AGU Ocean Sciences Meeting, Honolulu, HI, February 2002.

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

S. Emerson¹, C. Stump¹, B. Johnson² and D. M. Karl³

¹School of Oceanography, University of Washington, P.O. 355351, Seattle WA 98195, US

²Department of Oceanography, Dalhousie University, Halifax, NS B3H 4JI, Canada

³University of Hawaii, Department of Oceanography, Honolulu, HI 96822, US

Total gas pressure, oxygen, temperature and salinity were measured at 50 meters 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 CTD-oxygen 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 meters by 2-4 % in a series of episodes with durations of 1 - 3 months, indicating the discontinuous nature of net oxygen production and the probable decoupling of oxygen production and respiration in the euphotic zone. The GTD and O₂ 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.