1 research outputs found
Temporal variability of nitrogen fixation and particulate nitrogen export at Station ALOHA
We present nearly 9 yrs (June 2005āDecember 2013) of measurements of upperāocean (0 m to 125 m) dinitrogen (N2) fixation rates, coupled with particulate nitrogen (PN) export at 150 m, from Station ALOHA (22Ā° 45ā²N, 158Ā°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 15N2āenriched water was subsequently added to N2 fixation incubations. Direct comparisons between methodologies revealed a robust relationship, with the addition of 15N2āenriched seawater resulting in twofold greater depthāintegrated 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 230āĀ±ā136 Ī¼mol N mā2 dā1 for the full time series (nā=ā71). Analysis of the 15N isotopic composition of sinking PN, together with an isotope mass balance model, revealed that N2 fixation supported 26ā47% of PN export during calendar years 2006ā2013. The N export derived from these fractional contributions and measured N2 fixation rates ranged between 502 and 919 Ī¼mol N mā2 dā1, which are equivalent to rates of net community production (NCP) of 1.5 to 2.7 mol C mā2 yrā1, consistent with previous independent estimates of NCP at this site