Abstract
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 CO2 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 N2 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 N2 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.
Highlights
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 CO2 exchange between the ocean and atmosphere
On average, nitrate diffusion dominated over N2 fixation, and that diffusion mediated by salt fingers was responsible for ca. 20% of the new nitrogen supply in several tropical and subtropical provinces of the north and south Atlantic, and the Indian Ocean
Nitrate diffusive fluxes computed during the Malaspina expedition are in the range of previously reported values for open-ocean (34–850 mmol m À 2 d À 1)[3,4,21,22] and regions under the influence of equatorial upwelling (1,300 mmol m À 2 d À 1)[23]
Summary
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 CO2 exchange between the ocean and atmosphere. Recent studies indicate that biological fixation of atmospheric N2 by microbial diazotrophs could equal or even exceed nitrate diffusion as a mechanism for new nitrogen supply in the subtropical gyres[2,3,4] These vast biomes are responsible for approximately 30% of the global carbon export to the deep ocean[5,6]. Turbulent diffusivity is due to mechanical processes, such as shear instabilities and internal waves, and due to double-diffusive processes including salt fingers[7] These develop in the tropical and subtropical central oceans, where warm and salty layers overlie cooler and fresher waters[8]. On average, nitrate diffusion dominated over N2 fixation, and that diffusion mediated by salt fingers was responsible for ca. 20% of the new nitrogen supply in several tropical and subtropical provinces of the north and south Atlantic, and the Indian Ocean
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