Fully automated spectrophotometric approach to determine oxygen concentrations in seawater via continuous-flow analysis

Abstract

Oxygen consumption measurements are the most common approach to estimate the remineralization of organic carbon to CO2. A refined protocol of the spectrophotometric Winkler approach is presented, where a continuous-flow analyzer is coupled with a custom-made autosampler holding up to 30 oxygen bottles. The time required for analysis is 2 min per sample, and the precision is 0.05% at ~200 mmol O2 m-3. Thus, analysis speed and quality are significantly improved compared to the classic Winkler titration approach to determine O2 concentrations. The accuracy of the method is 99.7% ± 0.2% as determined by comparing the measured versus the theoretical oxygen concentration of saturated seawater at 20°C. The measured absorbance of the iodine at 460 nm wavelength was linear up to an equivalent of 320 mmol O2 m-3, which is within the range of open-ocean oxygen concentrations. The instrument was tested on a cruise in the subtropical North Atlantic where community respiration (CR) and bacterial respiration (BR) were determined. Both CR and BR decreased by ~85% from the Mauritanian upwelling region and the oligotrophic gyre. Along this transect, the contribution of BR to CR increased from 36% to 76%. The instrument proved highly suitable for work at sea and should allow more rapid and precise oxygen concentration measurements under open-ocean conditions.