Importance of salt fingering for new nitrogen supply in the oligotrophic ocean

The input of new nitrogen into the euphotic zone constrains the export of organic carbon to the deep ocean and thereby the biologically mediated long-term CO 2 exchange between the ocean and atmosphere. In low-latitude open-ocean regions, turbulence-driven nitrate diffusion from the ocean’s interior and biological fixation of atmospheric N 2 are the main sources of new nitrogen for phytoplankton productivity. With measurements across the tropical and subtropical Atlantic, Pacific and Indian oceans, we show that nitrate diffusion (171±190 μmol m −2 d −1 ) dominates over N 2 fixation (9.0±9.4 μmol m −2 d −1 ) at the time of sampling. Nitrate diffusion mediated by salt fingers is responsible for ca. 20% of the new nitrogen supply in several provinces of the Atlantic and Indian Oceans. Our results indicate that salt finger diffusion should be considered in present and future ocean nitrogen budgets, as it could supply globally 0.23–1.00 Tmol N yr −1 to the euphotic zone. The relative contribution of nitrogen fixation and nitrogen diffusion to marine biomes is presently debated. Here, the authors evaluate the contribution of these pathways across the tropics and subtropics of the global ocean and show that nitrogen diffusion, reinforced by salt fingers, is the dominant process.