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

Niels Peter Revsbech, Lars Hauer Larsen, Jens Gundersen, Tage Dalsgaard, Osvaldo Ulloa, Bo Thamdrup

Limnol. Oceanogr. Methods 7:371-381 (2009) | DOI: 10.4319/lom.2009.7.371

ABSTRACT: 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.