Temporal Variability in Nitrogen Fixation and Particulate Nitrogen Export at Station ALOHA

In order to constrain biological N₂ fixation rates in the North Pacific Subtropical Gyre we present 9 years (2005-2013) of near-monthly rates of upper-ocean N₂ fixation (0 – 125 m), coupled with the isotopic composition of particulate nitrogen (PN) export (150 m) at Station ALOHA (22˚ 45’N, 158˚ 00’W). Between June 2005 and June 2012, N₂ fixation rates were derived 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 both methodologies revealed a robust relationship, with the addition of ¹⁵N₂-enriched seawater resulting in 2-fold greater rates of N₂ fixation than those derived from adding a ¹⁵N₂ gas bubble. The correction of the initial period of measurements (2005 – 2012) based on this relationship resulted in rates of N₂ fixation that averaged 236 ± 107 µmol N m^-2 d^-1 for the full period of study. Furthermore, the analysis of the ¹⁵N isotopic composition of sinking PN, together with an isotope mass balance model, provided additional constraint on N₂ fixation rates. These model derived N₂ fixation rates varied from 70 to 128 mmol N m^-2 d^-1, representing 26 - 47% of the PN export. The resulting total N export derived from this analysis ranged between 506 and 921 µmol N m^-2 d^-1, equivalent to a net community production rate that ranged between 1.4 and 2.5 mol C m^-2 yr^-1, consistent with previous independent estimates at this site.