Low Net Community Production from Oxygen/Argon Mass Balance during the Southern Ocean Gas Exchange Experiment

Mass balance techniques using oxygen/argon ratios can yield high quality estimates of net community production. By ratioing with argon, the oxygen signal is corrected for physical influences such as bubble- mediated gas exchange and temperature change. We made near-continuous observations of dissolved oxygen/argon ratios and absolute oxygen concentrations in surface seawater pumped into the ship during the Southern Ocean Gas Exchange Experiment (SO GasEx). This experiment focused on an area to the east of the southern tip of South America during March and April 2008. Oxygen was measured by optode and calibrated with Winkler titrations. Oxygen/argon ratios were measured using a quadrupole mass spectrometer with a membrane equilibrator inlet. Ratios were standardized by comparison with air values and further calibrated by discrete measurements made by high-accuracy sector mass spectrometry. Repeated Winkler measurements of oxygen concentrations in water collected from Niskin bottles and the underway system confirmed the absence of heterotrophic activity in the ship's seawater lines. Spatial surveys showed significant variability in net community production around the study area. Preliminary estimates of productivity in the 3He/SF6 tracer patches laid down for the gas exchange experiment suggested that the first tracer patch had small but measurable rates of net community production but that the second tracer patch was characterized by net respiration or recent upwelling.