Nitrogen Isotopes of Sinking Particles Reveal the Seasonal Transition of the Nitrogen Source for Phytoplankton

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 (delta N-15) 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 delta N-15 of sinking particles, we used compound-specific delta N-15 analysis of amino acids (delta N-15(AA)) in sinking particles. The seasonal change in delta N-15(AA) revealed that most of the seasonal variation in delta N-15 of sinking particles arose from changes in the availability of nitrogenous nutrients in the euphotic layer. The delta N-15 of sinking particles suggested that the dominant nitrogen source for phytoplankton switches from nitrate to ammonium in autumn.

Automated summary

This study reveals the seasonal transition of the dominant nitrogen source for phytoplankton in the high-productivity subarctic ocean. The results show that the nitrogen source switches from nitrate to ammonium in autumn.