Oxygen is produced by phytoplankton photosynthesis and consumed by respiration of phytoplankton, zooplankton and bacteria. Oxygen also enters and exits the ocean via physical exchange with the atmosphere. Oxygen concentrations in the surface ocean may be supersaturated by photosynthesis and turbulence enhancing air-sea exchange via bubble injection; or undersaturated due to excess respiration. In cases where exchange with the atmosphere is limited, and/or respiration exceeds photosynthesis, oxygen concentration can be reduced to very low levels (hypoxia) or entirely depleted (anoxia). This is uncommon in cold Antarctic Seas where respiration is depressed and oxygen solubility is enhanced by low temperature. Different water masses have characteristic oxygen concentrations which serve as tracers for diagnosing physical mixing and advection. Dissolved oxygen was analyzed by Winkler Titration (see Methods) in CTD-Rosette bottle samples at all depths sampled until 2012. This measurement was discontinued in 2013. The CTD has duplicate oxygen electrodes that provide continuous vertical profiles of oxygen concentration at all depths on all casts. The vessel also has continuous underway, Optode determination of dissolved oxygen in the surface (ship's intake at 6 meters depth) on all cruises. Finally we now routinely measure net community production by Equilibrator Inlet Mass Spectroscopy (EIMS) on LTER cruises
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