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

Abstract

Abstract We present nearly 9 yrs (June 2005–December 2013) of measurements of upper‐ocean (0 m to 125 m) dinitrogen (N 2 ) 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, N 2 fixation rates were measured based on adding the 15 N 2 tracer as a gas bubble. Beginning in August 2012, 15 N 2 was first dissolved into filtered seawater and the 15 N 2 ‐enriched water was subsequently added to N 2 fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of 15 N 2 ‐enriched seawater resulting in twofold greater depth‐integrated rates than those derived from adding a 15 N 2 gas bubble. Based on this relationship, we corrected the initial period of measurements, and the resulting rates of N 2 fixation averaged 230 ± 136 μmol N m −2 d −1 for the full time series ( n = 71). Analysis of the 15 N isotopic composition of sinking PN, together with an isotope mass balance model, revealed that N 2 fixation supported 26–47% of PN export during calendar years 2006–2013. The N export derived from these fractional contributions and measured N 2 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.