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 O2 concentrations in oceanic oxygen minimum zones below 1–2 µmol L−1. We present a new amperometric sensor for the determination of ultra‐low O2 concentrations under in situ conditions. The electrochemical STOX O2 sensor contains a primary sensing cathode and a secondary cathode that, when polarized, prevents entry of O2 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 O2, 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 O2. Application in a reactor on board demonstrated that changes in O2 concentrations in OMZ water containing < 200 nmol L−1 O2 could be monitored over periods of hours to days. The linear decrease in O2 concentration in the reactor indicated very low (< 20 nmol L−1) half saturation constants for the O2 respiring microbial community.