In situ determination of oxygen and nitrogen dynamics in the upper ocean

Total gas pressure, oxygen, temperature and salinity were measured at 50 m on a mooring at the Hawaii Ocean Time series (HOT) station from January through September of 1997 and 1998 using a gas tension device (GTD) and a conductivity temperature depth and oxygen (CTD-O2) sensor (GTD–CTD-O2 instrument package). Our goal was to evaluate the precision and accuracy of the in situ total gas and oxygen measurements and to assess their utility for remotely determining net biological oxygen production in the euphotic zone. By calibrating the oxygen sensor approximately every month during periodic visits to the mooring it was possible to remotely measure the time history of the partial pressures of nitrogen and oxygen to within ±0.5% accuracy as assessed by independent determination of nitrogen concentration. Oxygen changed at 50 m by 2–4% in a series of episodes with durations of 1–3 months, indicating the discontinuous nature of net O2 production and the probable decoupling of oxygen production and respiration in the euphotic zone. The GTD and oxygen measurements together can be used to distinguish the effects of physical processes and net biological oxygen production on the oxygen concentrations. The largest uncertainty in the coupled O2 and total gas pressure measurements is the drift of the oxygen sensor, making frequent calibration presently a necessity. With more complete vertical coverage in the upper ocean it should be possible to use this approach to determine depth-integrated net oxygen production in the euphotic zone and upper thermocline respiration.