Evaluating the balance between vertical diffusive nitrate supply and nitrogen fixation with reference to nitrate uptake in the eastern subtropical North Atlantic Ocean. Issue 10 (22nd October 2013)

• Record Type: Journal Article
• Authors: Painter, Stuart C.; Patey, Matthew D.; Forryan, Alexander; Torres‐Valdes, Sinhue
• Is Part Of: Journal of geophysical research. Volume 118:Issue 10(2013:Oct.)
• Page Start: 5732
• Page End: 5749
• Publication Date: 2013-10-22
• Subjects: Oceanography -- Periodicals; 551.4605
• DOI: 10.1002/jgrc.20416
• Languages: English
• ISSNs: 2169-9275

Abstract

<abstract abstract-type="main"> <title> <x xml:space="preserve">Abstract</x> </title> <p>[1] The balance between N<sub>2</sub> fixation and diffusive NO<sub>3</sub><sup>−</sup> supply is a key determinant for assessing the importance of both processes for new production in subtropical waters. Here we report observations of integrated N<sub>2</sub> fixation rates from the eastern subtropical North Atlantic Ocean with coincident estimates of diffusive NO<sub>3</sub><sup>−</sup> supply. We find the average rate of N<sub>2</sub> fixation is equivalent to 62% of the diffusive NO<sub>3</sub><sup>−</sup> supply, though N<sub>2</sub> fixation could exceed the diffusive flux at individual stations. Turbulent diffusivity measurements across the nitracline indicate a mean diffusivity of 0.077 cm<sup>2</sup> s<sup>−1</sup>. If approximations for methodological underestimates in the dominant N<sub>2</sub> fixation technique are considered, the magnitude of N<sub>2</sub> fixation is shown to represent 100% of the NO<sub>3</sub><sup>−</sup> flux on average, and can be almost threefold higher at individual stations. As the study site is characterized by low rates of N<sub>2</sub> fixation compared to other sectors of the North Atlantic this confirms N<sub>2</sub> fixation as a major source term across the subtropical North Atlantic. The seasonal context of our observations suggests environmental factors underlie the in situ variability in observed N<sub>2</sub> fixation rates, and may well explain lower previous assessments of the importance of N<sub>2</sub> fixation relative to diffusive NO<sub>3</sub><sup>−</sup> supply in this region. The diffusive NO<sub>3</sub><sup>−</sup> supply provides <20% of measurable NO<sub>3</sub><sup>−</sup> uptake with the remainder supplied via other mechanisms, most notably nitrification. The mean integrated rate of N<sub>2</sub> fixation equates to just 8% of the NO<sub>3</sub><sup>−</sup> consumed on a daily basis by the phytoplankton community.</p> </abstract>