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© 2016 The Authors Limnology and Oceanography published by Wiley Periodicals, Inc. on behalf of Association for the Sciences of Limnology and OceanographyWe present nearly 9 yrs (June 2005–December 2013) of measurements of upper-ocean (0 m to 125 m) dinitrogen (N2) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ALOHA (22° 45′N, 158°W) in the North Pacific Subtropical Gyre. Between June 2005 and June 2012, N2 fixation rates were measured based on adding the 15N2 tracer as a gas bubble. Beginning in August 2012, 15N2 was first dissolved into filtered seawater and the 15N2-enriched water was subsequently added to N2 fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of 15N2-enriched seawater resulting in twofold greater depth-integrated rates than those derived from adding a 15N2 gas bubble. Based on this relationship, we corrected the initial period of measurements, and the resulting rates of N2 fixation averaged 230 ± 136 μmol N m−2 d−1 for the full time series (n = 71). Analysis of the 15N isotopic composition of sinking PN, together with an isotope mass balance model, revealed that N2 fixation supported 26–47% of PN export during calendar years 2006–2013. The N export derived from these fractional contributions and measured N2 fixation rates ranged between 502 and 919 μmol N m−2 d−1, which are equivalent to rates of net community production (NCP) of 1.5 to 2.7 mol C m−2 yr−1, consistent with previous independent estimates of NCP at this site.
Author(s): Bottjer D, Dore JE, Karl DM, Letelier RM, Mahaffey C, Wilson ST, Zehr J, Church MJ
Publication type: Article
Publication status: Published
Journal: Limnology and Oceanography
Year: 2017
Volume: 62
Issue: 1
Pages: 200-216
Print publication date: 01/01/2017
Online publication date: 25/08/2016
Acceptance date: 11/07/2016
ISSN (electronic): 1939-5590
Publisher: Wiley Blackwell
URL: https://doi.org/10.1002/lno.10386
DOI: 10.1002/lno.10386
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