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 (N-2) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ALOHA (22 degrees 45'N, 158 degrees W) in the North Pacific Subtropical Gyre. Between June 2005 and June 2012, N-2 fixation rates were measured based on adding the N-15(2) tracer as a gas bubble. Beginning in August 2012, N-15(2) was first dissolved into filtered seawater and the N-15(2)-enriched water was subsequently added to N-2 fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of N-15(2)-enriched seawater resulting in twofold greater depth-integrated rates than those derived from adding a N-15(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 mu molNm(-2) d(-1) for the full time series (n = 71). Analysis of the N-15 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 mu molNm(-2) d\(-1), which are equivalent to rates of net community production (NCP) of 1.5 to 2.7 molCm(-2) yr(-1), consistent with previous independent estimates of NCP at this site.