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

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.