High nitrogen fixation and Braarudosphaera presence in the North Sea

Abstract Nitrogen fixation, the microbial conversion of dinitrogen gas to ammonia, is a key nitrogen source and critical for sustaining marine productivity. We investigated the diversity and activity of nitrogen‐fixers in surface waters of the North Sea and Norwegian coast during July 2023. We detected, for the first time, the presence of Braarudosphaera /UCYN‐A (up to 10 4 nifH copies L −1 ) and measurable nitrogen fixation rates (up to 110 nM d −1 ) throughout the North Sea. Nitrogen‐fixing organisms may be transported from the northeast Atlantic across the northern North Sea by southward current systems, via Baltic Sea inflow or be a persistent but overlooked component of the phytoplankton community. We estimate nitrogen fixation may support between 4% and 24% of summertime net primary production. Nitrogen fixation is currently neglected in numerical models of shelf seas, yet may play a critical role in sustaining shelf sea productivity in the contemporary and future ocean.