Time series measurements of nitrogen fixation in the subtropical North Pacific (extended through 2019)

Rates of N₂ fixation were measured using the ¹⁵N₂ isotopic tracer technique. Sampling occurred during near-monthly Hawaii Ocean Time-series cruises. Whole seawater samples from six discrete depths (5, 25, 45, 75, 100, and 125 m) were subsampled into acid-washed 4.3 L polycarbonate bottles.  The ¹⁵N₂ gas was first dissolved into seawater and 100 mL of the resulting ¹⁵N₂-enriched water was added to 4.3 L polycarbonate sampling bottles. The resulting atom % enrichment of stocks of ¹⁵N₂-enriched seawater was measured using a membrane inlet mass spectrometer. Incubation bottles amended with the ¹⁵N₂ tracer were attached to a free-drifting array and incubated at the discrete depths from which samples had been collected. The array was deployed before dawn and samples were incubated at in situ light and temperature for 24 h. After recovery of the array, the entire volume from each bottle was filtered onto a pre-combusted glass microfiber filter (Whatman 25 mm GF/F) and filters were placed onto pre-combusted pieces of foil in Petri dishes and stored frozen at -20°C. Filters were dried for 24 h at 60°C, pelleted, and the total mass of N and its isotopic signature on each filter were analyzed on an elemental analyzer-isotope ratio mass spectrometer (Carlo-Erba EA NC2500 coupled with ThermoFinnigan Delta S).