nitrogen source for phytoplankton by using the nitrogen isotopic composition (δ 15 N) of nitrate, ammonium, and sinking particles in the northwestern North Pacific. To decouple the combined effects of nitrogen sources for phytoplankton and subsequent trophic transfers in δ 15 N of sinking particles, we used compound‐specific δ 15 N analysis of amino acids (δ 15 N AA ) in sinking particles. The seasonal change in δ 15 N AA revealed that most of the seasonal variation in δ 15 N of sinking particles arose from changes in the availability of nitrogenous nutrients in the euphotic layer. The δ 15 N of sinking particles suggested that the dominant nitrogen source for phytoplankton switches from nitrate to ammonium in autumn.