O2/Ar in seawater, pumped from the intake at 7 m below sea level, was measured using an equilibrator inlet mass spectrometer. The mass spectrometer measured a series of dissolved gases including O2 and Ar every 10 seconds. The data contain ion currents of those gases and total pressure in the mass spectrometer. Net community production (NCP), defined as the difference between autotrophic photosynthesis and (autrophic and heterotrophic) respiration, produces O2 proportional to the amount of net carbon. By measuring chemically and biologically inert Ar together with O2, it is possible to isolate O2 variation by physical processes (e.g., air temperature and pressure change and mixing of water masses) and deduce O2 variation by biological processes. To determine the net community (oxygen) production underway, we measured continuous O2/ Ar measurement system using an equilibrator inlet mass spectrometer.