Determination of ultra‐low oxygen concentrations in oxygen minimum zones by the STOX sensor

The methods used until now have not been able to reliably resolve O 2 concentrations in oceanic oxygen minimum zones below 1–2 µmol L −1 . We present a new amperometric sensor for the determination of ultra‐low O 2 concentrations under in situ conditions. The electrochemical STOX O 2 sensor contains a primary sensing cathode and a secondary cathode that, when polarized, prevents entry of O 2 into the sensor. This arrangement enables frequent in situ zero calibration and confers the sensor with a detection limit of 1‐10 nmol L −1 O 2 , even during application on a Conductivity‐Temperature‐Depth (CTD) profiler at great water depths. The sensor was used during the Galathea 3 Expedition to demonstrate that the core of the oxygen minimum zone (OMZ) off Peru contained < 2 nM O 2 . Application in a reactor on board demonstrated that changes in O 2 concentrations in OMZ water containing < 200 nmol L −1 O 2 could be monitored over periods of hours to days. The linear decrease in O 2 concentration in the reactor indicated very low (< 20 nmol L −1 ) half saturation constants for the O 2 respiring microbial community.