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

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.