Temporal variability of nitrogen fixation and particulate nitrogen export at Station ALOHA

Daniela Böttjer, John E. Dore, David M. Karl, Ricardo M. Letelier, Claire Mahaffey, Samuel T. Wilson, Jonathan Zehr, Matthew J. Church

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92 Scopus citations

Abstract

We 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.

Original languageEnglish
Pages (from-to)200-216
Number of pages17
JournalLimnology and Oceanography
Volume62
Issue number1
DOIs
StatePublished - Jan 1 2017

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