Gradients 4 - TN397 - 15N and 13C

<p>Standard protocols were used to measure in vitro primary productivity via the 13C radiotracer method [Lengendre et al., 1996] and N2-fixation rate measurements via 15N2 isotopic gas method described by Montoya et al., 1996. Briefly, water samples were collected before dawn in ~4.3L polycarbonate bottles, a total of 4ml of enriched 15N2 gas was added via syringe. Bottles were then injected with 1 ml of 47mM 13C bicarbonate stock and gently mixed by inversion. &nbsp;Bottles were incubated either on deck in incubators screened to 48% of surface irradiance and plumbed with flow through surface seawater for temperature regulation or on drifting arrays that allowed for incubations to happen at original light levels. All bottles were incubated for ~ 24hr. Time zero bottles were also collected and sacrificed for each station/depth. Following dawn to dawn incubations, each sample was filtered onto a combusted 25mm glass fiber filter, filters were folded in half and placed in combusted aluminum foil and stored at -20&deg;C for later analysis. Post-cruise, all filters were thawed and dried overnight at 60&deg;C. Samples were then balled into Sn boats and plated for analysis. The carbon and nitrogen isotopic composition (13C and 15N, respectively) were analyzed by continuous-flow isotope ratio mass spectrometry by the Biogeochemical Stable Isotope Facility at University of Hawaii (https://www.soest.hawaii.edu/GG/isotope_biogeochem/index_files/Page532.htm). Time stamp is in UTC.</p>